Thursday, February 26, 2015

Nuclear Power

Nuclear weapons have a singular purpose to kill as many people as possible. Nuclear power, however, based on the same physics, has the potential to satisfy the world's increasing demand for energy without contributing to global warming, which is perhaps, aside from the threat of nuclear war, the most pressing threat to our existence on this planet. As the effects of climate change grow more apparent around the world, countries are likely to start looking for energy alternatives to carbon-emitting non-renewable coal, natural gas and oil. It is definitely time for policy-makers to develop a scientific understanding of nuclear power and it is probably a good time for the rest of us to learn about it as well, so we can make informed choices about what kinds of energy we choose to support. I found this article fascinating to research and I hope it will help you develop a better conceptual understanding of nuclear power as well as the advantages, disadvantages and risks of different systems. Nuclear energy is complex and poorly understood by most of us but you don't have to be a nuclear physicist to grasp the essentials and even some of the nuances.

If you read some of the previous articles in this series (chemical/nuclear explosions and nuclear weapons), it will be easy to appreciate the enormous amount of energy available in a nuclear reaction. A very small amount of fuel delivers a vast amount of useable energy. A nuclear bomb releases all of this energy in one gigantic explosion. Nuclear power plants, on the other hand, harness the same energy by carefully controlling the reaction rate and capturing the heat emitted by the fission. But is it safe? Can a nuclear plant blow up like a nuclear bomb? What's a meltdown and when can it happen? What do we do about the inevitable radioactive waste? For these reasons, nuclear power is highly controversial around the world. In order to decide for ourselves what our position is on nuclear energy we need to learn how these reactors work.

There are significant similarities between a nuclear reactor and a nuclear weapon and there are critical differences as well. A nuclear weapon relies on a run-away nuclear reaction, whereas the nuclear reaction rate in a reactor is highly controlled.

Can A Reactor Blow Up?

The nuclear fission reaction that occurs in nuclear weapon is the same cascading reaction that takes place inside a nuclear reactor used for energy production. However, the designs of the two devices are vastly different. A nuclear weapon is carefully designed to maximize an uncontrolled chain reaction, releasing as much energy as possible as quickly as possible, before the weapon itself explodes and stops the chain reaction. A nuclear power reactor's reaction is controlled so that it releases a steady supply of energy over time. While a reactor can overheat and undergo meltdown, a very dangerous situation, it is impossible for a power reactor to undergo a nuclear explosion (though it can undergo a regular explosion as a result of steam pressure or hydrogen gas build-up). The fission chain reaction itself, whether it's in a nuclear weapon or a reactor, is the same reaction. Free neutrons emitted by one fission initiate other fissions in the material, and so on.

The reaction must be kept critical or it will spontaneously slow down and stop. What does critical mean? I offer a long explanation in the article Nuclear Weapons: Understanding Binging Energy about three quarters down the article starting at "as you can see . . . " but for this article all you need to know is that critical means that the nuclear reaction is self-sustaining where there is no increase or decrease in power, temperature or neutron population. Supercritical means the reaction is increasing in power, temperature and neutrons; subcritical means that it is decreasing.

This kind of reaction naturally fluctuates - it grows or shrinks exponentially and the trick for a reactor is to hold that reaction at a fairly constant rate. To do this, a reactor must be able to slow the reaction process down to the point that it can be controlled, whereas in a bomb you want the reaction to be run-away. In a reactor, the criticality itself must have a slowed down time-scale and the secret to that is to make use of delayed neutrons versus prompt neutrons, something we will explore in detail. A reactor is always kept just at criticality - in reality it is always fluctuating between delayed-supercritical and subcritical, but the important point (and the key reason why a reactor will never blow up in a nuclear explosion) is that it is always below prompt-critical. A nuclear weapon must be at or above prompt-critical in order to detonate. This means that for each fission event, one or more immediate or prompt neutrons is emitted, causing an additional event, which causes a very rapid exponential increase in fission, and therefore in power, heat and neutron number.

This being said, a reactor can experience an event that is akin to a pre-detonation in a nuclear bomb, where a low-powered uncontrolled chain reaction explosion occurs in one very small section and this would cause a lot of damage and a meltdown. This is not a full-scale nuclear explosion and it has never happened in practice. Even the Chernobyl disaster, which involved a run-away chain reaction, a meltdown and a low-powered steam explosion and fire (the last two of which threw a tremendous amount of radioactive debris into the air), did not include a nuclear explosion of any kind. A steam or hydrogen explosion with the release of radioactive fission products into the air is most often the greatest safety concern for any nuclear reactor.

Power Reactors Versus Research Reactors

There are two basic types of nuclear reactor based on type of use - power reactors, which most of the focus is on here, and research reactors. Research reactors differ from power reactors in that they use uranium that is more highly enriched - usually around 20% enrichment, but some reactors use very highly enriched weapon-grade 93% uranium. These reactors are designed differently from power reactors because they have a much higher core power density. Whereas power reactors are sources of useful heat, research reactors are most often used as fast neutron sources.

A brief mini-lesson on neutrons: All these neutron terms - delayed, prompt, fast and thermal (slow) - can be very confusing. After all a neutron is just a neutron. The terms delayed and prompt require a more involved explanation, given above, and we will talk more about them later. The terms fast and thermal, however, simply refer to the neutron's kinetic energy. A lot of kinetic energy means it is traveling fast. A fast neutron has an energy of around 1 MeV (million electron volts) and it is traveling at about 10% the speed of light (20,000 km/s), whereas a thermal neutron (we will be talking about both of these in detail) has an energy of about 0.025 eV, equivalent to about 2.2 km/s. That's about 10,000 times less energy than a fast neutron.

The free neutrons that research reactors produce are used for neutron scattering experiments and for the testing of new materials and they are often also used for the production of radioisotopes for medical and industrial use. Research reactors use either fast or thermal neutrons. Power reactors are used for electricity production, heat generation and for submarine propulsion and they usually but not always involve thermal neutrons.

Research Reactors

An example of a research reactor is the NRU thermal reactor at Chalk River. It's one of several thermal neutron research reactors in the world. CNL's (Canadian Nuclear Laboratories) Chalk River Laboratories in Ottawa, shown below, contains Canada's only major neutron beam source and material testing reactor, and it is one of the two largest producers of medical isotopes in the world, which are used for diagnostic applications as well as cancer treatments. Research here also focuses on increasing knowledge of the effects of radiation on humans to ensure worker safety at nuclear facilities around the world. A few decades ago my dad worked here.

Padraic Ryan;Wikipedia
SLOWPOKE - A Small Compact Fast Research Reactor

Universities in various provinces in Canada have research reactors, about half of which are SLOWPOKE reactors. The University of Edmonton (where I got my degrees and worked) has a SLOWPOKE 2 fast neutron research reactor that was developed in the late 1960's by Atomic Energy of Canada Limited (AECL). It uses highly enriched uranium (93%; sourced from the U.S.) as its fuel. It produces fast neutrons for radioisotope production, neutron activation (elemental) analysis, research and teaching. This compact low energy reactor was specifically designed for Canadian universities, using a beryllium reflected core with very low critical mass. It produces a very high neutron flux. The core is only 22 cm x 22 cm and it sits in a pool of light water 2.5 m in diameter and 6 m deep. The pool is large enough that the core can be cooled by natural convection. It has a high degree of inherent safety because it can regulate itself passively. The chain reaction slows down when the water heats up or when it starts to bubble (boil).

The U of A website is disappointingly (and perhaps reassuringly?) spare in detail. I was unable to find out if there are any plans underway to update this reactor into a more secure low-enriched (20%) uranium version, a process that is just getting underway for Canadian SLOWPOKE reactors.

A larger SLOWPOKE 3 reactor was designed in the 1980's. This reactor could supply one hundred times more power, enough to be used for a district heating (networked hot water) system for a remote community that currently relies on fossil fuels such as oil or natural gas for heating. The expected market for this system so far hasn't materialized and the single reactor that was built at Whiteshell Laboratories in Manitoba was shut down, likely because the prices of oil and especially natural gas have been relatively low.

Thermal Reactors Versus Fast Reactors

Another way of comparing reactors is by the way they carry out the fission reaction. Most power reactors are thermal reactors, which means they use slowed or thermal neutrons to maintain the fission chain reaction in their fuel. These reactors use a neutron modulator, a material that slows free neutrons emitted during fission until they are thermalized, which means the neutrons have the same average kinetic energy (temperature) as the surrounding particles in the core. The neutrons are slowed in order to increase their cross section, or probability of fissioning the fissile fuel (usually uranium-235), and to reduce their chance of being captured by fissionable but not fissile uranium-238, a process that would take the neutrons out of play. A fissile material is capable of sustaining a nuclear chain reaction (with neutrons of any energy). A fissionable material is capable of undergoing fission only by capturing a high-energy fast neutron, and once captured it cannot sustain a fission chain reaction.

Mined uranium naturally contains far more U-238 than U-235. Enriching uranium increases the percentage of the U-235 isotope present. U-235 is the fissible isotope, and when it fissions, it emits fast neutrons. These neutrons are more likely to pass right through the fuel than to interact with it and produce new fissions. In order to keep the fission reaction going, some kind of moderating material is required in order to slow these fast neutrons down to the point where they are likely to interact with other nuclei. Several materials can be used, most commonly regular (light, 1H20) water, but solid graphite, heavy water (2H20), and less commonly beryllium and other materials can be used. All of these substances have low mass (they contain fairly small atoms), high scattering cross section and low absorption cross section. This means that neutrons are scattered rather than absorbed, as they collide elastically with nuclei in the moderator. The neutrons bounce off nuclei but now have less energy, so this process in effect distributes their kinetic energy around until they reach thermal equilibrium with the moderator. Some reactors are more thoroughly thermalized than others. For example, in the NRU research reactor at Canadian Nuclear Laboratories in Chalk River, nearly all the fission reactions are produced by thermal neutrons, while in a pressurized water power reactor (we will discuss) a portion of the fissions are produced by fast neutrons. In a theoretical supercritical water power reactor (will also discuss), more than half the fissions may be produced by fast neutrons and this would technically make these reactors fast neutron reactors rather than thermal reactors.

Fast reactors in use today use no moderator at all. They rely on fission produced by fast neutrons to sustain the chain reaction. As we might suspect, ordinary natural unenriched uranium won't cut it in most fast reactors. The cross section is too small and the fission chain reaction cannot be sustained. A fast reactor can make use of non-fissile U-238 but it requires a more substantial proportion of fissile U-235 in its fuel as well. In these reactors, up to 20% of the fissions can come from the fission of U-238, which is not fissile at all with thermal neutrons. Fast neutrons, moving at around 10% the speed of light, have a very low chance of producing fission in neighbouring U-235 nuclei (low cross section), so richer fissile material (a higher density of U-235) is needed in order to maintain the chain reaction. Some of these reactors use highly enriched weapon-grade uranium, which is both very expensive (a very expensive to build enrichment plant is required) and presents a security issue. However, a fast reactor can be designed that actually produces more fissile material than it consumes. It breeds fuel in other words and is therefore called a fast breeder reactor. For example, it can breed fissile plutonium by fissioning non-fissile U-238.

Breeder Reactors

Several breeder reactor prototypes have been built around the world since the 1960's but only three are currently in use. The Superphénix reactor in France, brought online in 1984 and closed in 1998, could produce 20% more fuel than it consumed, and optimum breeding allowed about 75% of the energy of natural uranium to be used compared to just 1% used in a standard light water reactor. The fast neutrons used in most breeder reactors have enough energy to fission heavy medium and long half-life fission products (these are transuranic actinides on the periodic table) - these isotopes contribute significant radiotoxicity to ordinary spent nuclear fuel for over 500,000 years. In contrast, the fission products in breeder reactor spent fuel tends to remain radiotoxic only for a few hundred years, a huge advantage.

All reactors breed some new fissile material. The ratio of new fissile material to fissile material that is "burned" is called the conversion ratio. A conversion ratio of 1 means that the reactor is breaking even - it produces as much fissile material as it consumes. Light water reactors have a conversion ratio of 0.6. Pressurized heavy water reactors (such as CANDU reactors) have ratio of 0.8. In a breeder reactor the ratio is over 1. Eventually a breeder reactor will produce enough new fuel to supply a starting fuel load for another reactor. These reactors have a very high neutron economy and in principle they can be used to create new fissile fuel or run long-term without refueling or they can be used to burn nuclear waste.

Radioactive Waste: A Growing Problem

When we think about breeder reactors, it brings to the front the challenges of dealing with radioactive waste. What is it? Nuclear fuel usually consists of metal rods that enclose stacked up ceramic pellets, which are made of compacted uranium oxide powder that is sintered at high temperatures, though there are exceptions. The metal varies with the design of the reactor, as does the kind of fuel used, but the metal is often a zirconium alloy. This alloy has a very low cross section for thermal neutrons, a good thing. It is also very hard and it is very corrosion-resistant. Two fuel bundles from a CANDU power reactor are shown below right. The CANDU, a heavy-water pressurized power reactor, invented in Canada, is so far our only type of power reactor.

Nuclear waste looks just like the fuel that was put into the reactor. However, what was once uranium and oxygen is now a mixture of isotopes of almost all the transition metals in the periodic table, many of which are unstable and therefore radioactive. Surprisingly, almost all the mass (96%) of the waste is still uranium. Most of the original nonfissile U-238 is still present and even a tiny amount of the fissile U-235 fuel is still there because not all of it will fission. About 1% is plutonium - (fissile) Pu-239 and (fissionable) Pu-240, and just 3% consists of the fission products of U-235 and Pu-239. These products cover a wide variety of elements from zinc up to the lanthanides, many of which are either non-radioactive or very short-lived isotopes. Most hazardous in the waste are the medium and long-lived isotopes like strontium-90, cesium-137, technetium-99 and iodine-129.

In a natural uranium reactor such as the CANDU, the fissile component in the fuel starts off at 0.71% U-235 and ends up with almost the same amount of fissile material, 0.50%, which is now composed of 0.23 % U-235 and 0.27% Pu-239. What makes the waste "waste" is not the lack of fissile material but the build-up of neutron absorbing fission products, which make sustaining the fission reaction impossible. These percentages are small and it doesn't seem like a lot of radioactivity on paper, but if you stood unshielded a few metres from spent nuclear fuel that was just removed from the reactor you would receive a lethal dose in a few seconds. There would also be a tremendous amount of heat given off. Because most isotopes in the nuclear waste have a short half-life, decay heat from the waste decreases fairly rapidly and exponentially. The graph below shows how fast decay heat decreases over 10 days after total shutdown, using two different models.

Even though just a tiny percent of the original heat is now emitted after ten days, the fuel rods continue to emit deadly radiation. To shield the radiation (alpha, beta and gamma radiation emitted from decaying isotopes) and absorb the heat, spent fuel rods must be stored in a water pool for a minimum of one year and sometimes up to 20 years depending on the composition of the fuel used. If a reactor were shut down and no cooling system was working, the decay heat from the rods would cause the core of the reactor to reach an unsafe temperature within hours to days, depending on the type of core. It could cause a full meltdown in a light water reactor, as well as steam or hydrogen explosions.


The word "meltdown" is not a technical word but in practice it refers to the core melting and partially or completely collapsing. The temperature has risen enough that least one nuclear fuel element exceeds its melting point. Often the core cladding is breached as well, letting radioactive materials breach containment and escape into the environment. Lava-like core material, called corium, can react with oxygen or steam chemically, releasing even more heat into the system, and it can react with boric acid as well if that is used as an emergency coolant (more about this later). It may also release volatile elements into the air. The zirconium alloy metal rods can oxidize under extreme heat, releasing explosive hydrogen gas. The corium temperature can reach as high as 2400°C, and at this temperature when it comes into sudden contact with water, steam is explosively released, sending shards of core everywhere and possibly further damaging the core containment. Thermal decomposition of the concrete containment in contact with corium produces water vapour and carbon dioxide, which can further react with metals in the corium and create even more explosive gas. Core meltdown is clearly a very dangerous, and dangerously unpredictable, situation.

Even after full reactor shutdown, there is still a lot of heat production because the decay of the products will still be going on, contributing about 7% of full reactor power. This feature, common to all reactors, can have dire consequences. In the Fukushima Daiichi nuclear disaster, for example, residual decay heat from the core, after complete shutdown (the fission chain reaction was stopped), rose after a loss of coolant flow. Fuel rods exposed to air reacted chemically with it producing hydrogen gas, which is highly explosive when mixed with air. The inevitable resulting hydrogen explosions blew radioactive material everywhere, contaminating hundreds of square kilometres of land and part of the ocean.

CANDU Reactor: Waste Problem and Possible Solution

Most nuclear power reactors use low-enriched uranium as fuel but the heavy water design of the CANDU reactor means that it can use fuel with a lower percentage of fissile uranium than light water reactors. It can even use recovered uranium from spent light water reactor fuel. It can also burn a mixture of uranium and plutonium oxides (MOX fuel) as well as the plutonium from dismantled nuclear weapons. These are all big advantages of this kind of reactor. Still, the CANDU is not a breeder reactor. It creates a significant amount of radioactive waste that has to be stored somewhere. Currently Canada stores all of its spent nuclear fuel at the reactor sites in pools of water (for 10 years) and then in dry cask storage. As of 2011, Canada had 2.2 million spent fuel bundles in storage, each containing 20 kg of spent fuel, translating into about 44,000 tonnes of heavy metal waste. There are plans for an eventual deep geologic repository below the water table but nothing has been built as of yet.

Now for the bad news: Although decay heat drops exponentially as soon as the bundles are removed from the reactor and during this time much of the radioactive material has decayed into safe stable elements, heavy transuranium actinides, with half-lives of up to 25,000 years remain and become the dominant source of decay heat after about 100 years dry storage. Spent CANDU fuel remains radiotoxic for about 1 million years. Perhaps a better way of dealing with this waste, rather than storage, is to reuse it in future fast neutron reactors. A fast reactor can potentially convert the actinides into other fission products that have much shorter half-lives. The use of fast neutron reactors would not only reduce the effective half-life of the radioactive waste but it could make use of it to generate an enormous amount of additional electricity. A very interesting 2012 paper written by Dr. Peter Ottensmeyer for the Ontario Centre for Engineering and Public Policy claims a reduction in radiotoxicity to about 300 years (fast neutrons can fission the transuranic actinides) is possible, that is until the waste reaches the background level of natural uranium. This paper also claims that Canada's current 44,000 tonnes of total spent fuel could ultimately be converted into $48 trillion dollars worth of (non-carbon) electricity in fast neutron reactors because there is still significant fissile energy present in the waste that the fast neutron reactor can use. Understandably on reading this, one can tell that he is impatient with policymakers to get on this, rather than continue to store the waste above ground or spend resources to transport it and put it deep underground.

How A Nuclear Power Reactor Works

All nuclear power reactors utilize the same basic plan. Heat from nuclear fission is transferred to a fluid, which flows through a turbine, making it turn. The turbine then either drives a submarine's propellers or it spins an electrical generator, as shown in this brief 1-minute video produced by the Tennessee Valley Authority (requires the instalment of a VLC player).

Reactor Type Based on Coolant Used

In addition to categorizing nuclear reactors by their function - power versus research, or whether they use fast or thermal neutrons, reactors are categorized based on the kind of coolant they use. The following are some of the most common reactor designs in use. There are many designs and some are not included here, such as the gas-cooled reactor, of which Great Britain currently has two advanced designs and the still very experimental molten salt reactor.

Pressurized Water Reactor

A controlled uranium fission chain reaction produces lots of heat. In a pressurized water reactor, water is kept under pressure in the primary coolant loop (the orange loop in the diagram below). This water absorbs fission heat by thermal conduction through the core cladding. Now extremely hot but not able to boil as it's pressurized, the water is then pumped through a heat exchanger, which is also a steam generator (blue container with the orange tube in it). The water in this secondary system is completely separate from the pressurized core coolant water and in the diagram it is vastly simplified - the steam created is actually pumped through thousands of tubes. This secondary coolant water (blue) evaporates into pressurized steam, which drives the turbine (grey rotor assembly) and that spins the generator. This arrangement, using two completely separated fluids, ensures that the secondary coolant never becomes radioactive.

The animation below shows how energy is transferred through a typical pressurized water reactor. Primary coolant is shown in orange and secondary coolant is shown in blue

The steam turbine drives an electrical generator connected to the power grid, which distributes electrical energy. The secondary coolant is then cooled and condensed back into liquid water and then pumped back into the steam generator.

Most Western nuclear power plants, like the reactor just described, are pressurized water reactors that use ordinary (light) water as the primary coolant. The CANDU power reactor, mentioned earlier, is an exception because it uses pressurized heavy water. We will explore how it works in a moment but first I'd like to compare two additional kinds of light water reactors. One uses boiling water and the other, which is still in conceptual stage, uses supercritical water.

Boiling Water Reactor

The boiling water reactor, shown below, is a bit simpler in design than the pressurized water reactor. In this case, water is heated by thermal conduction from the reactor core (red vertical fuel rods) where it boils into steam (blue to violet transition) and then the steam is used directly to drive a steam turbine (green box). The steam is condensed back into water (the grey pipe contains cold water) and returned to the core. There is no secondary coolant and no pressurizer.

Robert Steffens (alias RobbyBer 8 November 2004), SVG: Marlus_Gancher, Antonsusi (talk) using a file from Marlus_Gancher;Wikipedia
Water around the core of any reactor is always contaminated with traces of radioisotopes, so the coolant in a boiling water reactor is considered contaminated. This means that the turbine must also be shielded and protective gear must be worn during regular maintenance of the turbine. This design is used entirely by Sweden and Mexico and most of Japan's newest builds are boiling water type reactors. It has several advantages over a pressurized system. It operates at a much lower nuclear fuel temperature, there is a lower risk of rupture and associated loss of coolant and there is a lower risk of core damage should a rupture occur. Additionally, because only one vendor sells the current boiling water reactor (GE/Hitachi), this means that all reactors of this type have predictable uniform designs, maintenance is standardized and replacement parts are easier to come by. In the U.S., for example, three companies sell very divergent pressurized water reactor designs, and this can complicate repair/maintenance, and even emergency procedures vary.

The Fukushima 1 nuclear power plants, which went into meltdown and exploded, were boiling water reactors. They experienced a so-called double-failure event in which the reaction could no longer be properly limited or moderated (power was lost to the water pumps) combined with a complete emergency core cooling system failure. Water vapour pressure continued to increase, heated by the nuclear fuel, and the Mark 1 containment that was used in these reactors failed, which allowed the release of highly pressurized radioactive steam. Modified containment is now designed to release steam in a controlled manner should such a double scenario happen again, with the addition of activated carbon filters to trap radioisotopes as the steam leaves.

A disadvantage of the boiling water reactor is that the newest designs have control rods that are inserted from the bottom of the reactor (the diagram above shows this new design). In most other reactor types, the rods are held above the reactor by electromagnets so if power is lost the rods fall into the reactor and the reaction is stopped. In the boiling water reactor two hydraulic power sources can drive the rods into the reactor in an emergency, but the system relies on at least one of them to work.

Supercritical Water Reactor

A third type of light water reactor is the supercritical water reactor, which is currently still in the concept stage of development (it's categorized as a Generation IV reactor).

What is supercritical water? Supercritical water, not to be confused with a supercritical mass of nuclear fuel, is water that is at its critical point. Above a specific temperature and pressure, the physical properties of water change very dramatically and these, now supercritical properties, can be utilized. To understand what critical point is, lets first consider the physical phases of water. A general phase diagram is shown below right (ignore the plasma phase for our purposes).

We know that water exists in three well-known physical phases - ice, liquid water, and gaseous water vapour - with each state having specific and unique physical properties. We usually think of all these phase changes happening under ordinary air pressure (1 atm), where just the temperature changes, but pressure is actually a significant factor in phase transition. A more complex pressure-temperature phase diagram for pure substances, including water, is shown below left. Now we can see where the phase transitions occur, and we can see that there is an additional phase called supercritical fluid that is only possible under high pressure.

Most pure substances follow a similar three-phase transition and each has its own unique critical point. The green lines mark the solid-liquid transition. The solid line applies to most substances, while the dotted green line marks water's anomalous behaviour: When cooled to 4°C, water as a liquid becomes more dense like other substances, but when it is cooled further, it becomes less dense until it freezes. That's why ice floats on water, a very unusual behaviour among pure substances. The blue line represents water's boiling point (its liquid to gas transition) and the red line is frozen ice. At water's triple point (lower left red dot; 0.01°C and 0.006 atm), all three phases of water exist in equilibrium. Even a very tiny deviation in pressure or temperature from this point will change all the water into liquid, solid or gas. Up past the blue line, the liquid-gas boundary ends at the critical point, which for water is 374°C and 218 atm. As water approaches this temperature and pressure, its liquid properties and vapour properties grow more and more similar. Its heat of vapourization approaches zero, which means that there is no change in enthalpy between its liquid and gas state. At critical point, water exists as one phase. Above critical point, the liquid and vapour phases no longer exist, and water now becomes a supercritical fluid. It can effuse through solids like a gas and it can dissolve materials like a liquid. Around critical point, tiny changes in pressure or temperature result in large changes in density.

A supercritical water reactor operates at higher pressure and temperature than pressurized water reactors and it has a direct once-through cycle like boiling water reactors. Its advantage is that it has a much higher thermal efficiency while keeping to a simple design, which looks a lot like the boiling water reactor plan except the pressure is very high and the water phase is different, shown below.

Because water is always in just one physical phase, there is no need for repressurizers or steam generators (as in pressurized water reactors) or for the recirculation pumps, steam separators and dryers used in boiling water reactors. By avoiding boiling, no chaotic voids or bubbles (areas with less density) are present, making heat transfer and water flow easier to predict and control. These design changes reduce costs and make the reactor safer to use. Supercritical water, however, has less moderating effect on free neutrons because it is less dense than liquid water, and this might limit it to being a fast neutron reactor, which is not necessarily a bad thing. A fast neutron reactor has advantages such as higher core power density and more efficient use of nonfissile uranium-238 (which is by far the more abundantly available isotope of uranium). The fast neutrons also split (radioactive) actinides and transmute long-lived radioactive fission products into isotopes with shorter half-lives. By doing this, this reactor will not only reduce the radiotoxicity of the nuclear waste it produces but it cuts short the waste's radioactive lifetime as well.

And it's highly efficient. A supercritical water reactor can in theory use almost all of the fuel present. Conventional fast reactors have the disadvantage that they are expensive to build and they require much more costly highly enriched fuel (some need weapons-grade fuel, which adds a security concern as well). The supercritical water reactor will likely need higher fuel enrichment as well but nowhere near weapon-grade. A disadvantage is that supercritical water is more corrosive than ordinary water, so all the core materials including the cladding will have to be of a higher standard (materials that resist corrosion but also do not absorb many neutrons). Hydrogen can also be added to the water to reduce its corrosiveness. Higher temperature and pressure in the core in general will mean increased mechanical and thermal stress on vessel materials as well.

On the other hand, supercritical water has far better heat transfer ability so less of it is needed, and this means a smaller core and a smaller containment structure is needed. However, in an accident, there is less water available to act as an emergency coolant. Temperature in the core could shoot up very high very fast, requiring cladding that will not readily melt and well-designed and redundant emergency cooling systems.

These are a couple of the challenges involved in developing this kind of reactor. Despite them, the reactor's minimal radioactive waste, low cost and simple design place this reactor high on the list of potential future design candidates.

The supercritical water reactor is just one of many theoretical reactor designs currently being researched, all of them categorized as Generation IV reactors. A simple comparison of reactor generations is shown below. CANDU, developed in the 1970's, is Generation II, for example.

Wikipedia has a convenient list of new potential thermal and fast reactors, complete with schematic diagrams for each of their designs. At the end of this article I will explore some more experimental designs.

Heavy Water Pressurized Reactor: CANDU

Edmonton is currently home to Alberta's only nuclear reactor - it's the research reactor SLOWPOKE 2, described earlier. In addition to research reactors, Canada has 20 nuclear power reactors - in Ontario and Quebec, and one in New Brunswick, accounting for about 15% of Canada's total electricity production. They are all CANDU-6 reactors, Canadian-invented pressurized heavy water reactors, which evolved from prototypes invented during and just after WW II to explore nuclear energy. While Canada lacked any expensive uranium enrichment facilities, it does have uranium ore, so a design evolved from the experimental ZEEP heavy water prototype, which used unenriched uranium and was designed by Canadian, British and French scientists as part of an effort to produce plutonium for nuclear weapons (Canada was part of the Manhattan Project). It then went through several earlier CANDU-type designs before the current CANDU was reached.

A more advanced Generation III+ CANDU reactor was designed but none were built. It was to be a light water cooled reactor that uses a separate heavy water moderator which is much like current CANDU reactors (described below). The key difference is that this reactor would use low enriched uranium fuel rather than natural unenriched uranium, which means more of the fuel would be "burned," reducing the amount of nuclear waste In 2007, Alberta Energy planned to use this newer prototype to process oil sands in Northern Alberta, but it was later scrapped. In 2011, SNC-Lavalin took over AECL and killed this reactor design. Some sources online claim that the design was unoriginal and flawed. Now, Toshiba has developed a small reactor, which Alberta plans to power oil sands extraction. Expected to come online in 2020, the Toshiba 4S, is a micro liquid sodium-cooled fast neutron reactor and it would replace the use of natural gas in oil sands extraction and therefore reduce carbon emissions. Not surprisingly there is significant concern among Albertans at the thought of having nuclear reactors dotting the northern landscape. I wonder if this will have any impact on the American decision whether or not to go forward with the Keystone Pipeline.

Meanwhile, several of SNC-Lavalin's new advanced fuel CANDU reactors might be sold to China. This design can use recycled uranium from light water reactors (as well as more abundant thorium) as fuel, so China could recycle the spent fuel from its existing 22 nuclear reactors and possibly from some of the additional 26 reactors now under construction.

A basic schematic diagram of a CANDU reactor is shown below.

Emoscopes;Wikipedia; the legend is a screen capture from the Wikipedia page
Like other pressurized water reactors, CANDU has two separate water systems. The steam generator (#5 above) is also the heat exchanger in the system. Fission heats the heavy water in the core (pressurized to keep it from boiling). The steam generator transfers that heat energy to the light water secondary cooling loop (the upside-down U-shaped tubes above). The pressurized steam powers a steam turbine (#11 above), which is connected to an electrical generator (would be to the right of 11, not shown above). The exhaust steam from the turbines is then condensed and cooled. This is often done using cool water from a nearby lake, river or ocean. The Darlington generating station in Ontario, for example, uses a diffuser to spread the warm water over a greater area in order to reduce the disruptive warming effect on Lake Ontario's ecosystem. Then the water is returned to the system.

CANDU reactors use natural unenriched uranium as fuel and heavy water (deuterium oxide) as the moderator. Designed much like a light water pressurized reactor, the use of heavy water, while much more expensive than light water, is a big advantage because it is a far better moderator (a ratio of 11,449 compared to 246 for light water). The light hydrogen in light water is very good at slowing fast neutrons into thermal neutrons. Because the proton is almost the same mass as a neutron it absorbs a lot of kinetic energy when the two collide in an elastic collision. However, light hydrogen also tends to absorb neutrons as well (into a heavy water nucleus), and this is undesirable because it results in fewer neutrons available for fission. Heavy hydrogen is also good at producing thermal neutrons and it has a much lower absorption cross section.

Technically put, the heavy water reactor has a high neutron economy - every neutron emitted is slowed down to the right kinetic energy to maximize the chance of a fission reaction and because it is not absorbed it stays in play, having a good chance of starting another fission reaction. The extra cost of the heavy water balances the savings of being able to use natural unenriched uranium. The use of heavy water allows the reactor to "burn" more uranium, using it more efficiently. However, a larger volume of fuel must go through the system, resulting in a larger volume of spent fuel. That being said, the spent uranium is less radioactive and can be stored more compactly. There is another downside to this - this reactor creates plutonium and tritium (although heavy hydrogen is fairly immune to neutron capture, some is captured to create tritium) as reaction byproducts, and these are two substances used to make boosted fission and fusion bombs. Spent CANDU fuel, once its radioactivity is reduced, can be a security threat.

The Benefit of Delayed Neutrons

The CANDU design has a built-in safety feature in that heavy water moderation stabilizes the fission chain reaction. This is how it works: The heavy hydrogen nucleus in heavy water is made of one proton and one neutron and it has quite low binding energy, which means it can be broken apart fairly easily. Some energetic neutrons and especially gamma rays (from the fission itself and from the decay of fission fragments) break the nuclei apart and add more free neutrons to the mix. The fission fragments have half-lives from seconds to hours to years, so gamma rays are emitted over time. This means that neutrons are jarred from heavy water over time in a more drawn out manner than they would be from light water.

Neutrons emitted over a period of time are called delayed neutrons. They are emitted anywhere from milliseconds to minutes after the fission event. During the fission event itself, the emission of neutrons is prompt - they are emitted almost instantaneously. The high power growth rate of nuclear weapons is hinged upon this fact, but if this happened in a reactor it would be almost impossible to control the reaction rate. An individual free neutron lasts only about one millisecond in the core before it is captured, but the emission of neutrons from the decay of fission products effectively extends that lifetime, affording a controllable overall rate of reaction. To maximize the benefit of delayed neutrons, the fuel is kept at such a level it would be subcritcal without the contribution of decay neutrons. The additional continuous contribution of delayed decay neutrons keeps the fuel just critical, allowing fission to continue but at a manageably slow rate.

The safety feature of this becomes apparent when the reaction accelerates for whatever reason in one part of the reactor. All nuclear reactors have an inherent challenge: the continuously replenished/lost cloud of neutrons inside a reactor core is subject to spatial and temporal fluctuations that are complex and difficult to predict, requiring sophisticated software to carefully control the reaction rate in various parts of the vessel. In operation, reactors are very complex systems. The neutrons interact differently with different isotopes present and the composition of isotopes is constantly changing, so neutron probes and temperature sensors are placed throughout the core. In the CANDU, a fluctuation in one zone will propagate relatively slowly to the rest of the core, and that delay should allow various feedback mechanisms built into the reaction to operate and allow technicians time to respond to the emergency. Still, this brings up the disturbing question of how good is the computer system (of any reactor) and how secure is it? In the case of the CANDU, there are two systems running simultaneously and independently, one as a backup for the other.

Is the CANDU safe?

This design has built-in features, as described above, that work to maintain a safe, steady and predictable fission reaction, but what happens if something goes very wrong and overcomes these features?

This design has a positive void coefficient. This means that if voids (steam bubbles) form in the reactor, its reactivity increases, a potentially devastating flaw in reactor design. In the CANDU, one can picture a nightmare scenario in which heavy water in the core gets too hot or there is a rupture that releases pressure. Sudden boiling would lead the reactor to become even more active, and then more steam is released and so on, creating a potential positive feedback loop leading to meltdown and a steam explosion. The Chernobyl disaster, involving a run-away chain reaction, was the result of a positive void coefficient. In the CANDU, however, there is a deliberately large mass of moderating heavy water in the reactor core so that, if this kind of event were set off, the effects should progress slowly, and that extra time along with the sluggish response of the fission process itself, should afford operators ample opportunity to deal with the problem, though this has not been proven in practice. While there is no international standard for nuclear reactors that prohibits designs with a positive void coefficient, the Chernobyl disaster, caused by a large positive void coefficient, makes some potential buyers and industry watchdogs wary of the CANDU reactor design. The latest advanced fuel CANDU, which may be sold to China, operates with more highly enriched fuel and has a negative void coefficient.

A design aspect that CADNU has going for it is the fact that the fuel bundles are held horizontally in the core. This means that if they get very hot, they will melt and sag and this change in shape will reduce their fission efficiency. There is very little excess reactivity in the core to start with so any deformation of the rods should bring the reaction to subcritical and stop the fission.

To deal with an emergency, the CANDU design has several built-in emergency systems: two independent shutdown systems, an emergency core cooling system and an emergency containment system. Shutdown system 1 uses neutron-absorbing rods that drop by gravity into the core, stopping the fission reaction. Shutdown system 2 injects high-pressure liquid neutron poison into the moderator. The poison is usually boric acid because boron nuclei have a high cross section for neutron capture. When dissolved in the moderator, boric acid provides spatially uniform neutron absorption, and in sufficient quantity it alone will stop the reaction. The emergency core cooling system can re-establish core cooling through high-pressure water injection, medium-pressure water supply from the building's dousing tank, and by recovering water from the building's sump. Again, Fukushima speaks caution. It was a situation where all primary, backup generator and battery power supplies were lost, and where valves to a similar dousing tank were also inexplicably closed and could not be reopened because of the loss of power. As a final safety measure, the CANDU reactor is contained by continuous concrete envelope.

Shortly after the Fukushima disaster, Canada's CANDU reactors underwent an extensive safety re-evaluation by the Canadian Nuclear Safety Commission. The full report can be read here. And here you can read the yearly national industry safety reports from 2008 to 2013.

Fast Breeder Reactors: Promise, Disappointment and New Promise

All large-scale breeder reactors around the world are currently fast breeder reactors, all of which use liquid sodium metal as the coolant, which can be circulated through heat exchangers outside the reactor tank (loop design, right) or inside the reactor tank (pool design, left), as shown below.

All fast breeder designs use liquid metal to cool the core, which in theory can be liquid helium, liquid sodium or liquid lead. Mercury, a liquid metal at room temperature, would be an obvious choice except that it is highly toxic and with a low boiling point it readily produces noxious fumes. The world's first fast neutron reactor called Clementine, brought online in 1946 and shut down in 1952, used liquid mercury. Liquid metals are used because they cool the reactor very well through heat transfer but they do not slow down or absorb neutrons. Water both slows and absorbs neutrons and that is why it can't be used in a fast neutron reactor (with the possible exception of the supercritical water design). An additional advantage is that liquid metal coolants do not have to be pressurized, whereas water used in many reactor designs needs to pressurized in order to perform as an effective coolant.

Liquid sodium cooled reactor designs have great promise but there is a long list of past failures. Over the decades, many experimental designs, including France's fairly disastrous Superphénix, have been built to use liquid sodium coolant but most of them have since been shut down. Sodium is very hazardous. It is so reactive that when it comes into contact with water it explodes and it burns upon exposure to air. Another serious concern is that liquid sodium can chemically react with many kinds of core cladding. The Monju loop-type power plant in Japan, brought online in 1994, was forced to shut down when a sodium leak caused a major fire. Restarted, it was again forced to shut down after another accident and it is now being decommissioned after costing the equivalent of over 9 billion dollars. Despite past failures, the promise of liquid sodium remains. As mentioned earlier, the Toshiba 4S micro reactor, being considered for processing oil sands bitumen in Northern Alberta, is an advanced liquid sodium fast reactor. An advanced (Generation IV) fast neutron breeder reactor design, called SFR, also calls for liquid sodium, as both the primary and secondary coolant. A diagram of this design is shown below.

Like other fast reactors, this design can use both fissile and fissionable materials, including depleted uranium, making it much more efficient than any thermal reactor. This new sodium-cooled fast reactor design could be used in conjunction with existing nuclear power plants to produce additional power from spent nuclear fuel.

Liquid lead also has been, and continues to be, considered as a reactor coolant. It has several advantages such as high neutron reflection and low neutron absorption cross sections and it is an excellent shield against gamma radiation as well. It has a high enough boiling point that it can effectively cool the core even at several hundred degrees C above normal operation conditions. The price to pay for this is that it is understandably difficult to refuel and service a molten lead cooled reactor core. Some Soviet nuclear submarine reactors built in the 1970's were lead-cooled, but other than that none has been used. However, a new Generation IV lead-cooled fast reactor is being developed, which overcomes the maintenance problem because the entire core can be replaced after many years of operation. In addition and unlike other reactor designs, no electricity is required to cool down the reactor after shutdown because of its natural circulation behaviour, a big safety advantage. If liquid lead-bismuth is used as the coolant, it would also quickly solidify in the case of a leak, eliminating the risk of an explosion. Liquid sodium reactors run the risk of a positive void coefficient but lead's nuclear properties eliminate that problem. Finally, lead is not very reactive and it is relatively cheap (bismuth if used, however, is expensive). The diagram below shows what this lead reactor might look like.
Despite the tantalizing promise of a reactor that actually makes more fuel than it consumes and can effectively recycle its own waste and the waste of other reactors, billions of dollars spent on liquid metal fast reactor development and research over six decades has, at best, delivered mixed results. Second, uranium has been found to be much more abundant than was assumed in the 1960's when many early design work was done based on what was thought to be a small and rapidly dwindling supply. Third, this kind of reactor tends to be far more expensive to build than light-water reactors, which they hope to replace.


The atomic nucleus, when harnessed as a nuclear weapon, has the power to cause unfathomable death and destruction, and that fear understandably spreads to a distrust of nuclear power as well. Several highly publicized nuclear reactor accidents have also driven fear into the world's population. Far less publicized are a stupendous number of military nuclear accidents and near accidents. An example that sticks out in my mind was the manual assembly a critical mass of plutonium during a 1946 demonstration, which killed the Canadian physicist, Louis Slotin, within days. These accidents and various nuclear reactor disasters tell us that our understanding of nuclear science has a long way to go. Yet despite all of that and many financially costly failures in reactor design, the green promise of nuclear energy continues to emerge, ironically, as one of the new technologies that might save mankind and our planet.

Well-designed with safety kept as a high priority, nuclear energy might finally experience its golden age, leaving carbon-emitting non-renewable oil, coal and natural gas technologies to history. I recommend a website called Created by a nuclear physicist and two nuclear engineers, it offers a series of interesting and easy to read articles written especially for people who want to learn more about nuclear energy and make an informed choice. It would also serve as an excellent primer for science teachers, as I hope this article does.

Next, in the last article in this series explore the potential and challenges of nuclear fusion power.

Friday, February 13, 2015

Nuclear Weapon Design and Operation

In the previous three articles we focused on the science behind chemical and nuclear bombs. The Science of Bombs and Chemical Versus Nuclear Weapons examined how chemical and nuclear explosions work. Then, Nuclear Weapons focused further on how nuclear weapons, utilizing fission and often fusion as well, tap into the incredible power of nuclear binding energy. Now that we understand the science behind a nuclear weapon, we can explore how that information is utilized to create a nuclear bomb.

There are two basic types of nuclear weapons based on the type(s) of nuclear reaction(s) involved. Their names can be confusing, much of the confusion having to do with the highly classified environment in which these weapons were developed. All manufactured nuclear devices utilize an explosively rapid fission reaction. Some bombs, particularly those developed in the past are pure fission bombs. These bombs are often called atomic bombs, atom bombs or A-bombs, which is misleading because the energy for these weapons comes from the nucleus inside the atom, as it does in fusion bombs as well. Fusion bombs never consist of just a nuclear fusion reaction in isolation. In these bombs, a fission reaction must also be used in order to trigger the fusion reaction, which then releases far more energy per kilogram than fission does. Fusion bombs are often called thermonuclear, hydrogen or H-bombs. Again, "thermonuclear" can be misleading, as both fission and fusion are nuclear reactions that release enormous heat energy.

The "thermo" in thermonuclear weapon is derived from the fact that a very high temperature is required to initiate the fusion reaction, ignoring the equally important requirement of high pressure. The fission reaction is required to heat the fusible atoms to a plasma state and to heat that plasma to its ignition temperature while keeping it all together long enough to initiate the fusion reaction. Here, a large input of energy from fission is required in order to release a far larger output of energy from fusion. The name "hydrogen" bomb comes from the fact that isotopes of hydrogen - deuterium (H-2) and tritium (H-3) - fuse together in the reaction.

Fission Weapons

These were the first nuclear weapons built and so far they are the only type of nuclear bomb ever used in warfare. All pure fission weapons involve some way of getting a subcritical mass of fissile material to supercritical mass, where as much of the fissile material as possible detonates in a very rapid fission chain reaction.

Two fissile elemental isotopes are commonly used in fission weapons - uranium-235 (used as an example in the previous article) and plutonium-239. The critical mass of a sphere of uranium-235 is 52 kg and for plutonium-239, it is 10 kg. In reality, these two metals are never found as pure isotopes. Instead, they exist as part of an isotopic mixture where other isotopes present are fissionable but not fissile (the difference is explored in the previous article). Enriched uranium, for example, means a uranium mixture in which the percentage of uranium-235 is high enough to sustain a chain reaction. The level of enrichment is an important factor in determining critical mass, along with various other factors including shape and pressure (or density). These factors are explored in the previous article.

There are two main types of fission bombs - gun-type and implosion-type. Little Boy, the fission bomb dropped on Hiroshima, was a gun-type uranium bomb, while Fat Man, dropped on Nagasaki, was an implosion-type plutonium bomb.

Gun-Type Fission Weapon: Little Boy

In a gun-type bomb, one subcritical mass is shot into another subcritical mass in order to reach supercritical mass and detonation. Little Boy was a test of this design. It was the first uranium-based detonation in history. The Trinity Test in July 1945 was the first nuclear test explosion; it tested the implosion-type Fat Man design and it used plutonium. A mock-up of Little Boy is shown below. This design was declassified in 1960.

Little Boy was about 300 cm long and 71 cm wide, weighing 4400 kg. It contained just 64 kg of enriched (about 80% urnanium-235 on average) uranium. It wasn't an efficient design, despite its horrific damage.

Below is a scale map of the extent of blast and fire damage to Hiroshima following Little Boy's detonation at about 600 m altitude, provided by the U.S. Strategic Bombing Survey. Both types of damage zones were roughly 3.2 km in diameter, where a surface temperature of 6000°C was reached. Countless people, along with all records of their existence, were gone. The lethal radius of intense neutron and gamma radiation was 1.3 km.

The legend is difficult to read. The largest version of the map is slightly clearer.

Less than 1 kg of the uranium actually underwent fission. Almost all of the entire output of three giant uranium enrichment plants in Oakridge, Tennessee blew apart before it could fission. The design was highly inefficient, allowing the fissioning uranium to rapidly expand and become subcritical before it could all fission, at which point it was no longer dense enough to sustain the chain reaction.

This is how Little Boy worked: inside the bomb a chemical explosive sends a 39 kg hollow uranium "bullet" down a barrel onto a cylinder of uranium, shown below.

Dake; Papa Lima Whiskey; Mfield; Wikipedia
The bullet, being hollow, is subcritical until it is driven onto the 26 kg uranium cylinder, powered by a cordite charge. Cordite is a smokeless low explosive that replaced gunpowder in the late 1800's. The combination of bullet and target becomes supercritical when the bullet is still 25 cm away. There are enough neutrons being spontaneously emitted from the 20% uranium-238 present to sustain the reaction when the pieces get close enough. One generation of the chain reaction takes less than a microsecond, during which time the bullet travels just 0.3 mm. This introduces a certain percentage risk of a pre-detonation, where the bomb blows apart before most of the fissile material has a chance to fission. A schematic diagram of pre-detonation is shown below right.

At the top of the diagram (right), one fissile mass is accelerated toward another fissile mass. If it is not accelerated fast enough, stray neutrons emitted from spontaneous fission will cause the two surfaces to start a chain reaction before the full critical mass is formed (before they are in contact). A much smaller explosion occurs (bottom image, right) blowing the containment apart and preventing the rest of the fissile material from fission. Two design elements prevent this. First, the cross section of the bullet is kept low enough to control the number of free neutrons being emitted (cross section is explored in the previous article). This is done by manipulating the isotopic makeup of the bullet. Second the bullet must be traveling sufficiently fast down the barrel. Speed can be increased by making a longer thicker barrel (more time to accelerate).

When Little Boy was designed there was only enough U-235 to create one bomb so it could not be tested beforehand. Little Boy detonated properly (though inefficiently) but there was a chance that it could have pre-detonated instead.

The creation of the Little Boy gun-type design was part of the top-secret Manhattan Project in Oakridge. Originally, researchers focused on using plutonium-239 as the fissile material. However, in 1944, the scientists discovered that reactor-bred Pu-239 was contaminated with Pu-240. P-240 had the effect of increasing the material's spontaneous neutron emission rate. This would have made pre-detonation inevitable. Additionally the barrel could not be made long enough to accelerate the bullet to high enough speed in order to prevent predetonation, at least not one that could fit onto an aircraft. Thereafter, they knew that gun-type bombs could only be made with enriched uranium.

Although still inefficient, the gun-type design was later refined and used to make earth-penetrating Mark 8 and Mark 11 bombs, commonly called nuclear bunker-busters. Researchers were confident that this type of bomb would reliably detonate even after being blasted into the ground. After the Mark bombs, a family of nuclear artillery shells also utilized this basic design. All gun-type weapons have since been retired and dismantled as of the early 1960's in favour of more efficient implosion-type weapons, now the only type of fission weapon in use worldwide.

Uranium-235 and plutonium-239 can be used separately or in combination in implosion-type nuclear devices, so the International Atomic Energy Agency attempts to monitor and control worldwide enriched uranium and plutonium supplies. Highly enriched uranium-235 is currently used in nuclear submarines (up to 96% enrichment) and in research reactors (up to 93%), in which maximal neutron production is desired. This high level of enrichment increases the reactor's power density and extends the life of the fuel but it is more expensive and is a greater risk to nuclear weapon proliferation. Uranium enriched to over 90% is called weapons-grade uranium. Supergrade plutonium (over 95% Pu-239) is used in U.S. Navy nuclear weapons while the U.S. Air Force uses conventional plutonium. Highly enriched supergrade plutonium, while more expensive, contains a very low amount of Pu-240, which is a high spontaneous fission isotope. Pu-240 is also a gamma radiation emitter, so submarine crewmembers would be exposed to too much gamma radiation from nuclear weapons stored onboard if conventional plutonium was used.

Implosion-Type Fission Weapon: Fat Man

The Trinity test device called the Gadget and the Fat Man bomb that dropped on Nagasaki were nearly identical designs. Both used plutonium-239 as the fissile material. Fat Man used only 6.2 kg of Pu-239, just 41% of spherical critical mass. Early research indicated that while an implosion-type mechanism would be far more efficient, a gun-type mechanism had less uncertainty associated with it. Therefore, a plutonium gun-type bomb called Thin Man was designed first. Soon after, researchers realized that Pu-239 could not work in a gun-type design so an implosion-type bomb called Fat Man was designed. A replica is shown below.

Weighing 4670 kg, it was 3.3 m long and 1.5 m wide.

High explosives in shaped charges could be used to implode a sphere of fissile plutonium. This meant a very rapid increase in plutonium density, making it all supercritical almost at once, and it had the additional advantage of requiring far less material. This mechanism created a much more efficient bomb. For an effective implosion, shaped high-explosive charges were designed into a three-dimensional explosive lens. Much like how an optical lens works, the spherical lens focuses several spherical diverging shock waves into a much more powerful singe converging shock wave. The following flash X-ray image sequence shows the converging compression shock wave formed during a high explosive lens test.

Wikipedia describes the interesting story of how this bomb was developed as part of the Manhattan Project. The troubleshooting processes involved are especially interesting to read. For this design to work, the plutonium in the centre needs to be cast into a sphere. During tests, the density of the sphere always came up as a different value, at first leading researchers to worry about contamination, but the problem turned out to be that plutonium comes in various allotropes, all of which vary in density, from 16 g/cm3 to about 20 g/cm3. This means that plutonium has a very complex and poorly understood phase diagram. Much like how water transitions from ice to liquid water to water vapour gas, plutonium transitions from a dense brittle alpha phase at room temperature to a much more malleable plastic beta phase at a slightly higher temperature and then to an even more malleable and even less dense gamma phase and so on, as shown in the graph below left, where increasing atomic volume means decreasing density.

This problem was solved by creating an alloy of plutonium by adding a small amount of gallium, which stabilized the density of the material and greatly reduced its thermal expansion as well.

As in the case of all nuclear weapons, the timing of the start of the chain fission reaction is critically important. If the chain reaction starts too soon, the bomb will pre-detonate, resulting in a reduced yield. For this reason the fissile material must have a low spontaneous neutron emission rate (low cross section). If the reaction starts too late, the fissile core will already be in the process of expanding into a less dense state in which the yield may be reduced or the reaction might not happen at all if the core is no longer at critical mass. To precisely control reaction initiation, the Fat Man was designed with a modulated neutron initiator. Located in the center of the plutonium pit, it kick-starts the fission reaction at just the right time. Activated by the converging shockwave from the synchronized high explosive detonations, it produces a sudden burst of free neutrons. The initiator is a combination of beryllium-9 and polonium-210. The two materials are held apart until the shockwave pushes them together. Polonium-210 is a radioactive alpha particle emitter. Much of its basic physics was classified until after WW II.  A silvery grey metal, it glows light blue because the alpha particles it emits excite the air around it. Beryllium-9 is a stable neutron-heavy isotope, with 5 neutrons. It releases neutrons (n, below) under bombardment by alpha particles, as shown below.

In the center of the plutonium pit, a beryllium pellet and a beryllium shell are separated from each other by a layer of polonium. Layers of gold and nickel over the beryllium shield it from alpha particles. The entire assembly is very small and weighs just 7 grams. The shock wave crushes the shell, mixing polonium with beryllium. The alpha particles from the polonium bombard the beryllium and cause it to emit a burst of neutrons. The neutrons start the fission chain reaction in the now compressed (to twice its normal density) and now supercritical plutonium core surrounding the initiator.

A simplified diagram of the fat Man assembly is shown left. The tamper (navy blue ring), composed of uranium-239, delays the outward expansion of the fissioning material. It holds the whole thing together for an additional few hundred nanoseconds. Because of its inertia, it increases the bomb's efficiency. U-239 also reflects neutrons very well, so it increases the bomb's yield. U-239 is not fissile but it is fissionable, so it undergoes fission by fast neutrons, contributing about 20% to the bomb's yield in this way as well. The animation below shows how the explosive lenses compress the fissile core and trigger the initiator.

Like Little Boy, Fat Man's detonation was not perfect, despite its more sophisticated design and increased efficiency. Only an estimated 20% of the plutonium underwent fission. The rest of it, about 5 kg, was scattered.

Since then, several improvements have been made to the implosion design, all in very short order, a matter of a few years. The thick layer of uranium-239 as well as the explosives made Fat Man fat and heavy. The levitated pit implosion design, creating a hammer-in-nail kind of impact to initiate the fission reaction, more than doubled the basic Fat Man design's yield but it was still fat and heavy. The two-point linear implosion design, though inefficient compared to the levitated pit design, effectively reduced the bomb's size, making small low yield battlefield missiles possible. A two-point hollow pit implosion design followed, increasing the efficiency of small tactical nuclear weapons.

A major improvement in design, which could used to make even smaller more powerful weapons, came with the boosted fission weapon design, first tested in 1951. Though not technically thermonuclear devices, these bombs utilize a small amount of fusion fuel to further increase efficiency and cut down on size and weight. The basic idea behind using fusion in addition to fission is to speed up the rate of fission in the fissile pit so that the required inertial confinement time can be reduced. Fusion is possible because once compressed, the very center of the pit is hot and under enough pressure to trigger this reaction. The newer hollow pit designs meant there is a perfect space to insert a tiny 50/50 fusion mixture of tritium/deuterium gas into the center of the pit.

Fusion-Boosted Fission Weapons

With this design, a thick heavy uranium tamper is no longer needed to contain the reaction. Instead, a lightweight beryllium shell can be used. The beryllium reflects neutrons back into the pit, an additional advantage. The mass of the fissile pit itself can be cut in half and smaller explosive charges can also be used. These weapons are called variable-yield weapons because the yield is highly dependent of the amount of fusible material inside the pit. At any time before detonation, the amount of tritium inserted into the pit can be reduced, reducing the weapon's yield. The first device to use this technology was the Swan device, a two-point, hollow-pit, fusion boosted implosion weapon. It was just 29.5 cm in diameter and 58 cm long. Developed from this design is the Mk-54 warhead, which could be launched from a Davy Crockett gun-type launcher, similar to the one shown below right. This was deployed by the U.S. during the Cold War. Just 11 inches in diameter and weighing 23 kg, it was the smallest nuclear device ever built It could deliver the equivalent of 10 to 20 tonnes of TNT, while also delivering lethal radiation within 150 m of its target, and a likely lethal dose up to a quarter mile away. Below, fat Man and a W54 warhead, the same size as a Mk-54 but with a larger 250 t yield, are shown to the same scale with their diameters in inches shown in red.

Nuclear bombs like the Mk-54 and the W54 warheads are sometimes called suitcase nukes. Developed both in Russia and the U.S. and possibly Israel as well, they are small and light enough to smuggle across borders in a suitcase. Whether these weapons have been or are currently deployed is classified.

As strategically useful as they are, this is not the best technology for making really large bombs. Even boosted with fusion, these weapons would require a great deal of expensive fissile material and tritium to produce yields of 100 or more kilotonnes. While many scientists, who knew of the then-recent devastation caused by Fat Man and Little Boy, were morally opposed to such efforts, work was quickly underway to create a far more efficient method to deliver even more devastating power. If you click the link just above, you will find in the third paragraph a very concise account of the controversy and reasoning used by those involved in the Manhattan Project at the then-secret Los Alamos National Laboratory in order to justify the development of the thermonuclear bomb. The most successful plan was to add on a second stage (called a secondary) to the boosted fission device (now called the primary). This is the general plan of the thermonuclear device or H-bomb.

Thermonuclear Weapons: The Teller-Ulam Design

As expected, detailed information about nuclear weapons, at least those that are in current use, is classified by the nation that possesses it. The basic designs described above have been declassified. Public knowledge such as that which I am relying on, mostly Wikipedia but other sources as well, comes from at least some speculation, some reverse engineering from known facts and comparison with other fields in nuclear physics by experts in the field. One design in particular, the Teller-Ulam design is the basic technical concept behind all modern nuclear weapons. While its details remain military secrets, this is the general blueprint for a thermonuclear bomb.

The first full test of this design was the test detonation of Ivy Mike (10.4 MT, 450 times more powerful than Fat Man dropped on Nagasaki) in 1952. The simple schematic diagram below shows how a thermonuclear warhead basically works. As we explored in the previous article, fusion is a spontaneous reaction but in order to happen, it requires tremendous heat under pressure. A thermonuclear device delivers those conditions. The legend that follows is taken directly from the image page on Wikipedia.

Teller-Ulam hydrogen bomb firing sequence, modified from Howard Morland, The Secret that Exploded (Random House, 1981).
A Warhead before firing; primary at top, first at top. Both components are fusion-boosted fission bombs.
B High-explosive fires in primary, compressing plutonium core into super-criticality and beginning a fission reaction.
C Fission in primary emits X-rays which channel along the inside of the casing, irradiating the polystyrene foam channel filler.
D Polystyrene foam becomes plasma, compressing secondary, and plutonium sparkplug inside the secondary begins to fission, supplying heat.
E Compressed and heated, lithium-6 deuterium fuel begins fusion reaction, neutron flux causes tamper to fission. A fireball is starting to form...

Energy is transferred from the primary to the secondary through X-rays from the fissioning primary. The X-rays compress the secondary fusion cell as well as the spark plug, which starts the fusion reaction. Exactly how this happens is secret.

Because this is the most efficient design for a high yield nuclear device, it is the only type of nuclear weapon deployed by the five nuclear weapon states under the Nuclear Non-Proliferation Treaty (at least officially): United States, Russia, United Kingdom, France and China. Other states that are not members of the treaty possess less well known numbers and types of devices, such as India, Pakistan and North Korea.

Perhaps the most advanced warhead design is the American W88 warhead, designed in the 1970's at the Los Alamos National Laboratory. The Trident submarine-launched ballistic missile can be armed with up to 12 W88 warheads (limited to 8 under the Strategic Offensive Reductions Treaty, as if this makes a difference). This device is shown below.
Dan Stober and Ian Hoffman;Wikipedia
Why Have a Nuclear Arsenal?

Just one W88 could completely destroy a city as large as London, Moscow or New York, killing millions of people. The aftermath of such devastation is unimaginable, considering too that all of the city's support infrastructures would also be obliterated. How vulnerable are these and other nuclear weapons to hacking, a military nuclear accident (sometimes called broken arrows; Wikipedia has compiled an astonishingly long list of worldwide accidents dating from the 1940's to the present), or to terrorism?

I find myself trying to imagine the general state of mind of those researchers in Los Alamos. It was the late 1940's and they were frantically working to design a better more destructive weapon, knowing that Russia at the time was also developing its nuclear program. The paranoid spectre of WW II was still fresh in everyone's mind and the extent of horror of the bombings of Nagasaki and Hiroshima could not have yet been fully realized in the west. At that time, the concept of mutual deterrence must have seemed reasonable to those in charge of the top-secret nuclear program. Today, over 60 years later, that thinking is outdated. The Cold War is over and attention is more focused on terrorist threats made by rogue militant groups not attached to any single country and the increasing threat of cyber-terrorism, rather than the threat of a full-scale nuclear exchange between the U.S. and Russia.

Yet mutual deterrence, still the only rationale, supports the world's present level of nuclear armament, one capable of destroying modern society as we know it. Two superpowers - the U.S. and Russia - have reduced their nuclear inventories, and while both the U.S. and Russia have signed the Treaty on the Non-Proliferation of Nuclear Weapons in which nuclear disarmament is one of the three "pillars' of the treaty, both countries as well as several others maintain large stockpiles of nuclear weapons. According to the Arms Control Association, an American organization the promotes public understanding and effective arms control policies, the U.S. alone currently has over 1600 deployed nuclear warheads, a number closely matched by Russia. China has 250 total warheads, France has 290 deployed warheads and the U.K. has 40 deployed warheads.

Unlike a conventional war, there can be no winner in a nuclear war. Even a limited regional exchange would have dire consequences for everyone on Earth. Current computer models reveal that Earth would suffer a 20-year-long nuclear winter as well as worldwide famine following a "limited" nuclear war. The model is based on an exchange between Pakistan and India, each detonating 50 15-kt nuclear weapons. The yield of each detonation is within the range of the Little Boy and Fat Man detonations (if you scroll up with this link, you will see a comprehensive list of nuclear testing treaties, well worth a look). 5 Mt of black carbon (soot) would be released into the atmosphere, absorbing sunlight. Black carbon rain would kill millions of people. Average surface temperature would drop by 1.5°C by year 5 postwar. This sounds minor, but growing seasons worldwide would be shortened by 10-40 days, causing a significant disruption to food production. The ozone layer would be 25% thinner and possibly up to 50% thinner over populated areas, increasing deadly UV radiation and rates of skin cancer would be substantially higher. There would be a 9% reduction in average rainfall. Even after 26 years, global rainfall would be decreased by 4.5%, leading to widespread crop failure and famine.

If this is not terrifying enough, I've come up with a list of possible post nuclear war effects, based on a variety of online sources.

1. Thermal radiation. The fires at Hiroshima accounted for half of the casualties.
2. Radiation contamination from either ground level or low detonations. The area contaminated by fallout exceeding a safe level of 300 roentgens from a one MT detonation (about the yield of a typical thermonuclear bomb) is thousands of square kilometres.
3. In a war, orderly evacuation is impossible. Millions of people might evacuate one contaminated area only to find themselves in another. Deteriorating sanitary conditions and hunger will lead to more deaths as well as genetic damage to all animal and plant life must be taken into account.
4. Ecological damage is difficult to estimate but entire ecosystems will be stressed or destroyed.
5. Continuous forest fires may destroy the majority of Earth's forests. Smoke could black out sunlight for weeks and a significant amount of oxygen in the atmosphere could be consumed. There is a very real threat of nuclear winter as well, which began to be studied in detail in the early 1980's. A study presented at the American Geophysical Union in 2006 found that even a small-scale regional nuclear war (100 Hiroshima-size nuclear detonations) could disrupt the global climate for a decade or more.
6.  High-altitude detonations could destroy the ozone layer, maximal estimates of the resulting UV radiation could destroy all life.
7. Disruption of transportation and communication, with serious economic impact.
8. Disruption of food distribution, water supplies, fuel and electricity, sewage and waste disposal, medicine, clothing. No assistance for millions of people who are wounded, burned or exposed to radiation.
9. Widespread hunger, the spread of diseases such as cholera, influenza, dysentery, typhus, etc., political chaos, disintegration of rules of law and order.


In short, a nuclear exchange could hurl mankind back into the dark ages, if our species survived. The conclusion of this is that nuclear warfare has no place among humankind. The technology, as fascinating as it is, needs to be mothballed. In the video, White Light & Black Rain: The Destruction of Hiroshima & Nagasaki, survivors said that the reason they agreed to relive and share their memories of the horror and to show their disfigurements on camera was to say that this could never happen again. The link to watch it online can be found in the previous article Nuclear Weapons: Understanding Nuclear Binding Energy.

Perhaps miraculously, no nuclear weapon has detonated since the bombings of Hiroshima and Nagasaki. It is disheartening however that nuclear weapon technology ever advanced at all, following what happened then. It also seems discouraging that once mankind harnessed the awesome power of the nuclear force, it was immediately turned into weapons of mass destruction. We can make a great deal of peaceful use out of nuclear energy. Nuclear fission power plants and possibly future nuclear fusion plants turn nuclear binding energy into heat, which can be turned into electrical power with great efficiency and little waste. However, there are many reasons why nuclear power may not reach a golden age. For the layman, it is not easy to separate in one's mind the threat of nuclear weapons from the promise of nuclear power. Catastrophic accidents such as Three Mile Island, Chernobyl and the Fukushima disaster have also, and with good reason, made many people wary of nuclear power. We will explore the possibilities of nuclear energy, and how it works, next.