Both plutonium and highly enriched uranium are difficult and expensive to produce. Their acquisition requires heavy capital investment and a mastery of a wide range of technologies. In other respects, their production processes are quite different. This section provides a brief introduction to their production. See section III for an introduction to the nuclear fuel cycle.
The production of plutonium is carried out in two main industrial stages. The first involves the irradiation of uranium fuels by neutrons in nuclear reactors. The second involves the chemical separation of plutonium from the uranium, transuranic elements and fission products contained in discharges of irradiated fuel. The second techniques is usually referred to as "reprocessing" when applied commercially and "plutonium separation" when carried out for military purposes.
Irradiation of Reactor Fuel. Although they can overlap, plutonium is produced in two different contexts. In the military context, the reason for irradiating nuclear fuel is to acquire stocks of weapon-grade material for use in nuclear warheads; plutonium supply is the raison d'être, and typically dedicated "production reactors" are used to make weapon-grade plutonium. In the civilian context, the purpose is to generate electricity, plutonium being a by-product which may or may not have further uses. The power reactor is typically optimized for electricity production. The isotopic content of discharged plutonium is a serious concern of nuclear weapon designers, but is less important to electricity producers.
As distinct from the production of uranium with high concentrations of uranium 235, the technique of isotopic enrichment has not been used to produce weapon-grade plutonium from lower-grade material. Research was carried out in the 1960s in the United States and the Soviet Union using calutrons (and centrifuges) to separate the plutonium isotopes. More substantial research and development programs were launched in the 1980s to develop laser techniques for enriching plutonium by these means. These plans came to naught because of the reduced demand for weapon-grade plutonium as weapon programs were curtailed.
Instead, the nuclear weapons producers have achieved the desired isotopic content of plutonium mainly by controlling the extent to which uranium fuel elements are irradiated with neutrons in nuclear reactors. This is known as the fuel burn-up, whose unit of measurement is megawatt-days per ton (MWd/t) of uranium fuel. Weapon-grade plutonium is produced by operating reactors at low burnup--400 MWd/t is typical--so that insufficient time elapses for a substantial build-up of plutonium 240 and other plutonium isotopes.
Civil power reactors are operated at higher burnups in order to optimize the energy output from a given amount of fissile material. Power reactors fueled with natural uranium, such as the gas-cooled, graphite-moderated reactor developed in Britain and France, and the Canadian deuterium-uranium (CANDU) reactor, have burnups in the range 3,000-8,000 MWd/t. The most common type of thermal power reactor, the pressurized water reactor (PWR) which is fueled with low-enriched uranium, is typically operated at 30,000-40,000 MWd/t. As table II.1 shows, the concentrations of the even-numbered isotopes become substantial at these burnups. It should also be noted that the concentrations of total fissile plutonium (plutonium 239 plus plutonium 241) are not dissimilar for these reactor types, but that PWR fuel contains relatively low concentrations of plutonium 239 and high concentrations of plutonium 241. Descriptions of major types of reactors are included in the appendix to this section.
The trend is towards still higher burnups, with many utilities aiming for 50,000-60,000 MWd/t for light water reactor (LWR) fuel in the coming decade. As the irradiation period is extended, the energy extracted from the fissioning of plutonium 239 and plutonium 241 increases as that from uranium 235 decreases. In effect, a high burnup strategy is a cheap and energy-efficient substitute for recycling plutonium from lower burnup fuels. These high burnup spent fuels will also have increasing concentrations of the isotopes plutonium 238, plutonium 240, plutonium 241 and plutonium 242, which have detrimental consequences for the economics of plutonium recycling in light water reactors. In general, the commercial attractions of reprocessing and plutonium recycling diminish with increasing burnups.
Spent Fuel Reprocessing. In the nuclear weapons context, all plutonium is routinely separated from the irradiated fuels discharged from production reactors. In contrast, most of the spent fuel emanating from civil power reactors is today held in store. By the end of the century, one-fifth or less of world plutonium arisings will have been separated. Nevertheless, the reprocessing of spent fuels, particularly at facilities in France, Russia, and the United Kingdom, is giving rise to large amounts of separated plutonium.
In contrast to enrichment, only one process is currently used to extract plutonium from spent reactor fuels. This is the Purex (plutonium-uranium extraction) process developed in the United States in the late 1940s and early 1950s. Plutonium separation occurs in three main stages. In the first, the spent fuel assemblies are dismantled and the fuel rods are chopped into short segments (after the cladding has been removed mechanically in the case of gas-graphite reactor fuel). In the second stage, the extracted fuel is dissolved in hot nitric acid. In the third and most complex stage, the plutonium and uranium are separated from other actinides and fission products, and then from each other, by a technique known as "solvent extraction." Tributyl phosphate is commonly used as the organic solvent in a kerosene-type dilutent in the Purex process. The plutonium and uranium are usually taken through several solvent-extraction cycles to reach the required levels of purity.
In modern reprocessing plants, less than 1 percent of the plutonium contained in spent fuel may end up in wastes. In older plants, the fraction was often several percent.
In order to use uranium in nuclear weapons or a fuel in nuclear reactors it is necessary to increase the concentration of uranium 235. This process is known as "enrichment."
There are several techniques that can be employed to enrich uranium. Today's enrichment industry is dominated by the techniques of gaseous diffusion and centrifuge enrichment. Gaseous diffusion exploits the property of gases whereby heavy molecules travel more slowly than light molecules. If parts of the vessel containing the gas are made permeable, in the form of a barrier, the lighter molecules will pass through the diffusion barrier more rapidly, causing escaping gas to be enriched in the lighter components. Thus, the uranium hexafluoride gas emanating at the end of the diffusion stage will be slightly enriched in the isotope uranium 235. The final degree of enrichment attained depends on the number of stages hooked together by pipes in a "cascade," and on the enrichment of the initial feed.
Centrifuge enrichment is a process whereby heavier molecules in a rotating gaseous mass move towards the outside of the fluid mass and are subjected to a counter-current flow. To first order, it is the same technique as that used in the separation of cream in the dairy industry. In the context of uranium enrichment, it requires high-precision engineering and sophisticated metallurgy because of the rotation speeds required. Russia inherited a huge centrifuge industry from the Soviet Union. Pakistan and Iraq also have employed this method when seeking nuclear weapons.
Aerodynamic enrichment has two variants. The Becker jet nozzle, developed in the Federal Republic of Germany and Brazil, exploits the mass dependence of the centrifugal force in a fast, curved flow of uranium hexafluoride. The gas expands into a curved duct and the flow is split into heavier and lighter fractions by means of a skimmer. In the South African process uranium hexafluoride is allowed to swirl in a separating element that acts as a stationary-walled centrifuge. Neither aerodynamic technique has been shown to be commercially viable.
Electromagnetic separation was used to produce HEU for the first U.S. atomic weapon and was recently applied in Iraq. In a device originally called a calutron, heavy and light uranium ions (atoms carrying electrical charges) follow trajectories with different curvatures in a strong magnetic field.
There has been speculation since the early 1970s that laser enrichment will provide the basis for the next generation of enrichment plants. In this process, high-energy lasers can selectively excite the isotopes of uranium. Two routes can be taken. In the first, the atomic route, uranium 235 is selectively excited using tunable lasers, and the resulting ionized atoms are separated electromagnetically. In the second process, the molecular route, selective infrared absorption of uranium 235 hexafluoride gas is followed by further irradiation at infrared or ultraviolet frequencies, allowing dissociation of the excited molecules or their chemical separation.
Two other enrichment techniques are in the research and development stage. The first is the plasma separation process. The second is called the chemical exchange process. Pilot plants employing chemical processes have been built in France and Japan.
(1) This section draws from Plutonium and Highly Enriched Uranium 1996, chapter 2, op cit.
Reactor-grade isotopic concentrations (typical)
(a) Pressurized water reactor
(b) Gas-cooled, graphite-moderated reactor
(c) Canadian deuterium-uranium reactor
(d) Megawatt-days per ton of uranium fuel
Reactors for Electricity Generation
In nuclear power plants the fission energy released in the reactor is used to produce steam, which in turn is used to generate electricity. There are several types of nuclear power reactors.
Light Water Reactor The most widely used reactor is the light water reactor (LWR) which is moderated and cooled by ordinary water (light water). Light water is an excellent moderator; however, light water reactors require uranium fuel that is enriched. There are two types of light water reactors, the pressurized water reactor (PWR) and the boiling water reactor (BWR). In the PWR, water passes through the reactor core but is under high pressure and does not boil. The water then passes through steam generators which exchange the heat, boiling the water in a secondary circuit which drives the turbines which produce electricity. Russia has developed a PWR known as the VVER. In the BWR the water boils as it passes through the reactor core and the resulting steam drives the turbines.
Heavy Water Reactor Another type of reactor is the heavy water moderated reactor (HWR). Heavy water (water in which hydrogen atoms have been replaced by deuterium) does not absorb neutrons as readily as light water and so natural uranium can be used as the fuel. Natural uranium fuel is readily available, but deuterium is expensive. The Canadian reactor design, known as the CANDU, has been successful and has been exported outside of Canada. HWRs do not have to be shut down to refuel but can be reloaded while running. Both Germany and Japan have developed their own HWR designs.
Gas-Graphite Reactors Other nuclear reactors use graphite as a moderator and a gas as the coolant. This type of reactor was the basis for nuclear power production in France and Great Britain. These gas-graphite reactors are cooled by carbon dioxide gas and use a natural uranium metal fuel that is clad in a magnesium alloy. This fuel corrodes when stored in water. The British gas-graphite, known as the Magnox reactor, was replaced by the advanced gas reactors (AGRs). AGRs are also cooled by carbon dioxide but use slightly enriched, ceramic uranium dioxide fuel.
Water-Graphite Reactor The Russian RBMK uses graphite as its moderator but light water as its coolant and slightly enriched uranium for fuel. The Chernobyl reactors are of this design.
Fast Breeder Reactors A breeder reactor makes more fissile material than it consumes and does not use a moderator. For example, a breeder core might contain 15 to 25 percent plutonium mixed with uranium, surrounded by a blanket of natural or depleted uranium. Plutonium in the core is consumed, but neutrons emanate from the core and produce plutonium 239 through the capture of neutrons by the uranium 238 in the blanket. This creates a net increase in plutonium. Breeder reactors can also have a core of highly enriched uranium. The liquid metal fast breeder reactor (LMFBR) uses sodium as a coolant and the gas-cooled fast breeder reactor (GCFR) uses helium as a coolant.