Weapons Materials Reactors

Type of physical science: Nuclear physics

Field of study: Nuclear reactors

Nuclear reactors have been designed to use the neutron flux from a nuclear fission chain reaction in order to produce plutonium and tritium, which are suitable for the manufacture of nuclear weapons.

Overview

Nuclear reactors are based on nuclear fission. In nuclear fission, a nucleus absorbs a neutron and splits into two unequal parts and several extra neutrons. The fission fragments are frequently highly radioactive. To construct a nuclear reactor, fissile nuclei are arranged so that the neutrons from each fission trigger another fission. This process, called a chain reaction, releases both energy and neutrons at a steady rate. If each fission triggers more than one additional fission, then energy and neutron release rapidly accelerate, as is the case in a nuclear weapon. In a reactor, the chain reaction is controlled by inserting neutron-absorbing rods into the core of the reactor, and energy and neutrons are released at a constant rate.

The chemical behavior of an atom is determined by the number of electrons that surround its nucleus when it is not ionized. This number of electrons equals the number of protons in the nucleus of the atom. Thus, all carbon atoms have six protons in their nuclei, and all uranium atoms have ninety-two. The neutrons that also occupy the nucleus do not affect the chemistry of the atom, but they do play a major role in determining the stability of the nucleus.

Nuclei with the same number of protons but different numbers of neutrons are said to be different isotopes of the same element. An isotope is designated by the total number of protons and neutrons in its nucleus. For example, the isotope carbon 12 has six protons and six neutrons.

The naturally occurring isotope uranium 235 undergoes fission, but it constitutes only 0.7 percent of the uranium found in nature. In order to construct a nuclear weapon, the concentration of uranium 235 must be increased to between 10 percent and 20 percent. Uranium that contains more than 0.7 percent uranium 235 is said to be enriched, while uranium that contains less than 0.7 percent is called depleted uranium. Most modern weapons-grade uranium is enriched until it is more than 90 percent uranium 235. The enrichment process is difficult and expensive, as the isotopes cannot be separated by chemical means. Nuclear reactors can operate with naturally occurring uranium that is 99.3 percent uranium 238 if they are cooled with graphite or heavy water. Reactors can operate with uranium enriched to 3 percent uranium 235 if they are cooled with normal water. Uranium 238, which constitutes 99.3 percent of naturally occurring uranium, does not fission unless it absorbs a high-energy neutron. In reactors, the neutrons are slowed down by collisions with the atoms of a substance called the moderator so that they do not have enough energy to cause fission in uranium 238.

A second fissionable isotope of element number 94, plutonium 239, does not occur in nature but can be produced in the cores of nuclear reactors. Plutonium 239 has a smaller critical mass than uranium 235 and produces slightly more neutrons in an average fission. Therefore, plutonium 239 offers some advantages over uranium 235 in the construction of nuclear weapons.

Plutonium 239 is produced when a nucleus of uranium 238 captures a neutron to form uranium 239. Uranium 239 emits an electron and an antineutrino to form neptunium 239. This β decay has a half-life of twenty-four minutes. The neptunium 239 in turn β-decays with a half-life of 2.4 days to form plutonium 239. Thus plutonium is formed in the core of a nuclear reactor if targets of natural or depleted uranium are inserted in the reactor core. The longer the target remains in the core and the higher the neutron flux to which it is exposed, the more plutonium 239 will be produced.

Two processes compete with the formation of plutonium 239. First, plutonium 239 itself fissions so that, as plutonium 239 builds up in the reactor core, an increasing percentage of it absorbs neutrons and therefore fissions. Second, plutonium 239 can capture an extra neutron without fissioning to form the isotope plutonium 240. Plutonium 240, in turn, can capture neutrons to form still heavier isotopes of plutonium. The longer the target element remains in the core and the higher the neutron flux of the reactor core, the more heavy isotopes will be produced. Plutonium 240 fissions spontaneously without absorbing a neutron. Thus, large concentrations of it in nuclear weapons may cause a spontaneous chain reaction inside the weapon before it is triggered. The explosion would be a "fizzle" in relation to the designed yield of the weapon. Nevertheless, preignition from large concentrations of plutonium 240 would be disastrous. The U.S. Department of Energy defines weapons-grade plutonium as that which contains less than 7 percent plutonium 240. If it is necessary to separate plutonium 240 from plutonium 239, then the separation must be done by physical means and will involve the same difficulties and expense as enriching uranium.

The burnup of a fuel element measures the percentage of the fissionable nuclei in the element that has undergone fission. The burnup depends on the neutron flux in the core of the reactor and on the amount of time that the fuel element has been left in the core. The neutron flux depends on the rate of the chain reaction in the core and thus on the power that is being generated by the reactor. Reactor engineers typically express the burnup of a fuel element in terms of megawatt-days per metric ton of fuel. The amount of time that targets of uranium 238 are left in the core of a production reactor is measured in the desired burnup of the element. Exact times will depend on the detailed design of the reactor, which determines the neutron flux at the location of the target for different power levels of operation.

A second material critical to the production of modern nuclear weapons is the isotope hydrogen 3, or tritium. Tritium and hydrogen 2 (deuterium) are used to "boost" the yield of nuclear-fission weapons. The fission explosion triggers the fusion of a small amount of deuterium and tritium, which produces high-energy neutrons. These energetic neutrons trigger fission in a uranium-238 blanket surrounding the weapon, producing large amounts of energy in addition to the small amount of energy from the fusion. Deuterium is stable and found in naturally occurring water. Tritium has a half-life of 12.3 years, so supplies of this isotope must be constantly renewed. Like plutonium 239, tritium is produced in nuclear reactors. A target of the isotope lithium 6 is inserted into the reactor core. Lithium 6 captures a neutron and splits into a helium-4 nucleus and a tritium nucleus. Lithium 6 is seventeen times as likely as uranium 238 to capture a neutron, but tritium and plutonium-239 production rates are limited by the neutron flux in the reactor core. Therefore, the production of a mole of tritium requires approximately the same burnup as the production of a mole of plutonium.

Applications

Reactors that are designed for the production of materials for nuclear weapons contain the same basic elements as conventional reactors that are designed for the production of electricity. In fact, the first electricity generated from nuclear fission was produced as a by-product by a weapons materials reactor. The fissile fuel is inserted in fuel rods, which are lowered into the core. The neutrons from the fissions are slowed down by a coolant to thermal energies. The coolant or moderator also carries excess heat away from the core.

The first nuclear weapons reactors were literally piles of graphite blocks surrounding slugs of uranium metal or oxide fuel. Heat was carried away by air. The graphite served to moderate the energy of the fission neutrons, but it also absorbed many of the neutrons.

Deuterium in the form of heavy water proved to be a much more efficient moderator than graphite: It not only absorbs fewer neutrons than the graphite but also slows the neutrons down faster. A deuterium nucleus has only twice the mass of a neutron, so neutrons lose more energy in each collision with a deuterium nucleus than in a collision with a more massive carbon nucleus. Weapons production reactors that are cooled with heavy water can be designed on a much smaller scale and sustain higher neutron fluxes than those cooled with graphite. Because they slow down faster in a reactor cooled with heavy water, neutrons are more likely to be captured by uranium-238 nuclei before they escape from the core. The reactor also uses control rods, which absorb neutrons and can be lowered into the core to retard the chain reaction.

Depending on the design of the individual reactor, target nuclei can be inserted as part of the fuel elements, used as part of the control rods (lithium 6 is particularly useful for this purpose, since it is an excellent neutron absorber), or placed in a blanket surrounding the reactor core. Nuclear weapons reactors have been run using a variety of fuels ranging from natural uranium to highly enriched uranium. Core designs and methods for loading fuel and target materials vary depending on the specific production campaign in progress. Nuclear weapons reactors can also be used to produce the fissionable isotope uranium 233, which does not occur naturally, or such exotic manmade nuclei as californium 252.

The target material must be easy to extract from the reactor core because operators wish to limit burnup in the targets in order to avoid the production of the heavy isotopes of plutonium. For example, a typical plutonium-239 production cycle at the Savannah River plant in Barnwell, South Carolina, calls for a sixty-day cycle at a power of 2,150 megawatts to produce plutonium that is 6 percent plutonium 240 and a thirty-day cycle for plutonium with 3 percent plutonium 240. Tritium production requires cycles on the order of two hundred days.

Adding or removing material from the core of the reactor tends to change the power level at which the chain reaction is stable. Thus, changing the target material in the reactor must be done with care in order to balance the supply of nuclear fuel and the target nuclei that are present. If there are not enough fissile nuclei in the core, then a chain reaction cannot be sustained. On the other hand, designers wish to maximize the number of target nuclei in the flux in order to breed as much plutonium as possible. Reactors are generally shut down during fuel and target changes because of the danger of instabilities developing during the process. In addition, the gradual accumulation of plutonium 239 inside the core increases the number of fissionable nuclei in the system. Fuel and target rods are usually closer together in the core of a production reactor than in a power reactor. This arrangement exposes the target nuclei to a higher neutron flux and helps to maintain a concentration of fissionable nuclei that is sufficiently high to sustain a chain reaction in the core. Engineers design cores known as charges for specific production tasks and burnup requirements.

After the target element has been irradiated and removed from the reactor, the plutonium must still be separated from the uranium 238 in the target. Because some of the plutonium in the target will have undergone fission, the material will be extremely radioactive from the fission fragments. The first step in extracting plutonium, uranium, or other isotopes for medical, military, or scientific uses is to place the target elements in cooling ponds for six months to allow short-lived isotopes to decay. Following the cooling period, the target elements must be chemically stripped of their protective coating or cladding and chopped into pieces for chemical processing.

There are several chemical processes that can be used to separate the elements in the still-radioactive rods. The most commonly used method in the United States is the "purex" process. The rods are dissolved in nitric acid, and the resulting solution is fed into a device in which the uranium and plutonium are concentrated in an organic solvent while the fission fragments are carried off in a weak solution of nitric acid. Uranium and plutonium are then separated from each other in a series of countercurrent processes using nitric acid and organic solvents. The purified plutonium is converted to an oxide or a metal for weapons manufacture.

The radioactive solutions are difficult to handle safely, and separation plants are expensive and difficult to design. The chemical process plants at Savannah River have two "canyons," each 9.0 meters wide at the top and 4.5 meters wide at the bottom and separated by four levels. The "hot" canyon is heavily shielded and contains equipment for the remote handling of radioactive material. The target material must be handled in this area until the highly radioactive fission products have been removed. After this removal, processing can be done in lightly shielded or unshielded areas. Tritium is recovered in a separate facility.

In addition to the danger that is intrinsic in handling highly radioactive materials, tight security must be maintained around nuclear weapons reactors to ensure that no one can steal the materials in order to manufacture a nuclear weapon.

Context

The first nuclear reactors developed during the Manhattan Project were nuclear weapons reactors. The second of two experimental graphite piles constructed in Chicago demonstrated plutonium production. An experimental air-cooled graphite pile at Oak Ridge, Tennessee, was the design prototype for the larger, water-cooled, graphite-moderated reactors constructed at Hanford, Washington, which produced the kilogram quantities of plutonium needed for the warheads detonated at the Alamogordo test site and over Nagasaki. The United States constructed other reactors at Hanford and Savannah River near Barnwell, South Carolina, during the 1950's. Each site also housed facilities for reprocessing fuel and target elements in order to separate out plutonium and tritium. Other nations, notably Great Britain, France, the Soviet Union, and the People's Republic of China, followed suit and constructed similar facilities in connection with the development of their own nuclear arsenals.

Many other nations have purchased or constructed nuclear reactors for electric-power production or for peaceful nuclear research. Experts have long been concerned that these reactors could be used to produce weapons-grade plutonium. Spent fuel from reactors that are designed to produce electric power contains a high concentration of plutonium 240, making it a poor candidate for a source of weapons material. The operation of electric-power reactors could be modified, however, so that fuel elements could be extracted after a much briefer processing period. Similarly, natural or depleted uranium targets could be irradiated in the cores of research reactors. Using a research reactor, India produced enough weapons-grade material for a nuclear test in 1974. It is believed that India has reprocessed enough fuel from civilian power reactors to construct a nuclear arsenal. Other nations may follow a similar model.

Concern over the spread of nuclear weapons prompted the signing of the Treaty on the Non-Proliferation of Nuclear Weapons in 1968. Although this treaty instituted international safeguards for the nuclear facilities of participating nations under the direction of the International Atomic Energy Agency, many of the nations whose potential for developing a nuclear arsenal is considered greatest have refused to sign the treaty. As of 1990, these nations included France, China, Israel, South Africa, India, and Pakistan. India had accepted international safeguards on the reactor fuel from which the material for its nuclear test device was extracted but seems to have developed nuclear weapons anyway. World reaction to India's nuclear test was not strong enough to deter other nations from doing likewise.

In the United States, nuclear weapons reactors were constructed following World War II. The materials from which they were constructed have undergone years of high-level irradiation. Many of these reactors have little or no external containment, and both Hanford and Savannah River are known to have severe problems with radioactive contamination of the environment. The aging reactors are becoming dangerous rapidly, forcing the United States either to rely on existing stocks of plutonium and tritium or to deploy a new generation of reactors for producing these materials quickly.

Principal terms

BETA DECAY: radioactive decay in which a nucleus emits either a positron or an electron (with the appropriate neutrino) and changes into a different chemical element with the same atomic mass

BURNUP: the percentage of fissionable nuclei in a sample that has undergone fission; measures the exposure of nuclear fuel elements to neutron flux

ISOTOPES: atoms of a particular chemical element that have different atomic mass numbers

NEUTRON CAPTURE: a nuclear reaction in which a nucleus absorbs a neutron and forms a heavier isotope of the same chemical element

NEUTRON FLUX: the number of neutrons crossing a unit area per unit time

PLUTONIUM: a man-made chemical element, an isotope of which (plutonium 239) is widely used in the manufacture of nuclear weapons

TRITIUM: an isotope of hydrogen which has one proton and two neutrons and which is used in the manufacture of boosted nuclear weapons

Bibliography

Cochran, Thomas B., William M. Arkin, Robert S. Norris, and Milton M. Hoenig. U.S. NUCLEAR WARHEAD PRODUCTION. Vol. 2 in NUCLEAR WEAPONS DATA BOOK. Cambridge, Mass.: Ballinger, 1987. Section 5 of this useful compendium of information contains a clear, simple summary of the operation of nuclear weapons reactors, including the design of the reactors and the principles governing the separation technologies. Written for nonspecialists, it provides a succinct, comprehensive picture of the operation of the reactors.

Cochran, Thomas B., U.S. NUCLEAR WARHEAD FACILITY PROFILES. Vol. 3 in NUCLEAR WEAPONS DATA BOOK. Cambridge, Mass.: Ballinger, 1987. This volume provides detailed portraits of the nuclear weapons reactors in the United States, including very specific data on the design of the cores used for various types of production projects and routine operating procedures for the reactors themselves. The data are probably more numerous than the casual reader will need, but they present a useful picture of U.S. production reactors.

De Volpi, Alexander. PROLIFERATION, PLUTONIUM, AND POLICY: INSTITUTIONAL AND TECHNOLOGICAL IMPEDIMENTS TO NUCLEAR WEAPONS PROPAGATION. New York: Pergamon Press, 1979. A more technical introduction to the problems of proliferation. Presents some of the practical steps that can be taken to ensure that plutonium intended for civilian use cannot be diverted to weapons production. This volume provides a very good introduction to the problems in weapons construction that are associated with heavy isotopes of plutonium.

Nero, Anthony V., Jr. "The Weapons Connection." In A GUIDEBOOK TO NUCLEAR REACTORS. Berkeley: University of California Press, 1979. This volume presents the production of materials for weapons from the point of view of a reactor specialist. Describes the origin of the problem of nuclear proliferation in connection with reactors designed for civilian use.

Patterson, Walter C. THE PLUTONIUM BUSINESS AND THE SPREAD OF THE BOMB. San Francisco: Sierra Club Books, 1984. This description of the production of plutonium for civilian use and the possibility of its diversion for nuclear weapons is very readable. It is strongly biased, however, against the widespread use of civilian nuclear power.

Rhodes, Richard. THE MAKING OF THE ATOMIC BOMB. New York: Simon & Schuster, 1986. This excellent history not only of the Manhattan Project but also of the physics that underlies nuclear weapons provides an excellent introduction to the technology of the production of plutonium for use in weapons. The discussion in part 2 includes the history of the first plutonium production reactors and presents their basic principles in a clear and readable manner.

Spector, Leonard S. THE UNDECLARED BOMB. Cambridge, Mass.: Ballinger, 1988. This volume traces the production of nuclear weapons and the materials that contribute to their manufacture throughout the world. A study in nuclear proliferation, it clarifies the relationship between civilian and military reactors that exists in practice. Includes an appendix on weapons material production.

Fission and Thermonuclear Weapons

Nuclear Reactions and Scattering

Nuclear Reactors: Design and Operation

Radioactive Nuclear Decay and Nuclear Excited States

Reactor Fuels and Waste Products