A nuclear fission process in which neutrons produced in earlier fission events cause successive fission events. Fission (splitting) of a large-mass atomic nucleus may be induced when the nucleus absorbs a neutron. Fission results in the release of approximately 200 MeV (200 × 106 electronvolts) of energy, divided among the kinetic energy of two (and rarely, three) medium-size fragments, 1 to 5 neutrons (most often 2 or 3), and some gamma rays (∼8 MeV). The fission neutrons can interact with nearby fissionable nuclei (the fission fuel), producing more fissions and more neutrons (see Fig. 1). If there is enough fuel assembled to interact with the fission neutrons, the number of fissions per second in this chain reaction can be sustained or increase. See also: Atomic nucleus; Electronvolt; Energy; Gamma rays; Mass; Neutron; Nuclear fission
The increase, decrease, or constant level of power (energy per second) produced in a chain reaction depends on the particular fissionable nuclide, the volume and geometry of the fuel, and the composition and positioning of other materials used to mechanically support and organize the fuel (the fuel structure). All of these factors combined determine the effective neutron multiplication factor, keff. This number is the average number of new fissions caused per neutron absorbed. If keff > 1, the power increases and the chain reaction is said to be supercritical; keff = 1 corresponds to constant power (critical); and keff < 1 means decreasing power (subcritical). The chain reaction can proceed at a controlled rate (as in power and research reactors) or uncontrolled (as in weapons), depending on details of the fuel structure. The neutrons from controlled reactors can be used to make radioisotopes for medical and research purposes. See also: Atomic bomb; Hydrogen bomb; Isotopic irradiation; Nuclear explosion; Nuclear medicine; Nuclear reactor; Nuclide; Reactor physics
Neutrons produced by fissions have high energies (∼2 MeV) when produced and are called fast neutrons. In most reactors, these are slowed down (moderated) to prevent escape from the fuel structure and to improve controllability. In weapons, fast neutrons are used in a rapidly increasing supercritical chain reaction. Most of the neutrons causing fissions in a commercial or naval reactor have very small (<1 eV) kinetic energy and are called thermal neutrons. The average time between “chained” fission events in a weapon is of the order of nanoseconds (10− 9 s) and between events in a thermal reactor is about 0.06 s because of the time required to moderate the neutrons. Typical moderators are water, heavy water (D2O), and graphite. See also: Graphite; Heavy water; Thermal neutrons; Water
The first human-made sustained chain reaction occurred on December 2, 1942, at the University of Chicago using an assembly of graphite and uranium (Fig. 2). Italian-born U.S. physicist Enrico Fermi supervised the construction of this “first” nuclear reactor for the Manhattan Project. Scientists also believe that a prehistoric (natural) fission reactor operated in Oklo, Gabon, Africa. See also: Uranium
Factors affecting the effective neutron multiplication
Each fissionable nuclide has a particular average number of neutrons produced when it fissions. If the average is higher, this increases keff. Several things can happen to these neutrons which will reduce keff: (1) escape from the fission fuel, (2) absorption or scattering by the fuel without causing fission, or (3) absorption by other materials (such as moderators, structural materials, or neutron control materials). See also: Absorption
The escape of neutrons depends on the surface area, volume, and distribution and density of the fuel. If the surface-to-volume ratio of the fuel is too large, too many neutrons will escape and keff < 1. This means that there is a minimum fuel mass (critical mass) required for a sustainable reaction. For most regular solid shapes, the volume will increase faster than the surface area, so a larger fuel volume will have a smaller escape probability allowing keff to be larger. Neutron reflector materials can be used to reduce escape and increase keff. If a fuel structure is built so that power can increase (supercritical), there must be a way to change keff and eventually establish a critical reaction. This is done by carefully planning the physical arrangement of the fuel and the moderator, along with adding moveable neutron absorbers (control rods). See also: Area; Density
Sometimes a fuel nucleus will absorb a neutron but will emit a gamma ray instead of fissioning. This neutron capture event happens 19% of the time for the fissionable nuclide 235U. Neutrons can also scatter from fuel nuclei and lose energy without causing fission.
Nuclides for chain reactions
Only a few nuclides can sustain a chain reaction, the most important being 235U, 233U, and 239Pu; only 235U occurs naturally, with 0.72% abundance. Induced fission of 235U can occur with thermal neutrons because the binding energy released is 6.5 MeV, greater than the 6.1 MeV activation energy of 235U. While 235U will undergo fission with fast neutrons, the probability of fission increases with decreasing neutron energy. This means that smaller densities of 235U can be used in thermal reactors. High purity 235U is required to maintain a chain reaction with fast neutrons because of the lower probability of fission. See also: Isotope; Nuclear binding energy; Plutonium
The uranium isotope most abundant in nature, 238U, can undergo neutron-induced fission only if the neutron has a kinetic energy greater than approximately 1 MeV. This is because the binding energy released when 238U absorbs a neutron is only 4.8 MeV, whereas the energy required to overcome the energy barrier holding the nucleus together (the activation energy) is about 6.2 MeV. (Although 1 MeV is less than the 1.4 MeV difference in binding energy and activation energy, the principle of quantum mechanical “tunneling” will allow fission to happen with a small probability. The probability of fission will increase rapidly between 1 and 2 MeV, becoming roughly constant at 2–6 MeV.) Even with fast neutrons, however, the probability of the neutron scattering without fission from the 238U nucleus is about 14 times larger than the fission probability. This means establishing a chain reaction based on 238U is practically impossible. See also: Nonrelativistic quantum theory; Quantum mechanics; Radioactivity; Tunneling in solids
The nuclides 233U and 239Pu can be used in thermal reactors and are more efficient than 235U in producing chain reactions because they have higher fission probabilities and produce more neutrons per fission. They can be produced when 232Th, and 238U, respectively, absorb a neutron and the products undergo radioactive decay. This behavior forms the basis for “breeder” reactors, which produce their own fuel for maintaining chain reactions. See also: Radioactivity
Natural water, used in reactors in the United States as a moderator, has a significant probability of absorbing neutrons, decreasing keff. To compensate for this, the percentage of 235U in the uranium fuel must be increased from 0.72% to over 3% to maintain a chain reaction. Canadian reactors can use natural uranium because heavy water has a very low neutron absorption probability. Heavy water is very expensive, however, and is not as efficient at moderating neutrons, so the reactor volume must be larger.
The safe control of the chain reaction in reactors depends on a process called beta-delayed neutron emission. Some fission fragments will emit neutrons following beta decay anywhere from 0.10 to 100 s after the fission. With proper design, a reactor can be subcritical based on the immediate (or prompt) fission neutrons, but critical or supercritical when the delayed neutrons are added. This design allows for slow and safe increases in power by motion of the control rods with reaction time factors of the order of 60 s. See also: Delayed neutron
Systems that become supercritical based on prompt neutrons will release energy rapidly, causing mechanical and thermal damage to the system. The fuel in a reactor is not dense enough to maintain such a reaction longer than a few nanoseconds. Fission weapons are designed for prompt supercritical chain reactions while maintaining the reaction for 10,000 times longer.