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The material (or fuel) that is generally used in a nuclear fission reactor is uranium metal or uranium dioxide. In some cases, plutonium dioxide is added. Reactors with oxide fuels operate at higher temperatures than those with metal fuels. This is linked to the thermodynamic efficiency of the generating station.
Natural uranium consists mainly of two isotopes: fissile 235U92 (0.7 %) and fertile 238U92 (99.3%). A fissile isotope (such as U-235) is capable of undergoing fission by thermal neutrons whereas a fertile isotope (such as U-238) is capable of absorbing neutrons to form a fissile material. Superscripts denote the mass numbers and the subscripts refer to the atomic number (number of protons). 239Pu94 (plutonium) is not found in nature but is produced artificially in a reactor from 238U92. Some reactors use natural uranium but most use enriched U-235 to a couple of percentage points. This enrichment may be even higher when it is used with plutonium.
Fission of uranium (U-235) or mixed U-235 and plutonium (Pu-239) provide the bulk of energy in most present-day reactors. Both U-235 and U-238 decay by alpha emission with half-lives of 0.7 and 4.5 billion years (pretty stable) and a small fraction undergo spontaneous fission.
Though U-235 can undergo spontaneous fission, these nuclei are particularly liable to fission if bombarded by slow (much less than 1 eV) neutrons. The fission reaction can be summarised as follows:
235U92 + 1n0 → fission products + 2 or 3 1n0 (≈ 1 MeV) + E (≈ 200 MeV)
Here 1n0 depicts a neutron and E is the energy released (in million electron volts). A typical pair of fission products formed is 92Kr36 (krypton) and 141Ba56 (barium).
The neutrons produced in a fission reaction could all go on to initiate other fission events, thereby liberating more neutrons, establishing a chain reaction. However, the energy (1 MeV) or the speed (~ 107 m/s) of neutrons released in a fission reaction is too high to cause further fission of U-235. They therefore have to be slowed down to practically thermal energies (much less than 1 eV).
As mentioned above, the fission neutrons are too fast (1 MeV) and need to be slowed down (≈ 0.025 eV or thermal energy) to cause more fission of U-235. A material called the moderator is used to slow down the neutrons without absorbing them. This material is made up of light nuclei such as hydrogen, deuterium or carbon. Ordinary water (H2O), heavy water (D2O), or graphite are often used in thermal reactors as a moderator. Neutrons strike with moderator nuclei many (perhaps 100) times without being absorbed before being thermalised.
Three different types (keff) of chain reaction can be identified:
(a) Sub-criticality (keff < 1): in such a case, the reaction may be started by external neutrons but it dies away fairly rapidly. keff values slightly below unity are used for experimental purposes.
(b) Criticality (keff = 1): the reaction continues at steady rate. This is the operating state of a power reactor.
(c) Super-criticality (keff > 1): due to a very high concentration of fissile material, the fission rate could be very high. The number of neutrons escalates rapidly and an uncontrollable chain reaction could be established with an explosive release of energy. This is the case in a fission bomb. In a power plant, there is thus a need to control the chain reaction.
How can the chain reaction be controlled?
We need to be able to reduce the concentration of neutrons at will by including some material that will absorb neutrons and thus reduce the neutron flux in the system. Cadmium and boron nuclei, for example, have a large cross section for the capture of neutrons, about 10 times that of U-235 for thermal neutrons. Rods of these materials are inserted into the nuclear reactor to slow or shut down the chain reaction. Withdrawing the rods restarts or speeds up the reaction. Some neutrons themselves are lost as they may be lodged in the surrounding material or fuel-rod structures.
The release of neutrons in the fission of U-235 is very prompt (10-16 s) and these neutrons are known as prompt neutrons. The control of the chain reaction (by the above mechanical control rods) in practice would be very difficult (practically impossible) if there were only prompt neutrons in the system. Fortunately, the fission products themselves decay with the release of neutrons with a delay time of a few seconds to a minute. These delayed neutrons also cause fission of U-235.
In the reactor design, it is ensured that the reactor can never be critical without the contribution of delayed neutrons. These delayed neutrons thus allow the critical control of the chain reaction by the control rods to be maintained more easily. The flux of delayed neutrons (about 1 for 100 prompt U-235 fission neutrons) maintains the balance between a runaway exponential reaction and a reaction that would quickly grind to a halt.
How is the safety of the nuclear reactor ensured?
The need to guarantee the safe shutdown of the fission chain reaction is an obvious aspect of reactor safety. About 7% of heat output is typically derived from delayed neutron fission, the decay of fission products and other transuranic (heavier than uranium) elements. When the chain reaction is stopped, this source of heat – known as decay heat – remains. This heat (about 200 MW of a 3 000-MW plant) has to be adequately removed after the shutdown of the reactor.
Certain transient conditions have to be carefully considered. For example, the fission product iodine-135 decays with a 7-hour half-life to Xe-135 (xenon) which itself decays with a half-life of 9 hours. Xe-135 is a strong neutron absorber and is present in a transient fashion. This aspect must be carefully evaluated during shutdown and start-up.
In extreme situations, very rapid loss of coolant or the malfunctioning of control rods or other transient conditions may cause a rapid rise of temperature in the system leading to melting of the core of the reactor containing the fuel and moderator – a situation known as meltdown. However, modern reactor designs incorporate several levels of redundancies in the system so that accidental conditions should not occur and the reactors operate safely.
During the operation of a reactor, a part of the U-235 or plutonium is gradually consumed. A small part of the huge amount of U-238 present in the fuel is transmuted into plutonium by the capture of neutrons that are released during the fission reaction. Moreover, fission products (somewhat lighter elements) build up and some of them absorb neutrons, which affects the neutron economy. The result is that the critical chain reaction becomes more and more difficult to maintain. Eventually rods containing the fuel have to be changed well before the entire amount of fissionable U-235 has been consumed.