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Nuclear fission is a process in which the neutron-rich nucleus of a heavy atom (such as uranium-235) splits spontaneously or on the impact of another particle such as neutron. This process is accompanied by the release of energy.
Nuclei decay because this allows them to achieve a lower energy state and become more stable. Neutron-rich heavy nuclei become more stable by splitting to form two lighter nuclei. In this process, the mass of the resulting particles is less than the initial mass of the heavy nuclei. The difference in mass (or the mass defect) is equivalent to the energy released. This energy is related to the mass defect by Einstein’s famous equation E = mc2, where E is the energy in joules, m is the mass in kg, and c is the velocity of light.
In the common reaction in a nuclear reactor, a nucleus of uranium-235 (U-235) captures a neutron and then undergoes a fission event releasing two or three prompt neutrons of about 1 MeV (million electron volts) energy. A pair of fission products is formed which is accompanied by the release of huge amounts of energy (about 200 MeV). The fission reaction of U-235 can be summarised as follows:
235U92 + 1n0 → fission products + 2 or 3 1n0 (≈ 1 MeV) + E (≈ 200 MeV)
Here 1n0 depicts a neutron, E is the energy released, superscripts are the mass numbers and the subscripts refer to the atomic number (number of protons). A typical pair of fission products formed is 92Kr36 (krypton) and 141Ba56 (barium).
This energy released is vastly greater than the few electron volts that are released when an inter-atomic bond is broken in the conventional combustion of fuels such as coal and oil.
As seen above, in a fission reaction, two or three neutrons are released. In principle, these fission neutrons could all go on to initiate other fission events that would liberate more neutrons, and so on. The number of neutrons rises rapidly and a chain reaction could then be established releasing large amounts of energy. However, the neutrons produced during fission travel extremely fast (energy = 1 MeV or speed ~ 107 m/s). At such speeds, they are very unlikely to be captured by another U-235 nucleus, and therefore need to be slowed down to roughly thermal level (much less than 1 eV). At such levels, their probability of capture rises dramatically.
A material called the moderator is used to slow down the neutrons. This material is made up of light nuclei such as hydrogen, deuterium or carbon. Ordinary water (H2O), heavy water (D2O), or graphite is often used in thermal reactors as the moderator. Neutrons strike with moderator nuclei many (perhaps 100) times without being absorbed before they are thermalised.
For a self-sustaining chain reaction, we need a balance between the rate at which neutrons are produced by fission and the rate at which they are lost, i.e. by causing further fission, by absorption by the surrounding materials, or by leaving the core. When the rate of production of neutrons equals the rate of loss, the reaction is said to be critical. This is the operating state of a reactor. If the losses are higher, the reaction dies away. In the opposite case, an uncontrollable chain reaction could be established and the core may melt, leading to what is popularly known as meltdown (an extremely unlikely event in power reactors).
Among the parameters required to achieve criticality is the critical mass – the minimum mass needed for a critical reaction to be established. If the fissionable material’s mass is too small, the loss of fission neutrons is too large to achieve criticality. The size of the critical mass depends on the enrichment of the fissionable isotope and the type of moderator used. In reactors which use natural uranium (U-235 fraction 0.7%), the size needed is much larger than those reactors which use enriched U-235 or plutonium fuel. When highly enriched fuel is used as in a nuclear weapon, the critical mass could be much smaller – perhaps the size of an orange!
We need to be able to reduce the concentration of neutrons at will by including in the system a material that will absorb neutrons and thus reduce the neutron flux in the system. Cadmium and boron are ideal for this as their nuclei have a large cross section for the capture of neutrons, about 10 times larger than that of U-235 for thermal neutrons. Control rods of these materials are inserted into the nuclear reactor to slow or shut down the chain reaction. Withdrawing the rods from the core restarts or speeds up the reaction.
The short answer is no. In a nuclear bomb, all the enriched fissile material is compactly and rapidly assembled and undergoes fission very promptly, releasing the energy explosively. In a power reactor, however, the fissile material is spread out within the moderator. Moreover, the concentration of the fissile isotope U-235 is only a few percent of the fertile isotope U-238. With the presence of the control rods, the energy cannot be released explosively. Moreover, there are other natural negative feedback mechanisms that are at work which reduce the reaction rate automatically when the temperature starts to rise.
The basic ingredients of a reactor are a large enough concentration of fissile material and a moderator. This could have happened on Earth in the past in an area where the concentration of U-235 was sufficiently large and which was percolated by water. Evidence of such a reactor operating in nature some 2 billion years ago was found by chance in 1972 in the Oklo mines in Gabon (Africa) by routine measurements of U-235 isotopic concentrations. The current concentration of 0.7% U-235 in natural uranium found on Earth is insufficient for criticality in any system moderated by ordinary water.
Some atoms, the building blocks of elements, are unstable and can decay into other types of atoms releasing particles or electromagnetic radiation. These nuclides are termed as radioactive. Radioactivity is expressed in a unit called becquerel (Bq), where 1 Bq equals one decay per second. Radioactivity represents changes in individual atoms. It is not possible to predict when an individual atom will disintegrate as the decay is statistical in nature.
The half-life of an isotope represents the time necessary for half of the nuclei present in a sample of material to decay. The most common particles that are released are alpha (α), beta (β), neutrons (n), and gamma (γ) rays. The half-life varies from a fraction of a second to millions of years or even longer. Atoms with long half-lives emit little radiation. Those with a short half-life are much more radioactive.
A radioactive atom can decay into another unstable type with its own half-life. This begins a radioactive decay chain, which may continue until a stable atom is finally reached. Half-life is a unique property of radioactive elements and is used to a great advantage in techniques such as carbon dating.