Nuclear Fission and Fusion - Part VI

7. Nuclear reactors, atom bombs  
We have seen in the earlier section how nuclear fission reactions can be controlled. The chain reaction, if uncontrolled can lead to catastrophic explosion which we know as the atom bomb. On the other hand, controlled nuclear fission reaction can be utilized usefully to release energy for use of nuclear reactors that are used for thermal power generation, propulsion of submarines, space crafts, etc.

The basic unit of a nuclear reactor is the chain reaction of U-235. This is started by a slow moving thermal neutron. To sustain the chain reaction, a minimum quantity of U-235 is needed, which is called the critical mass. A nuclear reactor produces immense amount of heat energy; we have seen earlier that there is a mass difference between the U-235 and its fission products which leads to release of energy. This energy is converted into electricity in a nuclear reactor.  

Moderator and control rods : A nuclear fission reaction releases more neutrons which themselves start more fission, thereby causing a chain reaction. To control the chain reaction, so that only a required amount of fission occurs, the extra, unwanted neutrons have to be removed from the nuclear fuel. This is done by materials that are good neutron absorbers. Since they control or moderate the rate of reactions, these materials are called moderators. Sometimes the moderators also slow the fast moving neutrons so that the chain reaction is sustained by slow thermal neutrons. Graphite, heavy water (D₂O : deuterium water) are good examples of materials which are used as moderators. They are able to slow the neutrons. On the other hand, materials such as cadmium (Cd) or boron (B) are called control rods, which absorb neutrons and thereby remove them from the core of the nuclear reactor.

Coolant : Enormous amount of energy is released from a nuclear chain reaction. The nuclear reactor core has to be sufficiently cooled so that the core does not melt and cause radiation hazard. Fluids are circulated through the core to cool it; these fluids are called coolants. Carbon dioxide gas, water, heavy water, liquid sodium, etc. are used as coolants. Heavy water has a larger advantage because it also acts as a moderator.

The diagram below shows a nuclear reactor along with an arrangement to show how a reactor is used as a nuclear power plant.  

Nuclear reactor has a core consisting of U-235.  Bombarding slow neutrons start a fission reaction. Energy, neutrons and fission products are released. The coolants absorb the energy. Moderators slow neutrons down; some neutrons are removed from the core by control rods. The fission products are removed as spent fuel in the form of spent fuel rods. The entire reactor is housed in a thick and strong steel vessel. There are several layers around this vessel. The last layer is made out of thick concrete. This is because a nuclear fission reaction also releases radioactivity that has to be contained. These strong layers prevent the escape of emitted harmful, energetic - particles, - particles and  - rays.

Uranium fuel rods are inserted in between a solid graphite core of a reactor. Cadmium or boron rods are also placed suitably. These rods can be inserted or removed, depending on the rate of reaction that is needed. A coolant such as liquid sodium is made to flow around the core. In fact the core is immersed in the coolant. The coolant removes heat and is put through a heat exchanger. Water in the heat exchanger absorbs heat and becomes steam. This steam is made to rotate a turbine. The turbine is connected to a generator. Generator is magnetic so when it rotates inside a conducting loop, electricity is generated. This electricity is then transmitted to a grid where it is then distributed for domestic or industrial consumption.

Atom Bomb : An uncontrolled fission chain reaction is called an atom bomb. To sustain a chain reaction, a minimum quantity of fissionable material is needed. Neutrons produced during fission may escape the sample and may not be able to keep the chain reaction going. To contain the neutrons produced and to increase the probability of emitted neutrons to start next fission, some minimum amount of fissionable material is required. This minimum mass is called the critical mass. The minimum mass of fissionable material that can support and sustain a chain reaction is called the critical mass of the fissionable material. If the chain reaction is not sustained, then the mass of fissionable material is called sub-critical mass. In an atom bomb, two sub-critical masses of uranium are initially kept apart. Just before the explosion, the sub-critical masses are imploded together. An atom bomb has no moderator or control rods. Along with the uranium core, explosive chemicals surround an atom bomb. The extent of death and damage caused by an atom bomb surpasses all conventional war weapons. Bombs dropped on Hiroshima and Nagasaki during World war II were made out of uranium and plutonium fission chain reactions.

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