with the fissionable nuclei for slowing down fast neutrons. The moderators commonly used are water, heavy water (D O) and graphite. The Apsara reactor at the Bhabha Atomic Research Centre (BARC), Mumbai, uses water as moderator. The other Indian reactors, which are used for power production, use heavy water as moderator.
Because of the use of moderator, it is possible that the ratio, K , of number of fission produced by a given generation of neutrons to the number of fission of the preceeding generation may be greater than one. This ratio is called the multiplication factor; it is the measure of the growth rate of the neutrons in the reactor. For K = , the operation of the reactor is said to be critical , which is what we wish it to be for steady power operation. If K becomes greater than one, the reaction rate and the reactor power increases exponentially.
Unless the factor K is brought down very close to unity, the reactor will become supercritical and can even explode. The explosion of the Chernobyl reactor in Ukraine in is a sad reminder that accidents in a nuclear reactor can be catastrophic. The reaction rate is controlled through control-rods made out of neutron-absorbing material such as cadmium. In addition to control rods, reactors are provided with safety rods which, when required, can be inserted into the reactor and K can be reduced rapidly to less than unity.
The abundant U isotope, which does not fission, on capturing a neutron leads to the formation of plutonium. The series of reactions involved is – U+ n U Np+ e – Np Pu+ e ( . ) Plutonium is highly radioactive and can also undergo fission under bombardment by slow neutrons. The broad outlines of a typical nuclear power plant based on pressurised-water reactor are shown in Fig.
. . In such a reactor, water is used both as the moderator and as the heat transfer medium. In the primary-loop , water is circulated through the reactor vessel and