19.3 Nuclear Reactor

19.3 Nuclear Reactor

In a nuclear power station, the reactor plays the same part as does furnace in a thermal power station. In a furnace, coal or oil is burnt to produce heat, while in a reactor fission reaction produces heat. When fission takes place in the atom of uranium or any other heavy atom, then an energy at the rate of about $200\,\mathrm{MeV}$ per nucleus is produced. This energy appears in the form of kinetic energy of the fission fragments. These fast moving fragments besides colliding with one another also collide with the uranium atoms. In this way, their kinetic energy gets transformed in heat energy. This heat is used to produce steam which in turn rotates the turbine. Turbine rotates the generator which produces electricity. A sketch of a nuclear power station is shown in Fig. 19.5.

(Fig. 19.5: Nuclear Power Station)

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An induced nuclear reaction in which $_{92}^{235}\mathrm{U}$ is transmuted into the transuranium element plutonium $_{94}^{239}\mathrm{Pu}$.

A reactor usually has four important parts. These are:

1. The most important and vital part of a reactor is called core. Here the fuel is kept in the shape of cylindrical tubes. Reactor fuels are of various types. Uranium was used as fuel in the elementary reactors. In this fuel, the quantity of is enriched from $2.4$ to $3.0$ percent. It may be remembered that the quantity of in the naturally occurring uranium is only $0.7$ percent. Nowadays plutonium-$239$ and uranium-$233$ are also being used as fuel.
2. The fuel rods are placed in a substance of small atomic weight, such as water or heavy water. They are called moderators. The function of these moderators is to slow down the speed of the neutrons produced during the fission process and to direct them towards the fuel. Heavy water, it may be remembered, is made of $_{2}\mathrm{H}$, a heavy isotope of hydrogen instead of $_{1}\mathrm{H}$. The neutrons produced in the fission reaction are very fast and energetic and are not suitable for producing fission in reactor fuel like or $_{94}^{239}\mathrm{Pu}$ etc. For this purpose, slow neutrons are more useful. To achieve this moderators are used.
3. Besides moderator, there is an arrangement for the control of number of neutrons, so that of all the neutrons produced in fission, only one neutron produces further fission reaction. The purpose is achieved either by cadmium or by boron rods because they have the property of absorbing fast neutrons. The control rods made of cadmium or boron are moved in or out of the reactor core to control the neutrons that can initiate further fission reaction. In this way, the speed of the chain reaction is kept under control. In case of emergency or for repair purposes, control rods are allowed to fall back into the reactor and thus stop the chain reaction and shut down the reactor.
4. Heat is produced due to chain reaction taking place in the core of the reactor. The temperature of the core, therefore, rises to about $500\,^{\circ}\mathrm{C}$. To produce steam from this heat, it is transported to heat exchanger with the help of water, heavy water or any other liquid under high pressure. In the heat exchanger, this heat is used to produce high temperature steam from ordinary water. The steam is then used to run the turbine which in turn rotates the generator to produce electricity. The temperature of the steam coming out of the turbine is about $300\,^{\circ}\mathrm{C}$. This is further cooled to convert it into water again. To cool this steam, water from some river or sea is, generally, used.

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This symbol is universally used to indicate an area where radioactivity is being handled or artificial radiations are being produced.

In Karachi nuclear power plant (KANUP), heavy water is being used as a moderator and for the transportation of heat also from the reactor core to heat exchanger, heavy water is used. However, to cool steam coming out of the turbine, sea water is being used. 5. At present, three nuclear power plants are operational in Karachi as well as Chashma (Mianwali) in Pakistan with a total output capacity of more than $3000\,\mathrm{MW}$. They are all pressurized water reactors (PWR). Their cooling source is the water from nearby Indus river.

The nuclear fuel once loaded into the reactor can operate it for a few months. There after the fissile material begins to decrease. Now the used fuel is removed and fresh fuel is fed instead. In the used up fuel, intense radioactive substances still remain present. The half-life of these radioactive remnant materials is many thousand years. The radiations and the particles emitted out of this nuclear waste are very injurious and harmful to the living things. Unfortunately, there is no proper arrangement of the disposal of the nuclear waste. This cannot be dumped into oceans or left in any place where they will contaminate the environment, such as through the soil or the air. They must not be allowed to get into the drinking water. The best place so far found to store these wastes is in the bottom of old salt mines, which are very dry and are thousands of metres below the Earth surface. Here, they can remain and decay without polluting the environment on the surface of the Earth.

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Radioactive wastes are of three types i.e., high level, medium and low level. All these wastes are dangerous for ground water and land environment.

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It is very difficult to dispose off radioactive waste safely due to their long half-lives for example, Pu-$239$ half life is $24{,}000$ years, therefore, it remains dangerous for about $1{,}82{,}000$ years.