Nuclear Reactor (or) Pile

NUCLEAR REACTOR (OR) PILE

Reactors, which are designed to carry out nuclear reactions for generation of heat, electricity and radioactive isotopes are called nuclear reactors.

Definition

The arrangement or equipment used to carry out fission reaction under controlled conditions is called a nuclear reactor.

1. Light Water Nuclear-Power Plant

Definition

Light-water nuclear-power plant is the one, in which U²³⁵ fuel rods are submerged in water. Here the water acts as coolant and moderator.

I. Components of a Light water nuclear power plant

The main components of the light water nuclear power plant are

1. Fuel rods

The fissionable materials used in the nuclear reactor is enriched U²³⁵. The enriched fuel is used in the reactor in the form of rods or strips.

Example : U²³⁵; Pu²³⁹

Function: It produces heat energy and neutrons, that starts nuclear chain reaction.

2. Control rods

To control the fission reaction (rate), movable rods, made of cadmium (or) boron, are suspended between fuel rods. These rods can be lowered or raised and control the fission reaction by absorbing excess neutrons.

If the rods are deeply inserted inside the reactor, they will absorb more neutrons and the reaction becomes very slow. On the other hand, if the rods are pushed outwards, they will absorb less neutrons and the reaction will be very fast.

Ca¹¹³₄₃ + n¹₀ → cd¹¹⁴₄₃ + ɤ - ray

B¹⁰₅ + n¹₀ →+B¹¹₅ + ɤ  - ray

Example

cd¹¹³ : 3₁₀

Function: It controls the nuclear chain-reaction and avoids the damage of the reactors.

3. Moderators

The substances used to slow down the neutrons are called moderators.

Fig. 7.3 Functions of a moderator

When the fast-moving neutrons collide with moderator, they lose energy and gets slow down.

Example

Ordinary water, heavy water, graphite, beryllium.

Function: The kinetic energy of fast moving neutrons (1 meV) is reduced to slow neutrons (0.25 eV).

4. Coolants

In order to absorb the heat produced during fission, a liquid called coolant is circulated in the reactor core. It enters the base of the reactor and leaves at the top. The heat carried by out-going liquid is used to produce steam.

Example

Water (act as moderator & coolant), heavy water, liquid metal (Na or K), air (CO₂).

Function: It cools the fuel core.

5. Pressure vessel

It encloses the core and also provides the entrance and exit passages for coolant.

Function: It withstand the pressure as high as 200 kg/cm².

6. Protective shield

The nuclear reactor is enclosed in a thick massive concrete shield (more than 10 meters thick).

Function: The environment and operating personnels are protected from destruction in case of leakage of radiation.

7. Turbine

The steam generated in the heat exchanger is used to operate a steam turbine, which drives a generator to produce electricity.

Fig. 7.4 Light water nuclear power plant

II. Working

The fission reaction is controlled by inserting or removing the control rods of B¹⁰ automatically from the spaces in between the fuel rods. The heat emitted by fission of U²³⁵ in the fuel core is absorbed by the coolant (light water). The heated coolant (water at 300°C) then goes to the heat exchanger containing sea water. The coolant here, transfers heat to sea water, which is converted into steam. The steam then drives the turbines, generating electricity.

Pollution

Though nuclear power plants are very important for production of electricity, they will cause a serious danger to environments.

Problem on disposal of reactor waste

Disposal of reactor waste is another important problem because the fission products viz., Ba¹³⁹ & Kr⁹² are themselves radioactive. They emit dangerous radiation for several hundred years. So the waste is packed in concrete barrels, which are buried deep in the sea. Salt water will not allow the radiation to come out.