ASSIGNMENT ON NUCLEAR POWER

PLANT

SUBMITTED TO: SUBMITTED BY:

PROF: ASHOKAN M.A. VINAY KUMAR RAI

REG NO: 10BME0340

INTRODUCTION

A nuclear power plant is a thermal power station in which the heat source is one or more nuclear reactors. As in a conventional thermal power station the heat is used to generate steam which drives a steam turbine connected to a generator which produces electricity.

Nuclear power plants are usually considered to be base load stations, which are best suited to constant power output.

NUCLEAR FUEL

Nuclear fuel is any material that can be consumed to derive nuclear energy. The most common type of nuclear fuel is fissile elements that can be made to undergo nuclear fission chain reactions in a nuclear reactor.

The most common nuclear fuels are 235U and 239Pu. Not all nuclear fuels are used in fission chain reactions.

WORKING PRINCIPLE

NUCLEAR FISSION:

When a neutron strikes an atom of uranium, the uranium splits into two lighter atoms and releases heat simultaneously.

Fission of heavy elements is an exothermic reaction which can release large amounts of energy both as electromagnetic radiation and as kinetic energy of the fragments.

NUCLEAR CHAIN REACTION :

A chain reaction refers to a process in which neutrons released in fission produce an additional fission in at least one further nucleus. This nucleus in turn produces neutrons, and the process repeats. If the process is controlled it is used for nuclear power or if uncontrolled it is used for nuclear weapons.

FIG: SCHEMATIC PRESENTATION OF NUCLEAR CHAIN REACTION

If each neutron releases two more neutrons, then the number of fissions doubles each generation. In that case, in 10 generations there are 1,024 fissions and in 80 generations about 6 x 10 23 (a mole) fissions.

U235 + n → fission + 2 or 3 n + 200 MeV

COMPONENTS OF NUCLEAR POWER PLANT

1. NUCLEAR REACTOR: A nuclear reactor is a device in which nuclear chain reactions are initiated, controlled, and sustained at a steady rate, as opposed to a nuclear bomb, in which the chain reaction occurs in a fraction of a second and is uncontrolled causing an explosion.

TYPES OF NUCLEAR REACTORS

Pressurized Water Reactor (PWR) Boiling Water Reactor (BWR)

Pressurized Water Reactor (PWR)

Heat is produced in the reactor due to nuclear fission and there is a chain reaction.

The heat generated in the reactor is carried away by the coolant (water or heavy water) circulated through the core.

The purpose of the pressure equalizes is to maintain a constant pressure of 14 MN/m2. This enables water to carry more heat from the reactor.

The purpose of the coolant pump is to pump coolant water

under pressure into the reactor core.

The steam generator is a heat exchanger where the heat from the coolant is transferred on to the water that circulates through

the steam generator. As the water passes through the steam generator it gets converted into steam.

The steam produced in the steam generator is sent to the turbine. The turbine blades rotate.

The turbine shaft is coupled to a generator and electricity is produced.

After the steam performing the work on the turbine blades by expansion, it comes out of the turbine as wet steam. This is converted back into water by circulating cold water around the condenser tubes.

FIG: SCHEMATIC PRESENTATION OF PRESSURIZED WATER REACTOR POWER PLANTS

Boiling Water Reactor (BWR)

The water is circulated through the reactor where it converts to

water steam mixture. The steam gets collected above the steam separator.

This steam is expanded in the turbine which turns the turbine

shaft. The expanded steam coming out of the turbine is condensed and

is pumped back as feed water by the feed water pump into the

reactor core. Also the down coming recirculation water from the steam

separator is fed back to the reactor core.

FIG: SCHEMATIC PRESENTATION OF BOILING WATER REACTOR POWER PLANTS

2. CONTROL RODS

Control rods made of a material that absorbs neutrons are inserted into the bundle using a mechanism that can rise or lower the control rods.

The control rods essentially contain neutron absorbers like, boron, cadmium or indium.

FIG: CONTROL RODS

3. STEAM TURBINE

A steam turbine is a mechanical device that extracts thermal energy from pressurized steam, and converts it into useful mechanical.

Various high-performance alloys and super alloys have been used for steam generator tubing.

FIG: STEAM TURBINE

4. COOLING SYSTEM

A cooling system removes heat from the reactor core and transports it to another area of the plant, where the thermal energy can be harnessed to produce electricity or to do other useful work. Typically the hot coolant is used as a heat source for a boiler, and the pressurized steam from that boiler powers one or more steam turbine driven electrical generators.

5. SAFETY VALVES

In the event of an emergency, two independent safety valves can be used to prevent pipes from bursting or the reactor from exploding. The valves are designed so that they can derive all of the supplied flow rates with little increase in pressure. In the case of the BWR, the steam is directed into the condensate chamber and condenses there. The chambers on a heat exchanger are connected to the intermediate cooling circuit.

6. FEED WATER PUMP

Steam coming out of the turbine, flows through the condenser for condensation and recirculated for the next cycle of operation.

The feed pump circulates the condensed water in the working fluid loop.

FIG: FEED WATER PUMP

7. CONDENSER

Condenser is a device or unit which is used to condense vapor into liquid.

The objective of the condenser are to reduce the turbine exhaust pressure to increase the efficiency and to recover high quality feed water in the form of condensate & feed back it to the steam generator without any further treatment.

FIG: SURFACE CONDENSOR

8. COOLING TOWER

Cooling towers are heat removal devices used to transfer process waste heat to the atmosphere.

Water circulating through the condenser is taken to the cooling tower for cooling and reused.

9. GENERATOR

The generator converts kinetic energy supplied by the turbine into electrical energy. Low-pole AC synchronous generators of high rated power are used.

FIG: STEAM GENERATOR

WORKING OF NUCLEAR POWER PLANT

Control rods made of graphite have an extremely important role to play in maintaining the normal operation of the nuclear plant. The primary neutron on reaction with Uranium-235 atom releases 3 more neutrons into the system. If these 3 neutrons are left free, they react with 3 more U-235 atoms and release 9 more neutrons and the process continues. So if this process is not stopped, the reaction will proceed on to become what one calls a Chain reaction. The control rods absorb the 2 excess neutrons resulting from this process and permit only one to pass through. Hence, the nuclear reaction is kept under control.

The coolant also has an important role to play. A lot of heat is produced from the nuclear reaction causing the reactor to heat up excessively. The coolant helps to extract this heat and maintains the temperature within an optimal range, making the operation of the reactor smoother. The coolant usually used is water, but in some cases carbon dioxide gas or a liquid metal like sodium may also be used.

The heat produced from the fission reaction is used to heat the water into steam. The steam is then passed through a steam turbine, rotating it and hence leading to production of electricity.

The nuclear reactor is housed in a concrete structure which serves two purposes. Firstly, it prevents the nuclear reactor from damage due to external forces. Secondly, it prevents the radioactive emissions from the nuclear reactor from escaping into the atmosphere.

FIG: WORKING OF NUCLEAR POWER PLANT

ADVANTAGES

Nuclear power generation does emit relatively low amounts of carbon dioxide (CO2). The emissions of greenhouse gases and

therefore the contribution of nuclear power plants to global warming is therefore relatively little.

This technology is readily available, it does not have to be developed first.

It is possible to generate a high amount of electrical energy in one single plant.

DISADVANTAGES

The problem of radioactive waste is still an unsolved one. High risks: It is technically impossible to build a plant with

100% security. The energy source for nuclear energy is Uranium. Uranium is a

scarce resource, its supply is estimated to last only for the next 30 to 60 years depending on the actual demand.

Nuclear power plants as well as nuclear waste could be preferred targets for terrorist attacks..

During the operation of nuclear power plants, radioactive waste is produced, which in turn can be used for the production of nuclear weapons.

SAFETY ASPECTS

Design aspects. Dose limit. Radiation protection. Disposal of nuclear waste.