Nuclear Power Plant

Necessary Parts of Plant and Properties

Necessary Parts of Plant
*1. Nuclear Fuel
*2. Moderator
*3. Control Rods
*4. Reflector
*5. Reactors Vessel
*6. Biological Shielding
*7. Coolant.

Nuclear Fuel
*Fuel of a nuclear reactor should be fissionable material It can be one or all of the following
*U233, U235 and Pu239.
*U238 — 99.3%
*U235 — 0.7%
*U234 — Trace
*Out of these U235 is most unstable and is capable of sustaining chain reaction and has been given the name as primary fuel.

Cladding
*In order to prevent the contamination of the coolant by fission products, a protective coating or cladding must separate the fuel from the coolant stream.
*Fuel element cladding should possess the following properties :
*1. It should be able to withstand high temperature within the reactor.
*2. It should have high corrosion resistance.
*3. It should have high thermal conductivity.
*4. It should not have a tendency to absorb neutrons.
*5. It should have sufficient strength to withstand the effect of radiations to which it is subjected.

UO2
*Uranium oxide (UO2) is another important fuel element.
*Uranium oxide has the following advantages over natural uranium
*1. It is more stable than natural uranium.
*2. It can be used for higher temperatures.
*3. It does not corrode as easily as natural uranium.
*4. It is more compatible with most of the coolants and is not attacked by H2

UO2
*Uranium oxide possesses following disadvantages :
*1. It has low thermal conductivity.
*2. It is more brittle than natural uranium and therefore it can break due to thermal stresses.
*3. Its enrichment is essential.
*Uranium oxide is a brittle ceramic produced as a powder and then sintered to form fuel pellets.

Moderator
*In the chain reaction the neutrons produced are fast moving neutrons. These fast moving neutrons are far less effective in causing the fission of U235 and try to escape from the reactor.
*To improve the utilization of these neutrons their speed is reduced. It is done by colliding them with the nuclei of other material which is lighter, does not capture the neutrons but scatters them.

Moderator Properties
*1. It should have high thermal conductivity.
*2. It should be available in large quantities in pure form.
*3. It should have high melting point in case of solid moderators and low melting point in case of
*liquid moderators. Solid moderators should also possess good strength and machinability.
*4. It should provide good resistance to corrosion.
*5. It should be stable under heat and radiation.
*6. It should be able to slow down neutrons

Control Rods
*Control rods in the cylindrical or sheet form are made of boron or cadmium.
*These rods can be moved in and out of the holes in the reactor core assembly. Their insertion absorbs more neutrons and damps down the reaction and their withdrawal absorbs less neutrons.
*Thus power of reaction is controlled by shifting control rods which may be done manually or automatically.

Control Rods Properties
*1. They should have adequate heat transfer properties.
*2. They should be stable under heat and radiation.
*3. They should be corrosion resistant.
*4. They should be sufficient strong and should be able to shut down the reactor almost instantly under all conditions.
*5. They should have sufficient cross-sectional area for the absorption.

Reflectors
*The neutrons produced during the fission process will be partly absorbed by the fuel rods, moderator, coolant or structural material etc.
*Neutrons left unabsorbed will try to leave the reactor core never to return to it and will be lost. Such losses should be minimized.
*It is done by surrounding the reactor core by a material called reflector

Reactor Vessel
*It is a strong walled container housing the cure of the power reactor. It contains moderator, reflector, thermal shielding and control rods.

Biological Shielding
*Shielding the radioactive zones in the reactor roan possible radiation hazard is essential to protect, the operating men from the harmful effects.
*During fission of nuclear fuel, alpha particles, beta particles, deadly gamma rays and neutrons are produced. A protection must be provided against them. Thick layers of lead or concrete are provided round the reactor for stopping the gamma rays.
*Thick layers of metals are sufficient to stop the alpha and beta particles.

Coolant
*Coolant flows through and around the reactor core.
*It is used to transfer the large amount of heat produced in the reactor due to fission of the nuclear fuel during chain reaction.
*The coolant either transfers its heat to another medium or if the coolant used is water it takes up the heat and gets converted into steam in the reactor which is directly sent to the turbine.

Coolant Cycles
*(a) Direct Cycle. In this system coolant which is water leaves the reactor in the form of steam. Boiling water reactor uses this system.
*(b) Single Circuit System. In this system the coolant transfers the heat to the feed water in the steam generator. This system is used in pressurized reactor.

*(c) Double Circuit System. In this system two coolant are used.
*Primary coolant after circulating through the reactor flows through the intermediate heat exchanger (IHX) and passes on its hest to the secondary coolant which transfers its heat in the feed water in the steam generator.
*This system is used in sodium graphite reactor and fast breeder reactor.