Basic Civil & Mechanical Engineering: UNIT IV: a. Power plants

Nuclear power plant

Working Principle, Classification, Layout Diagram, Advantages, Disadvantages, Location

The need of Nuclear Power Plant lies in the fact that the hunger for electricity is virtually unending and unsatisfied, as it is needed to lighten man’s burden and to brighten the world.

NUCLEAR POWER PLANT

The need of Nuclear Power Plant lies in the fact that the hunger for electricity is virtually unending and unsatisfied, as it is needed to lighten man’s burden and to brighten the world.

Working Principle

A Nuclear Power Plant differs from a thermal power plant only in the steam generating part. There is no change in the turbine-generator and the condensing systems. The nuclear fuel which is at present in commercial use is Uranium. Scientists say that 1 kg of U235 can produce as much energy as can be produced by burning 4500 tonnes of high grade coal.

 

1. NUCLEAR FISSION


Refer Fig. 4. Uranium exists in the isotopic form of U235 which is unstable. When a neutron enters the nucleus of U235, the nucleus splits into two equal fragments and also releases 2 or 3 fast moving neutrons with a velocity of 1.5 × 107 meters/sec producing a large amount of energy, nearly 200 million electron-volts. This is called Nuclear Fission.

Chain Reaction

The neutrons released during the fission can be made to fission other nuclei of U235 causing a Chain Reaction. A chain reaction produces enormous amount of heat, which is used to produce steam. [ The chain reaction under uncontrolled conditions can release extremely large amounts of energy causing Atomic Explosion.]

Energy liberated in chain reaction, according to Einstein Law, is E = mv2, where E = energy liberated, m = mass in grams, v = speed of light = 3 × 1010 cm/sec.

Out of 2.5 neutrons released in fission of each nuclei of U235, one neutron is used to sustain the chain reaction, 0.9 neutron is converted into fissionable material Pu239-and 0.6 neutron is absorbed by Control Rod to control the rate of reaction inside the reactor.

Function of the Moderator is to control the neutrons speed inside the reactor. The moderators used may be liquid (ordinary water and heavy water) or solid (graphite).

 

2. NUCLEAR REACTOR

A Nuclear Reactor may be regarded as a substitute for the Boiler Furnace of a thermal power plant. Heat is produced in the reactor due to nuclear fission of the fuel U235.

A Nuclear Reactor is the heart of the nuclear plant. It is a neutron machine for starting and controlling a self-sustaining fission chain reaction, that is, to control the emission and absorption of neutrons. Nuclear reactors are used for several purposes, viz., to produce heat for power generation, to produce fissionable materials, etc., to propel sub-marines, aircrafts, etc.

Parts of Nuclear Reactor (Fig. 5)


1. Reactor Core and Fuel Rods

Fuel Rod of a nuclear reactor should be a fissionable material. A Fissionable Material is an element or isotope whose nuclei can be caused to undergo nuclear fission by nuclear bombardment, thus producing a fission chain reaction. Fuel rods of the reactor can be one or all of the following: U233, U235 and Pu239.

Out of these, U235 is most unstable. It is capable of sustaining chain reaction and is called Primary Fuel. It is the only naturally occurring fuel. Plutonium is produced when U238 (which is the predominant isotope of the naturally occurring element) is irradiated by neutrons. Similarly, U233 is produced by neutron irradiation of the element Thorium.

2. Moderators

Moderator is a material that slows down fast neutrons quickly. The process of slowing down is called Moderation or Thermalizing. The slowing down of fast neutrons is desirable, because slow moving neutrons are more effective than fast neutrons in triggering fission. Note that where reactor operation is designed with fast neutrons, such as in reactors using highly enriched fuel, a moderator is not needed.

Materials containing light weight atoms, specially Hydrogen, are found to be most effective as moderators.

3. Control Rods

Control rods are inserted into the reactor core from the top of the reactor vessel. Their function is to regulate the fissioning in the reactor by absorbing the excess neutrons. These rods can be moved in and out of the holes in the reactor core assembly. If the fissioning rate of the chain reaction is to be increased, the rods are moved out slightly so that they absorb less number of neutrons and vice versa.

Control rods are made of Boron or Cadmium.

4. Reflector

Compact reactor design is achieved by a material called Reflector. Reflector surrounds the reactor Core. Reflector has good scattering properties and weak absorbing power. Therefore, it reflects neutrons back into the core. Thus, neutron leakage is reduced. As a result, less fuel is needed to generate sufficient neutrons to sustain the chain reaction. Materials which make good moderators generally also make good reflectors.

5. Reactor Vessel

Reactor Vessel is a strong walled vessel, housing the core of the reactor. It contains reflector, moderator, control rods and fuel rods. It provides the entrance and exit for the coolant and also the passages for its flow through and around the reactor core. There are holes at the top of the vessel through which the control rods pass.

6. Radiation Shielding

Among the nuclear radiations produced in a reactor, the a and B particles, thermal (slow) neutrons, fast neutrons and y rays are the harmful ones and must be shielded against. Shielding is necessary to prevent the escape or leak away of the above harmful rays and to protect the walls of the reactor vessel from radiation damage. It also protects the operating personnel from exposure to radiation. The first known as the Thermal Shield is provided through Steel Lining of the reactor vessel. The other is called Biological Shield, made of Thick Concrete.

7. Coolant

Coolant is the medium through which the heat produced in the reactor is transferred to the Steam Generator to produce steam. In passing through the reactor, the coolant picks up heat which is transferred to the working medium (water) in the Boiler/Steam Generator. On the other hand, by keeping the temperature inside the reactor within limits, it prevents overheating of the core. Gases, viz., Air, Helium, Hydrogen & CO2 and liquids including Light and Heavy Water may be used as coolant.

Light Water Reactors use ordinary water, technically known as Light Water as coolant and moderator. They are simpler and cheaper. But, they require enriched Uranium (U235) as their fuel. For this, Uranium enrichment plant is needed, which requires huge investment. Heavy Water Reactors use heavy water as their coolant and moderator. They have the advantage of using natural Uranium as their fuel.

 

3. TYPES OF REACTORS

i) Pressurized Water Reactor (P.W.R) shown in Fig. 6.

[Advantages: It is compact. Normal turbine maintenance techniques can be used, as the steam is not contaminated by radiation.]

ii) Boiling Water Reactor (B.W.R)

iii) Fast Breeder Reactor (F.B.R.)

iv) Heavy Water-Cooled Reactor

v) Gas Cooled Reactor

vi) Sodium Graphite Reactor (S.G.R.)

 

4. LAYOUT OF NUCLEAR POWER PLANT (Fig. 6)


1. Nuclear Reactor – Pressurized Water Reactor (PWR)

It has Primary Circuit or Coolant Circuit and Secondary Circuit or Water-Steam Circuit. Primary circuit consists of the Reactor, Steam Generator, Coolant Pump and Pressurizer. Secondary circuit consists of Steam Turbine, Electric Generator, Condenser and Feed Pump.

Nuclear Reactor is similar to the furnace of thermal power plant. Heat is liberated in the reactor due to nuclear fission of the fuel. This heat energy is absorbed by the coolant circulating through the reactor core. Coolant Pump is provided to maintain the flow of coolant.

Hot coolant leaves the reactor at top and then flows to the Steam Generator (Boiler). The steam produced is passed to the Turbine. The turbine drives a Generator (TurbineAlternator Assembly), thereby generating electricity.

The Exhaust Steam in the Turbine is then re-circulated back to the steam generator by a Feed Pump, after condensation in the Condenser.

Pressurizer

The Coolant Circuit (Primary Circuit) pressure is to be maintained at a higher level than that of the Water-Steam Circuit (Secondary Circuit) to maintain the coolant at liquid state. The purpose of this is to ensure that no boiling of coolant takes place. Thus, the coolant remains in liquid phase throughout its path in the Primary Circuit.

Whenever the load on the reactor changes, the coolant temperature as well as its volume change. This causes pressure changes in the Coolant Circuit. Whenever the pressure increases, water flashes into steam. This results in the disruption of the safe working of the reactor and even burnout of the nuclear fuel elements. Whenever the pressure decreases, it causes cavitations which are again harmful.

To avoid these happenings, the Pressurizer is provided. Pressurizer is a surge chamber which accommodates the changes in volume and at the same time, maintaining the pressure limits.

Electrical Heating Coil in the pressuriser boils the water to form steam. The steam is collected in the dome and it pressurizes the entire Coolant Circuit before starting the reactor.

Radiation Hazards and Shieldings

The water becomes radio-active in passing through the reactor. Hence, the reactor is a source of intense radio-activity. These radiations are very harmful to human life. It requires strong control to ensure that this radio-activity is not released into the atmosphere to avoid atmospheric pollution.

Thermal Shielding: Thermal Shield is made of Steel Lining. It surrounds the entire reactor core and absorbs some of the radiations in the form of a-rays, B-particles and escaping neutrons produced by fissioning. Thus, it gets heated and prevents the reactor wall from getting heated. Coolant flows over the shielding to take away the heat.

Concrete Shielding: A thick Concrete Shielding is provided to prevent the escape of these radiations to atmosphere.

2. Steam Generator (Boiler or Heat Exchanger)

The heat liberated in the reactor is taken up by the coolant circulating through the core. The purpose of the coolant is to transfer the heat generated in the reactor core and use it for steam generation. Ordinary water or heavy water is a common coolant.

Hot Coolant Water leaves the reactor at the top. It flows into the steam generator (boiler) in the Secondary Circuit and transfers the heat to the Feed Water in the steam generator. The feed water evaporates to become steam.

3. Feed Pump

Feed Pump supplies feed water to the steam generator.

4. Steam Turbine

The steam produced in the steam generator is passed to the turbine. Work is done by the expansion of steam in the turbine.

5. Condenser

The exhaust steam from the turbine flows to the Condenser where cooling water is circulated. The exhaust steam is condensed to water in the condenser by cooling. The condensate is pumped again into the steam generator by the feed pump.

 

5. ADVANTAGES OF NUCLEAR POWER PLANT

1. Less Fuel: Nuclear plant requires very small quantity of fuel than a fossil-fuel plant. 1 metric ton of Uranium fuel = 3 million metric tons of coal = 12 million barrels of oil.

2. Less Fuel Transportation Cost: Fuel transportation cost is less.

3. Fuel storage : fuel storage facilities are not needed as in the case of the thermal power plant. Less Space Requirement: Space requirement is less compared to other power plants of equal capacity.

5. Weather Conditions: Nuclear plant is not affected by adverse weather conditions. The plant can function throughout the year. Hydel power plant depends on monsoon.

6. Manpower: Number of workmen required at the nuclear plant is far less than thermal plant.

7. Location: Nuclear plant can be easily located where coal resources or dams are not available.

8. Reliability of Operation: There is increased reliability of operation of nuclear plant.

9. Conservation of Fossil Fuels: By using nuclear fuel, we can conserve the fossil fuels like coal, oil, gas, etc., for other purposes. For example, coal can be used to power steam engines, oil can be used for running vehicles and gas can be used for cooking.

10. Environment: A thermal power plant burns thousands of tonnes of coal per year. It looks like a madhouse of coal mines, conveyers, trucks, ashes, etc. But, a nuclear plant of the same capacity will be very neat, clean, sophisticated and is hospital quiet.

 

6. DISADVANTAGES OF NUCLEAR POWER PLANT

1. Waste Disposal

In the nuclear plant, the major problem faced is the disposal of highly radioactive wastes in the form of liquid, solid and gas without any injury to the atmosphere. Radio-active wastes, if not disposed very carefully, have adverse effect on the health of workmen and the population surrounding the plant.

2. Varying Load

Nuclear plant is not suited for varying load conditions.

3. Well-Trained Personnel

It requires skilled and well-trained personnel for operation.

4. Control of Chain Reaction

It is necessary to control the chain reaction effectively.

5. High Initial Cost

It requires high initial cost compared to hydro or thermal power plants.

6. Maintenance Cost

Maintenance cost of the plant is high

 

7. NUCLEAR POWER PLANT Vs THERMAL POWER PLANT


 

8. LOCATION OF NUCLEAR POWER PLANT

The detailed investigations for selecting suitable site for nuclear power plant include geological, meteorological, hydrological, topographical, special transport and radiological investigations. The factors which control the site selection for the nuclear power plant are as follows:

1. Distance from Load Center: The nuclear plant should be located near the Load Center. This will reduce the power loss as well as the cost of long transmission lines.

2. Availability of Cooling Water: The nuclear plant requires large quantity of cooling water. Therefore, it must be located near a river, lake or sea.

3. Distance from Populated Area: A nuclear plant should be located at a reasonable distance from the populated area to avoid the radiation hazards.

4. Radioactive Waste Disposal Facility: The wastes of the nuclear plant are highly radioactive. Therefore, sufficient space must be available near the plant for short-time storage as well as for long-time burial of the radioactive waste.

 

Basic Civil & Mechanical Engineering: UNIT IV: a. Power plants : Tag: : Working Principle, Classification, Layout Diagram, Advantages, Disadvantages, Location - Nuclear power plant