Showing posts with label Nuclear reactor. Show all posts
Showing posts with label Nuclear reactor. Show all posts

Types of Nuclear Reactor Power Plants

There are many concepts for the nuclear reactors, however, we will discuss few of following types of power plants that have been developed commercially.

Pressurized Water Reactor (PWR) Power Plant :

Pressurized Water Reactor uses water or heavy water as both the coolant as well as moderator. It uses enriched uranium as fuel. Water in the reactor absorbs the heat generated and converts it into steam. This steam generated is used for thermal power plant to drive its turbine and produce energy.

Boiled Water Reactor (BWR) Power Plant :

Boiled water reactor uses water as both coolant as well as moderator. It uses enriched uranium as fuel. In this reactor, water is directly converted into saturated steam. This steam is supplied to the steam turbine in conventional power plant and electrical energy is produced.

CANDU Reactor Power Plant :

CANDU stands for Canadian Deutorium Uranium reactor. It uses pressurized heavy water as moderator and primary coolant. Fuel used is natural uranium.
  For more details see - CANDU Reactor Power Plant

Gas Cooled Reactors (GCR) Power Plant :

These type of reactors uses gas as coolant and graphite as moderator. The advantage of using gas as coolant is that it is safe, easy to handle and it can be heated up to any temperature without change of phase at any pressure.
For more details see - Gas Cooled Reactor (GCR) Power Plants

Sodium Graphite Reactor (SGR) Power Plant :

This power plant uses graphite as moderator and sodium as coolant. Here, sodium is used as primary coolant and sodium potassium (NaK) is used as secondary coolant.

Fast Breeder Reactor (FBR) Power Plant :

In this reactor, moderator is not used. The primary fuel U-235 is surrounded by a blanket of fertile material. Fertile material like U-238 is kept there. Here, fast speed electrons are absorbed by U-238 which produces Pu-239.
  For more details see - Fast Breeder Reactor (FBR) Power Plant
   

Sodium Graphite Reactor (SGR) Power Plant

Sodium graphite reactor is a typical liquid metal reactor. The arrangement of sodium graphite reactor power plant is shown in figure.
Sodium Graphite Reactor Power Plant | enggarena.net
Sodium Graphite Reactor Power Plant

It uses graphite as a moderator and liquid sodium as coolant which reach a temperature of about 850 degree Celsius at a low pressure of only 7 bar.
In the primary circuit, the heat is absorbed by liquid sodium in the reactor. The sodium becomes radioactive while it passes through the core and reacts chemically with water.
Therefore, the heat absorbed by sodium is transferred to secondary coolant sodium potassium (NaK) in the primary heat exchanger which in turn transfers the heat in the secondary heat exchanger called Steam generator.
 Water leaving the generator is converted into super-heated steam up to a temperature of 540 degree Celsius. This steam is used for power generation in the steam plant circuit in the usual manner. The reactor vessel, primary circuit and the primary heat exchanger have to be shielded from radiations.
The liquid metal is required to be handled under the cover of an inert gas like helium to prevent the contact with air while charging or draining in the primary and secondary heat exchangers.

Advantages of Sodium Graphite Reactor (SGR) : 

1.High temperature of the steam can be obtained due to the use of liquid sodium as coolant.
2. Thermal efficiency is high.
3. System need not to be pressurized.
4. Cost of pressure vessel and piping system is reduced due to the use of low pressure sodium in primary circuit.

Disadvantages of Sodium Graphite Reactor :

1. Any leakage of sodium coolant is highly dangerous.
2. Sodium is highly violent with air and water.
3. Primary and secondary heat exchangers are needed to be shielded with concrete blocks against radiations.
4. Intermediate heat exchanger is required to separate radioactive sodium with water and steam.

Fast Breeder Reactors (FBR) Power Plant

When uranium U-235 is fissioned by slow neutrons it produces heat and an additional neutron. In case a fertile material like U-238 is kept in the same reactor surrounding the core of U-235, the fast moving additional neutron is absorbed by U-238 and converts it into plutonium (Pu-239), a fissile material. This man made fuel Pu-239 can be used for further fission.
Therefore, these type of reactors are important since they not only produce heat but also produce more secondary fissile fuels like plutonium more than fuel consumed in the reactor.
This is known as Breeding.
Similarly, Thorium (Th-232) can be converted into U-233 which is also a secondary fissile material.
Since, India has massive reserves of thorium and limited resources of uranium, development of these fast breed reactors are important.
As Fast breeder reactor is shown in figure below.
Fast Breeder Reactor Power plant | enggarena.net
Fast Breeder Reactor Power Plant
The enriched uranium U-235 or plutonium Pu-239 is kept without a moderator in the reactor core surrounded by a thick blanket of depleted U-238.
One additional neutron available from fission of U-235 is used to convert U-238 or Th-232 into U-233 as secondary fuels.
As in case of sodium graphite reactor, this reactor also uses two liquid metal coolants in which sodium is used as primary coolant and sodium potassium as secondary coolant.
In fast breeder reactors, the neutron shielding is provided by the use of boron or graphite. In order to protect against gamma radiations, a shield is provided made of lead or concrete or of other materials.

Advantages of Fast Breeder Reactor :

1. Moderator is not required.
2. Absorption of neutron is slow.
3. Secondary fissile material by breeding are obtained.
4. Small core is sufficient since it gives high power density as compared to other reactors.

Disadvantages of Fast Breeder Reactor :

1. Requires enriched uranium as fuel.
2. Neutron flux is high at the center of the core.
3. Thick shielding is necessary against radioactive radiations in primary and secondary circuits as in case of sodium graphite reactors.

CANDU Reactor Power Plant

A reactor developed and designed by Canadian is called as CANDU (CANadian Deutorium Uranium ) reactor.
It uses pressurized heavy water (which has 99.8% deutorium oxide ) as moderator and primary coolant. Natural Uranium is used as fuel for this reactor.
A Canadian Deutorium Uranium Pressurized Heavy Water power plant is shown in figure.
CANDU Reactor Power Plant | enggarena.net
CANDU Reactor Power Plant

Natural uranium used as fuel is in the form of small cylinder palletes. These are packed in corrosion resistant Zirconium alloy tubes of 0.5 cm long and 1.3 cm in diameter to form a fuel rod.
These short rods are combined in 37 bundles of 37 rods and 12 bundles are placed end to end in each pressure tubes.
This type of arrangement helps in refuelling the reactor while in operation.
Reactor vessel is a cylinder called Calandria.It is placed horizontally . It has pressure tubes penetrating the reactor vessel. The active core is about 6 m high abd 7-8 m diameter.
In the primary circuit deutorium coolant enters the array of pressure tubes at 110 bar pressure and 260 degree Celsius temperature. It flows through the fuel element and leaves the pressure tubes at about 370 degree Celsius after absorbing the heat generated by fission of fuel material.
The coolant at 110 bar and 370 degree leaving the reactor enters the steam power plant where the generated steam is used in conventional steam power plant.
Control rods are made up of cadmium. These control rods are used to start and shut down the reactor. In addition, there are other absorbing rods which are used to control the power output during reactor operation.

Advantages of CANDU Reactor :

1. Enriched fuel is not required.
2. Cost and time of construction is less.
3. It has good neutron economy resulting into good breeding ratio.
4. Heavy water is used as moderator which has low fuel consumption.

Disadvantages of CANDU Reactor :

1. Heavy water used as moderator is costly.
2. Leakage problem may occur.
3. It has critical temperature limitations.
4. It requires has standard of design, manufacture and maintenance.
5. Size of plant is large.

Gas Cooled Reactors (GCR) Power Plant

A gas cooled reactor was first developed in U.K. which uses C02 as coolant instead of water and graphite as moderator. It is also called as Gas Cooled Graphite Moderated (GCGM) reactor. The basic diagram of gas cooled reactor power plant is shown in figure,
Gas Cooled Reactor Power plant | enggarena.net
Gas Cooled Reactor Power Plant
It uses natural Uranium as fuel. The coolant pressure is about 7 bar and temperature about 336 degree Celsius.
There was another gas cooled reactor developed in U.S.A which uses helium as coolant and graphite as moderator. It is called high temperature gas cooled (HTGC) reactor. The pressure and temperature of the coolant are 15 bar to 30 bar and 800 degree Celsius respectively.
HTGCR may work up to a thermal efficiency of 40%.
The advantage of using gas as coolant as compared to water is that it is safe, easy to handle and most important it can be heated up to any temperature without change of phase at any pressure though the gas has low heat transfer properties as compared to water.
However, while working with gas ass coolant in the reactors, large rate of gas circulation is required which is supplied with the help of blowers for affecting the required rate of energy transfers.
Therefore, for driving the large number of blower, large amount of energy is required as compared to the power needed for running the feed water pumps in case water is used as coolant. Thus, it reduces the thermal efficiency of the plant as compared to water cooled or liquid cooled power plants.

Advantages of Gas Cooled Reactors :

1. It has no corrosion problems.
2. As mentioned earlier, gases are easy to  handle.
3. These can be operated at high temperatures.
4. Gases can be pressurized easily.
5. Graphite remains stable at high temperatures and radiation problems are minimum.

Disadvantages of Gas Cooled Reactors :

1. Fuels have to be operated at high temperatures.
2. If helium is used as in case of HTGC reactor, leakage is a major problem.
3. Gases have lower heat transfer coefficient thus it requires large heat exchangers.
4. Large amount of fuel loading is required.
5. More power is needed for coolant circulation compared to water cooled reactors.

Boiled Water Reactor (BWR) Power Plant

In Boiled Water Reactor (BWR) , water is used both as coolant as well as moderator.
The arrangement of boiling water reactor is shown in figure.
Boiled Water Reactor Power Plant | enggarena.net
Boiled Water Reactor Power Plant

It uses enriched Uranium as fuel. These reactors do not need heat exchangers as needed in Pressurized Water Reactor (PWR) since water is directly converted into saturated steam at about 285 degree centigrade at 70 bar pressure.
For above these reasons, this system is also called as Direct cycle Boiled Water Reactor power plant.
The feed water circulated in the reactor is converted into saturated vapor or steam by transfer of heat energy released in reactor core in fission process. This steam is supplied to the steam turbine in a conventional power plant working on the cycle.
Thus, the mechanical power developed by the turbine is converted into electrical energy by the generator. The exhaust of the steam from the turbine is condensed into condenser. The condensate is returned to the reactor as feed water by feed water pump.

Advantages of Boiled Water Reactor :

1. It eliminates the use of heat exchanger, pressurizer, circulating pump and piping. Therefore, the system is simple and cheap.
2. Efficiency of the system is high.
3. Use of low pressure reactor further reduces cost of the plant.

Disadvantages of Boiled Water Reactor :

1. It cannot meet sudden changes in load on the plant.
2. It has the possibility of radioactive contamination of steam turbine.
3. System requires extensive safety devices against radioactive radiations which are costly.

Pressurized Water Reactor (PWR) Power Plant - The Basic Concept

In Pressurized Water Reactor (PWR) water or heavy water is used as both coolant as well as moderator. The below figure shows basic diagram of Pressurized Water Reactor.
Pressurized Water Reactor Power Plant | enggarena.net
Pressurized Water Reactor Power Plant

This reactor uses enriched Uranium.
Before starting the reactor, water in pressurizer is boiled and converted into steam by electric heating coil. In order to prevent the boiling of water in the core, it is kept under the pressure of about 130-150 bar. It helps in absorbing the heat by water in the liquid state in the reactor. The heat energy absorbed by the water in the reactor is used for converting the water in to the steam in heat exchanger.
This steam is used in a conventional way in the steam power plant cycle. Thus, power in not only generated through reactor but also through steam power plant.
The condenser condenses hot steam from steam power plant and cools it down.
The water coolant from heat exchanger is recirculated to the reactor with the help of coolant pump.
These power plants are compact and its cost is reduced since it uses water as coolant as well as moderator.However, high pressure in primary circuit of water absorbing heat in the reactor requires a stronger shell which increases its cost.
In these reactors, the water flowing through the reactor becomes radioactive.
Therefore this primary circuit must be heavily shielded to protect the operators.

Advantages of Pressurized Water Reactor (PWR):

1. Water is used as both coolant as well as moderator which is cheap and easily available.
2. This reactor is compact.
3. Fission product remains contained in the reactor.
4. Small number of control rods are required.

Disadvantages of Pressurized Water Reactor (PWR):

1. Its capital cost is high since the reactor and primary circuit works under pressure.
2. Costly shielding is required to shield the operators in primary circuit since the coolant becomes radioactive.
3. Fuel suffers from radiations. Therefore, its reprocessing is difficult.
4. Only saturated steam can be generated in secondary circuit, therefore the efficiency of the plant is low.
5. Sever corrosion problems.
6. Plant needs to be shut down for fuel charging.