👉NUCLEAR REACTION AND CLASSIFICATION OF NUCLEAR REACTOR 📝

🎪CLASSIFICATION OF NUCLEAR REACTORS🎪

✏️A nuclear reactor can be classified in different ways such as on the basis of types of cores used, moderator used, coolant used, fuel used and neutral energy.

👉On the basis of type of core used:

1️⃣Homogenous reactors

2️⃣Heterogenous reactors

👉On the basis of moderator used:

1️⃣Graphite reactors

2️⃣Beryllium reactors

3️⃣Light water reactors

4️⃣Heavy water reactors

👉On the basis of coolant used:

1️⃣Ordinary water cooled reactors

2️⃣Heavy water cooled reactors

3️⃣Gas cooled reactors

4️⃣Liquid metal cooled reactors

5️⃣Organic liquid cooled reactors

👉On the basis of neutron energy used:

1️⃣Thermal reactors

2️⃣Fast reactors

👉On the basis of fuel material used:

1️⃣Enrich uranium

2️⃣Natural uranium

3️⃣Plutonium

4️⃣Thorium

✏️A hetero-genous reactor has a large number of fuel rods with a coolant circulating around them and carrying away the heat produced during the fission process. In a homogenous reactor, the moderator and fuel are mixed together the solution is critical in the core.


✏️Due to difficulties in maintenance, induced radioactivity, erosion and corrosion, homogeneous reactors are not commonly used natural uranium also cannot be used. Light-water cooled moderated reactors using slightly and enriched uranium fuel are the most commonly used for power production.

📝Power reactors in use:
✏️power reactor is commonly employed in nuclear power plants are described as follows: 

📝Boiling water reactor (BWR)
✏️This is the simplest type of water reactor as the name suggests this other reactors where coolant balls in the reactor Pressure Vessel itself the boiling water reactor utilizes enriched uranium as the fuel and light water as both moderator and coolant.  A boiling water reactor operates at a relatively lower pressure of about 76 bar in such a way that the water balls in the core at about 285 degree Celsius.  Steam water mixture leaves the core with water separator from steam in a steam separator.  Steam dryer is used to produce dry steam required to run the turbine for power generation.  A boiling water reactor assembly consists of 90-100 fuel rods and their up to 750 assemblies in a core holding up to 140 tons of Uranium.  A number of safety devices are installed to achieve immediate isolation of the reactor from the engine house in case of a malfunction.  The diagram of the boiling water reactor is shown in the figure below:

📝ADVANTAGES:
✏️The reactor vessel and associated components operate at a substantially lower pressure of about 70-75 bars (1020-1090 psi).

✏️Can operate at lower core power density levels using natural circulation without forced flow.

✏️Boiling water reactor may be designed to operate using only natural circulation so that recirculation pumps are eliminated entirely.

✏️Simple in construction.

✏️Elimination of heat exchanger circuit resulting in reduction in cost and gain in thermal efficiency.

📝DISADVANTAGES:
✏️Since steam is produced from water which is passing through the reactor the radioactive contamination of turbine mechanism is possible and better steam pipes are required to avoid the radioactive fear.

✏️Waste of steam will also lower the efficiency of the plant.


✏️It is not suitable for meeting a sudden increase in the load.



📝PWR (PRESSUROZED WATER REACTOR) 

✏️Water can be used as a moderator and coolant for power reactors. The fuel used is slightly and enriched uranium in the form of thin rods or pallets and cladding is either of stainless steel or zircaloy.  Water under pressure is used as both moderator and coolant. This type of reactor is extensively developed in USA. the most important limitation in power pressurized water reactor is the critical temperature of water.  The current pressure must be greater than the saturation pressure to suppress boiling which is maintained at about 155 bars.  A circulating pump is used to maintain the water around the core which absorbs heat. the pressurized water reactor power plant is composed of two loops in series the coolant loop, also called as primary look and water steam or working fluid loop. the cool and pics of the heat from the reactor and transfer it to the working fluid to generate steam to run the turbine generator system similar to that of the steam power plant.  a pressure Riser and search tank which is tapped into the pipe loop is used to maintain the constant pressure in the water system throughout the load range.  There is also some modification to increase the efficiency of the cycle by the inclusion of an oil fired super-heater between the main heat exchangers and the turbine.  There is also an economy is there along with some feed water heaters since the water passing through the reactor becomes a radioactive and therefore the entire primary circuit including the heat exchanger has to be shielded.

📝ADVANTAGES:
✏️It is cheap as ordinary water is used as moderator and coolant.

✏️It is very compact in size compared to other reactors.

✏️Power density of a reactor is relatively high.

✏️Reactor takes care of the load variation by using the pressurizer and surge tank.

📝DISADVANTAGES:
✏️Low thermal efficiency about 20 percentage.

✏️Greater heat loss due to use of heat exchanger.

✏️Due to high pressure a strong Pressure vessel is required.

✏️There is lack of flexibility in recharging.

✏️More safety devices is required.


📝HEAVY WATER COOLED AND MODERATED REACTOR (CANDU TYPE REACTOR) 

✏️Heavy water has almost the same characteristics as that of the ordinary water. Is boiling point at atmospheric pressure is 101.4 degree Celsius and its density at room temperature is only 10 % above the density of water.  Heavy water moderated and cooled reactors are extensively developed and used in Canada and are called Canadian deuterium uranium (CANDU) reactors.  this reactors use pressurized heavy water and suitable for those countries which do not produce the enriched uranium and the figure of the CANDU reactor is shown in the figure below: 

✏️This reactor uses the natural uranium as fuel which is comparatively cheaper than the enriched uranium. Other advantages are low Pressure Vessel no control rods and low fuel consumption the moderators being at low temperature is more effective in slowing down the neutrons.  The construction of equipment requires less time than the others. The main drawback of this type of reactor is its cost, as a heavy cost of heavy water is extremely high and also there is also the problem of leakage and a proper safety design is required.

📝GAS COOLED REACTOR
✏️The gas cooled reactor in which the gas is used as a coolant and graphite is used as the moderator and is shown in the figure below:

✏️The advantages of gas cooling is that the maximum temperature of the working fluid can be selected independently, rising temperature does not necessarily imply raising of cool and pressure as with the liquid cooled reactors.  Normally carbon dioxide or helium is used as the coolant all the gases are inferior to water in heat transfer but offer several advantages such as safer than water cooled reactor less severe erosion problem and natural uranium can be used as fuel.  Helium a suitable coal and compared to carbon dioxide as it is chemically inert has good heat transfer capability and a low neutron absorption. Being a monoatomic gas it can produce more power for the given temperatures in the Brayton cycle and higher efficiency.

📝FAST BREEDER REACTOR

✏️A fast breeder reactor is different from the thermal reactors on the basis of their thermal reactor uses Fissile nuclear fuel and produces heat, whereas a fast breeder reactor produces heat and at the same time convert fertile material into fissile material.  It is possible to make a fast reactor producing more physical material than it consumes.

✏️The advantage of using high energy neutrons in a reactor has been known since early days of the nuclear science.  In a fast breeder reactor the average neutron yield of a pigeon caused by a fast neutron is greater than in the thermal reactors.  The absorption cross sections are low and conversion factor is high also no moderator is needed in this reactor.

✏️Coolant with excellent heat transfer properties is required to minimise the temperature drop from the fuel surface to the coolant and also it must be non-moderating.  This rules out water and the best coolants for fast breeder reactors are liquid metal such as sodium.  Such reactors are also called as liquid metal cooled reactor or LMCR.  Due to induced the radioactive of liquid Sodium an intermediate loop also use a sodium or sodium potassium as coolant and between the primary radioactive coolant and the steam cycle.  Therefore there is a need of two heat exchangers. The primary loop design can be either pool type / loop type. In pool type system, the reactor core, primary pumps and intermediate heat exchangers are placed in a large pool of liquid sodium contained reactor vessel, whereas in loop type system all are placed outside the vessel. The diagram of liquid metal fast breeder reactor is shown in figure below: