Classification of Nuclear Reactors

Classification of Nuclear Reactors
Nuclear Reactors, specifically fission reacors, are classified by several methods, a brief outline of these classification schemes is given below.
Classification by use
Research reactors : Typically reactors used for research and training, materials testing, or the production of radioisotopes for medicine and industry. These are much smaller than power reactors or those propelling ships, and many are on university campuses. There are about 280 such reactors operating, in 56 countries. Some operate with high-enriched uranium fuel, and international efforts are underway to substitute low-enriched fuel.
Production reactors
Power reactors
Propulsion reactors 

Classification by moderator material
Graphite moderated reactors
water moderated reactors 

• Light water moderated reactors (LWRs)
• Heavy Water moderated reactors

Classification by coolant
Gas cooled reactor
Liquid metal cooled reactor
Water cooled reactor

• Pressure water reactor
• Boiling water reactor

Classification by type of nuclear reactionFast Reactors
Thermal reactors
Classification by role in the fuel cycleBreeder reactors
burner reactors
Classification by Generation
Generation II reactor
Generation III reactor
Generation IV reactor
Classification by phase of fuelSolid fueled
Fluid fueled
Gas Fueled

Boiling Water Reactor (BWR) - Advantages and Disadvantages

Boiling Water Reactor (BWR)
A boiling water reactor (BWR) is a type of light-water nuclear reactor developed by the General Electric Company in the mid 1950s.


1.Reactor pressure vessel 2.Fuel rods 3. Control rod 4.Circulating pump 5.Control rod drive 6.Fresh steam 7. Feedwater 8.High pressure turbine 9.Low pressure turbine 10.Generator 11.Exciter 12.Condenser 13.Cooling water 14.Preheater 15.Feedwater pump 16. Cooling water pump 17.Concrete shield

The above diagram shows BWR and its main parts.The BWR is characterized by two-phase fluid flow (water and steam) in the upper part of the reactor core. Light water (i.e., common distilled water) is the working fluid used to conduct heat away from the nuclear fuel. The water around the fuel elements also "thermalizes" neutrons, i.e., reduces their kinetic energy, which is necessary to improve the probability of fission of fissile fuel. Fissile fuel material, such as the U-235 and Pu-239 isotopes, have large capture cross sections for thermal neutrons.

In a boling water reactor, light water (H2O) plays the role ofmoderator and coolant, as well. In this case the steam is generted in the reactor it self.As you can see in the diagrm feed water enters the reactor pressure vessel at the bottom and takes up the heat generated due to fission of fuel (fuel rods) and gets converted in to steam.

Part of the water boils away in the reactor pressure vessel, thus a mixture of water and steam leaves the reactor core. The so generated steam directly goes to the turbine, therefore steam and moisture must be separated (water drops in steam can damage the turbine blades). Steam leaving the turbine is condensed in the condenser and then fed back to the reactor after preheating. Water that has not evaporated in the reactor vessel accumulates at the bottom of the vessel and mixes with the pumped back feedwater.
Since boiling in the reactor is allowed, the pressure is lower than that of the PWRs: it is about 60 to 70 bars. The fuel is usually uranium dioxide. Enrichment of the fresh fuel is normally somewhat lower than that in a PWR. The advantage of this type is that - since this type has the simplest construction - the building costs are comparatively low. 22.5% of the total power of presently operating nuclear power plants is given by BWRs.

Feedwater Inside of a BWR reactor pressure vessel (RPV), feedwater enters through nozzles high on the vessel, well above the top of the nuclear fuel assemblies (these nuclear fuel assemblies constitute the "core") but below the water level. The feedwater is pumped into the RPV from the condensers located underneath the low pressure turbines and after going through feedwater heaters that raise its temperature using extraction steam from various turbine stages.
The feedwater enters into the downcomer region and combines with water exiting the water separators. The feedwater subcools the saturated water from the steam separators. This water now flows down the downcomer region, which is separated from the core by a tall shroud. The water then goes through either jet pumps or internal recirculation pumps that provide additional pumping power (hydraulic head). The water now makes a 180 degree turn and moves up through the lower core plate into the nuclear core where the fuel elements heat the water. When the flow moves out of the core through the upper core plate, about 12 to 15% of the flow by volume is saturated steam.
The heating from the core creates a thermal head that assists the recirculation pumps in recirculating the water inside of the RPV. A BWR can be designed with no recirculation pumps and rely entirely on the thermal head to recirculate the water inside of the RPV. The forced recirculation head from the recirculation pumps is very useful in controlling power, however. The thermal power level is easily varied by simply increasing or decreasing the speed of the recirculation pumps.
The two phase fluid (water and steam) above the core enters the riser area, which is the upper region contained inside of the shroud. The height of this region may be increased to increase the thermal natural recirculation pumping head. At the top of the riser area is the water separator. By swirling the two phase flow in cyclone separators, the steam is separated and rises upwards towards the steam dryer while the water remains behind and flows horizontally out into the downcomer region. In the downcomer region, it combines with the feedwater flow and the cycle repeats.
The saturated steam that rises above the separator is dried by a chevron dryer structure. The steam then exists the RPV through four main steam lines and goes to the turbine.
Control systems
Reactor power is controlled via two methods: by inserting or withdrawing control rods and by changing the water flow through the reactor core.
Positioning (withdrawing or inserting) control rods is the normal method for controlling power when starting up a BWR. As control rods are withdrawn, neutron absorption decreases in the control material and increases in the fuel, so reactor power increases. As control rods are inserted, neutron absorption increases in the control material and decreases in the fuel, so reactor power decreases. Some early BWRs and the proposed ESBWR designs use only natural ciculation with control rod positioning to control power from zero to 100% because they do not have reactor recirculation systems.
Changing (increasing or decreasing) the flow of water through the core is the normal and convenient method for controlling power. When operating on the so-called "100% rod line," power may be varied from approximately 70% to 100% of rated power by changing the reactor recirculation system flow by varying the speed of the recirculation pumps. As flow of water through the core is increased, steam bubbles ("voids") are more quickly removed from the core, the amount of liquid water in the core increases, neutron moderation increases, more neutrons are slowed down to be absorbed by the fuel, and reactor power increases. As flow of water through the core is decreased, steam voids remain longer in the core, the amount of liquid water in the core decreases, neutron moderation decreases, fewer neutrons are slowed down to be absorbed by the fuel, and reactor power decreases.
Steam Turbines
Steam produced in the reactor core passes through steam separators and dryer plates above the core and then directly to the turbine, which is part of the reactor circuit. Because the water around the core of a reactor is always contaminated with traces of radionuclides, the turbine must be shielded during normal operation, and radiological protection must be provided during maintenance. The increased cost related to operation and maintenance of a BWR tends to balance the savings due to the simpler design and greater thermal efficiency of a BWR when compared with a PWR. Most of the radioactivity in the water is very short-lived (mostly N-16, with a 7 second half life), so the turbine hall can be entered soon after the reactor is shut down.
Safety Like the pressurized water reactor, the BWR reactor core continues to produce heat from radioactive decay after the fission reactions have stopped, making nuclear meltdown possible in the event that all safety systems have failed and the core does not receive coolant. Also like the pressurized water reactor, a boiling-water reactor has a negative void coefficient, that is, the thermal output decreases as the proportion of steam to liquid water increases inside the reactor. However, unlike a pressurized water reactor which contains no steam in the reactor core, a sudden increase in BWR steam pressure (caused, for example, by a blockage of steam flow from the reactor) will result in a sudden decrease in the proportion of steam to liquid water inside the reactor. The increased ratio of water to steam will lead to increased neutron moderation, which in turn will cause an increase in the power output of the reactor. Because of this effect in BWRs, operating components and safety systems are designed to ensure that no credible, postulated failure can cause a pressure and power increase that exceeds the safety systems' capability to quickly shutdown the reactor before damage to the fuel or to components containing the reactor coolant can occur.
In the event of an emergency that disables all of the safety systems, each reactor is surrounded by a containment building designed to seal off the reactor from the environment.
Comparison with other reactors Light water is ordinary water. In comparison, some other water-cooled reactor types use heavy water. In heavy water, the deuterium isotope of hydrogen replaces the common hydrogen atoms in the water molecules (D2O instead of H2O, molecular weight 20 instead of 18).
The Pressurized Water Reactor (PWR) was the first type of light-water reactor developed because of its application to submarine propulsion. The civilian motivation for the BWR is reducing costs for commercial applications through design simplification and lower pressure components. In naval reactors, BWR designs are used when natural circulation is specified for its quietness. The description of BWRs below describes civilian reactor plants in which the same water used for reactor cooling is also used in the Rankine cycle turbine generators. A Naval BWR is designed like a PWR that has both primary and secondary loops.
In contrast to the pressurized water reactors that utilize a primary and secondary loop, in civilian BWRs the steam going to the turbine that powers the electrical generator is produced in the reactor core rather than in steam generators or heat exchangers. There is just a single circuit in a civilian BWR in which the water is at lower pressure (about 75 times atmospheric pressure) compared to a PWR so that it boils in the core at about 285°C. The reactor is designed to operate with steam comprising 12–15% of the volume of the two-phase coolant flow (the "void fraction") in the top part of the core, resulting in less moderation, lower neutron efficiency and lower power density than in the bottom part of the core. In comparison, there is no significant boiling allowed in a PWR because of the high pressure maintained in its primary loop (about 158 times atmospheric pressure).

Advantages

• The reactor vessel and associated components operate at a substantially lower pressure (about 75 times atmospheric pressure) compared to a PWR (about 158 times atmospheric pressure).
• Pressure vessel is subject to significantly less irradiation compared to a PWR, and so does not become as brittle with age.
• Operates at a lower nuclear fuel temperature.
• Fewer components due to no steam generators and no pressurizer vessel. (Older BWRs have external recirculation loops, but even this piping is eliminated in modern BWRs, such as the ABWR.)
• Lower risk (probability) of a rupture causing loss of coolant compared to a PWR, and lower risk of a severe accident should such a rupture occur. This is due to fewer pipes, fewer large diameter pipes, fewer welds and no steam generator tubes.
• Measuring the water level in the pressure vessel is the same for both normal and emergency operations, which results in easy and intuitive assessment of emergency conditions.
• Can operate at lower core power density levels using natural circulation without forced flow.
• A BWR may be designed to operate using only natural circulation so that recirculation pumps are eliminated entirely. (The new ESBWR design uses natural circulation.)

Disadvantages

• Complex operational calculations for managing the utilization of the nuclear fuel in the fuel elements during power production due to "two phase fluid flow" (water and steam) in the upper part of the core (less of a factor with modern computers). More incore nuclear instrumentation is required.
• Much larger pressure vessel than for a PWR of similar power, with correspondingly higher cost. (However, the overall cost is reduced because a modern BWR has no main steam generators and associated piping.)
• Contamination of the turbine by fission products.
• Shielding and access control around the steam turbine are required during normal operations due to the radiation levels arising from the steam entering directly from the reactor core. Additional precautions are required during turbine maintenance activities compared to a PWR.
• Control rods are inserted from below for current BWR designs. There are two available hydraulic power sources that can drive the control rods into the core for a BWR under emergency conditions. There is a dedicated high pressure hydraulic accumulator and also the pressure inside of the reactor pressure vessel available to each control rod. Either the dedicated accumulator (one per rod) or reactor pressure is capable of fully inserting each rod. Most other reactor types use top entry control rods that are held up in the withdrawn position by electromagnets, causing them to fall into the reactor by gravity if power is lost.

For flash animation of BWR operation Click here

Pressurized Water Reactor (PWR)

Pressurized Water Reactor (PWR)
Pressurized water reactors (PWRs) (also VVER if of Russian design) are generation II nuclear power reactors that use ordinary water under high pressure as coolant and neutron moderator. The primary coolant loop is kept under high pressure to prevent the water from boiling, hence the name. PWRs are one of the most common types of reactors and are widely used all over the world. More than 230 of them are in use to generate electric power, and several hundred more for naval propulsion. They were originally designed by the Bettis Atomic Power Laboratory as a nuclear submarine power plant.The below diagram shows the PWR and its main parts.

1.Reactor vessel 2.Fuel elements 3.Control rods 4.Control rod drive 5.Pressurizer 6.Steam generator 7.Main circulating pump 8.Fresh steam 9.Feedwater 10.High pressure turbine 11.Low pressure turbine 12.Generator 13.Exciter 14.Condenser 15.Cooling water 16.Feedwater pump 17.Feedwater pre-heater 18.Concrete shield 19.Cooling water pump

The pressurized water reactor belongs to the light water type: the moderator and coolant are both light water (H2O). It can be seen in the figure that the cooling water circulates in two loops, which are fully seperated from one another.
The primary circuit water (dark blue) is continuously kept at avery high pressure and therefore it does not boil even at the high operating temperature. (Hence the name of the type.) Constant pressure is ensured with the aid of the pressurizer (expansion tank). (If pressure falls in the primary circuit, water in the pressurizers is heated up by electric heaters, thus raising the pressure. If pressure increases, colder cooling water is injected to the pressurizer. Since the upper part is steam, pressure will drop.) The primary circuit water transferes its heat to the secondary circuit water in the small tubes of the steam generator, it cooles down and returns to the reactor vessel at a lower temperature.
Since the secondary circuit pressure is much lower than that of the primary circuit, the secondary circuit water in the steam generator starts to boil (red). The steam goes from here to the turbine, which has high and low pressure stages. When steam leaves the turbine, it becomes liquid again in the condenser, from where it is pumped back to the steam generator after pre-heating.
Normally, primary and secondary circuit waters cannot mix. In this way it can be achieved that any potentially radioactive material that gets into the primary water should stay in the primary loop and cannot get into the turbine and condenser. This is a barrier to prevent radioactive contamination from getting out.
In pressurized water reactors the fuel is usually low (3 to 4 percent) enriched uranium oxide, sometimes uranium and plutonium oxide mixture (MOX). In today's PWRs the primary pressure is usually 120 to 160 bars, while the outlet temperature of coolant is 300 to 320 °C. PWR is the most widespread reactor type in the world: they give about 64% of the total power of the presently operating nuclear power plants.

Two things are characteristic for the pressurized water reactor (PWR) when compared with other reactor types:

• In a PWR, there are two separate coolant loops (primary and secondary), which are both filled with ordinary water (also called light water). A boiling water reactor, by contrast, has only one coolant loop, while more exotic designs such as breeder reactors use substances other than water (i.e., liquid metal as sodium) for the task.
• The pressure in the primary coolant loop is at typically 15-16 Megapascal, notably higher than in other nuclear reactors. As an effect of this, the gas laws guarantee that only sub-cooled boiling will occur in the primary loop. By contrast, in a boiling water reactor the primary coolant is allowed to boil and it feeds the turbine directly without the use of a secondary loop.

Coolant
Ordinary water is used as primary coolant in a PWR and flows through the reactor at a temperature of roughly 315 °C (600 °F). The water remains liquid despite the high temperature due to the high pressure in the primary coolant loop (usually around 2200 psig [15 MPa, 150 atm]). The primary coolant loop is used to heat water in a secondary circuit that becomes saturated steam (in most designs 900 psia [6.2 MPa, 60 atm], 275 °C [530 °F]) for use in the steam turbine.
Moderator
Pressurized water reactors, like thermal reactor designs, require the fast fission neutrons in the reactor to be slowed down (a process called moderation) in order to sustain its chain reaction. In PWRs the coolant water is used as a moderator by letting the neutrons undergo multiple collisions with light hydrogen atoms in the water, losing speed in the process. This "moderating" of neutrons will happen more often when the water is more dense (more collisions will occur). The use of water as a moderator is an important safety feature of PWRs, as any increase in temperature causes the water to expand and become less dense; thereby reducing the extent to which neutrons are slowed down and hence reducing the reactivity in the reactor. Therefore, if reactor activity increases beyond normal, the reduced moderation of neutrons will cause the chain reaction to slow down, producing less heat. This property, known as the negative temperature coefficient of reactivity, makes PWR reactors very stable.
FuelThe uranium used in PWR fuel is usually enriched several percent in 235U. After enrichment the uranium dioxide (UO2) powder is fired in a high-temperature, sintering furnace to create hard, ceramic pellets of enriched uranium dioxide. The cylindrical pellets are then put into tubes of a corrosion-resistant zirconium metal alloy (Zircaloy) which are backfilled with helium to aid heat conduction and detect leakages. The finished fuel rods are grouped in fuel assemblies, called fuel bundles, that are then used to build the core of the reactor. As a safety measure PWR designs do not contain enough fissile uranium to sustain a prompt critical chain reaction (i.e, substained only by prompt neutron). Avoiding prompt criticality is important as a prompt critical chain reaction could very rapidly produce enough energy to damage or even melt the reactor (as is suspected to have occurred during the accident at the Chernobyl plant). A typical PWR has fuel assemblies of 200 to 300 rods each, and a large reactor would have about 150-250 such assemblies with 80-100 tonnes of uranium in all. Generally, the fuel bundles consist of fuel rods bundled 14x14 to 17x17. A PWR produces on the order of 900 to 1500 MWe. PWR fuel bundles are about 4 meters in length.Refuelings for most commercial PWRs is on an 18-24 month cycle. Approximately one third of the core is replaced each refueling.
ControlGenerally, reactor power can be viewed as following steam (turbine) demand due to the reactivity feedback of the temperature change caused by increased or decreased steam flow. Boron and control rods are used to maintain primary system temperature at the desired point. In order to decrease power, the operator throttles shut turbine inlet valves. This would result in less steam being drawn from the steam generators. This results in the primary loop increasing in temperature. The higher temperature causes the reactor to fission less and decrease in power. The operator could then add boric acid and/or insert control rods to decrease temperature to the desired point.
Reactivity adjustments to maintain 100% power as the fuel is burned up in most commercial PWR's is normally controlled by varying the concentration of boric acid dissolved in the primary reactor coolant. The boron readily absorbs neutrons and increasing or decreasing its concentration in the reactor coolant will therefore affect the neutron activity correspondingly. An entire control system involving high pressure pumps (usually called the charging and letdown system) is required to remove water from the high pressure primary loop and re-inject the water back in with differing concentrations of boric acid. The reactor control rods, inserted through the top directly into the fuel bundles, are normally only used for power changes. In contrast, BWRs have no boron in the reactor coolant and control the reactor power by adjusting the reactor coolant flow rate.Due to design and fuel enrichment differences, naval nuclear reactors do not use boric acid.
Advantages

• PWR reactors are very stable due to their tendency to produce less power as temperatures increase, this makes the reactor easier to operate from a stability standpoint.
• PWR reactors can be operated with a core containing less fissile material than is required for them to go prompt critical. This significantly reduces the chance that the reactor will run out of control and makes PWR designs relatively safe from criticality accidents.
• Because PWR reactors use enriched uranium as fuel they can use ordinary water as a moderator rather than the much more expensive heavy water.
• PWR turbine cycle loop is separate from the primary loop, so the water in the secondary loop is not contaminated by radioactive materials.
• The reactor has high power density.
• The reactor responds to supply more power when the load increases.

Disadvantages

• The coolant water must be heavily pressurized to remain liquid at high temperatures. This requires high strength piping and a heavy pressure vessel and hence increases construction costs. The higher pressure can increase the consequences of a Loss of Coolant Accident.
• Most pressurized water reactors cannot be refueled while operating. This decreases the availability of the reactor- it has to go offline for comparably long periods of time (some weeks).
• The high temperature water coolant with boric acid dissolved in it is corrosive to carbon steel (but not stainless steel), this can result in radioactive corrosion products to circulate in the primary coolant loop. This not only limits the lifetime of the reactor, but the systems that filter out the corrosion products and adjust the boric acid concentration add significantly to the overall cost of the reactor and radiation exposure.
• Water absorbs neutrons making it necessary to enrich the uranium fuel, which increases the costs of fuel production. If heavy water is used it is possible to operate the reactor with natural uranium, but the production of heavy water requires large amounts of energy and is hence expensive.
• Because water acts as a neutron moderator it is not possible to build a fast neutron reactor with a PWR design. For this reason it is not possible to build a fast breeder reactor with water coolant.
• Because the reactor produces energy more slowly at higher temperatures, a sudden cooling of the reactor coolant could increase power production until safety systems shut down the reactor.

For flash animation of PWR operation Click here

CANDU Reactor

CANDU Reactor
The CANDU reactor is a Pressurized Heavy Water Reactordeveloped initially in the late 1950s and 1960s by a partnership between Atomic Energy of Canada Limited (AECL), the Hydro-Electric Power Commission of Ontario (now known as Ontario Power Generation), Canadian General Electric (now known as GE Canada), as well as several private industry participants. The acronym "CANDU", a registered trademark of Atomic Energy of Canada Limited, stands for "CANada Deuterium Uranium". This is a reference to its deuterium-oxide (heavy water) moderator and its use of natural uranium fuel. This type of reactor is meant for those countries which do not prodce enriched uranium.Enrichment of uranium is costly and this reactor uses natural uranium as fuel and heavy water as moderator.

In heavy water reactors both the modeartor and coolant are heavy water (D2O). A great disadvantage of this type comes from this fact: heavy water is one of the most expensive liquids. However, it is worth its price: this is the best moderator. Therefore, the fuel of HWRs can be slightly (1% to 2%) enriched or even natural uranium. Heavy water is not allowed to boil, so in the primary circuit very high pressure, similar to that of PWRs, exists.

CANDU fuel is made from uranium that is naturally radioactive. Small amounts of uranium can generate large amounts of energy in the form of heat. The uranium is mined, refined and made into solid ceramic pellets (two pellets are the size of an AA battery). The pellets are put in metal tubes, which are welded together to form a fuel bundle that weighs around 23 kg.The bundle is about the size of a fireplace log and can provide enough energy for an average home for 100 years. The figure below shows the CANDU reactor and its main parts.


In CANDU reactors, the moderator and coolant are spatially separated: the moderator is in a large tank (calandria), in which there are pressure tubes surrounding the fuel assemblies. The coolant flows in these tubes only.

The advantage of this construction is that the whole tank need not be kept under high pressure, it is sufficient to pressurize the coolant flowing in the tubes. This arrangement is called pressurized tube reactor. Warming up of the moderator is much less than that of the coolant; its is simply lost for heat generation or steam production. The high temperature and high pressure coolant, similarly to PWRs, goes to the steam generator where it boils the secondary side light water. Another advantage of this type is that fuel can be replaced during operation and thus there is no need for outages.
Fission reactions in the reactor core heat a fluid, in this case heavy water (see below), which is kept under high pressure to raise its boiling point and avoid significant steam formation in the core. The hot heavy water generated in this primary cooling loop is passed into a heat exchanger heating light (ordinary) water in the less-pressurized secondary cooling loop. This water turns to steam and powers a conventional turbine with a generator attached to it. Any excess heat energy in the steam after flowing through the turbine is rejected into the environment in a variety of ways, most typically into a large body of cool water (lake, river, or ocean). More recently-built CANDU plants (such as the Darlington station near Toronto, Ontario) use a discharge-diffuser system that limits the thermal effects in the environment to within natural variations.
CANDU reactors employ two independent, fast-acting safety shutdown systems. Control rods penetrate the calandria vertically and lower into the core in the case of a safety-system trip.A second shutdown system is via gadolinium nitrate liquid "neutron poison" injection directly in to the low pressure moderator. Both systems operate via separate and independent trip logic.

Nuclear Power Plant Operation

Nuclear Power Plant Operation
The below diagram shows the schematic of nuclear power plant.Nuclear power generation is much similar to that of conventional steam power generation.The difference lies only in the steam generation part i.e coal or oil boiling furnance and boiler are replaced by nuclear reactor.

Thus a nuclear power plant consists of a nuclear reactor,steam generator,turbine, generator, condenser etc. as shown in the above figure.As in a conventional steam plant, water for raising steam forms a closed feed system.However, the reactor and the cooling circuit have to be heavily shielded to eliminate radiation hazards.

A nuclear power plant uses the heat generated by a nuclear fission process to drive a steam turbine which generates usable electricity.Fission is the splitting of atoms into smaller parts. Some atoms, themselves tiny, split when they are struck by even smaller particles, called neutrons. Each time this happens more neutrons come out of the split atom and strike other atoms. This process of energy release is called a chain reaction. The plant controls the chain reaction to keep it from releasing too much energy too fast. In this way, the chain reaction can go on for a long time.

Few natural elements have atoms that will split in a chain reaction. Iron, copper, silver and many other common metals will not split, or fission. There are isotopes of iron, copper, etc. that are radioactive. This means that they have an unstable nucleus and they emit radioactivity. However, just being radioactive does not mean that they will fission, or split. But uranium will. So uranium is suitable to fuel a nuclear power plant.

As atoms split and collide, they heat up. The plant uses this heat to create steam.The heat is transfered to the water through heat exchanging tubes in steam generator in the primary loop.After extractig this heat, water is converted in to steam and collected at the top of steam generator.The pressure of the expanding steam turns a turbine which is connected to a generator in the secondary loop.After rotating turbine - generator set steam passes to the condenser.After that the function of condenser and coling towers is same as that of thermal plant.

After the steam is made, a nuclear plant operates much like a fossil fuel fired plant: the turbine spins a generator. The whirling magnetic field of the generator produces electricity. The electricity then goes through wires strung on tall towers you might see along a highway to an electrical substation in your neighborhood where the power is regulated to the proper strength. Then it goes to your home.

In the case of nuclear power plant operation the following factors must be considered

• Control -- Keeping the nuclear reaction from dying out or exploding.
• Safety -- If something goes wrong it can be contained.
• Refueling -- Adding more nuclear fuel without stoping the reactor.
• Waste production -- The byproducts of the reaction must be manageable.
• Efficiency -- Capture as much of the heat as possible.

Control is the most important aspect to a design. When an atom of nuclear fuel (uranium) absorbs a neutron, the uranium will fission into two smaller atoms (waste) and release one to three neutrons. The kinetic energy of the waste is used to heat the water for the steam turbine. The neutrons are used to fission the next lot of uranium atoms and the process continues. If none of these neutrons are absorbed by another uranium atom then the reaction dies out. If too many neutrons are absorbed then the reaction grows extremely quickly and could explode. Current reactor designs are most usefully classified by how they ensure this nuclear reaction is kept at a level which produces power without getting out of hand.
The Nuclear Regulatory Commission (NRC), part of our government, makes sure nuclear power plants in the United States protect public health and safety and the environment. The NRC licenses the use of nuclear material and inspects users to make sure they follow the rules for safety.
Since radioactive materials are potentially harmful, nuclear power plants have many safety systems to protect workers, the public, and the environment. These safety systems include shutting the reactor down quickly and stopping the fission process, systems to cool the reactor down and carry heat away from it and barriers to contain any radioactivity and prevent it from escaping into the environment.
One of the greatest benefits of nuclear plants is that they have no smoke stacks! The big towers many people associate with nuclear plants are actually for cooling water used to make steam. (Some other kinds of plants have these towers, too.) The towers spread the water out so as much air as possible can reach it and cool it down. Most water is then recycled into the plant.
Nuclear power plants are very clean and efficient to operate. However, nuclear power plants have some major environmental risks. Nuclear power plants produce radioactive gases. These gases are to be contained in the operation of the plant. If these gases are released into the air, major health risks can occur. Nuclear plants use uranium as a fuel to produce power. The mining and handling of uranium is very risky and radiation leaks can occur. The third concern of nuclear power is the permanent storage of spent radioactive fuel. This fuel is toxic for centuries, handling and disposal is an ongoing environmental issue.

General applications of transmission lines

Transferring signals from one point to anotherElectrical transmission lines are very widely used to transmit high frequency signals over long or short distances with minimum power loss. One familiar example is the down lead from a TV or radioaerial to the receiver.
Pulse generation
Transmission lines are also used as pulse generators. By charging the transmission line and then discharging it into a resistive load, a rectangular pulse equal in length to twice the electrical length of the line can be obtained, although with half the voltage. A Blumlein transmission line is a related pulse forming device that overcomes this limitation. These are sometimes used as the pulsed energy sources for radar transmitters and other devices.
Stub filters
If a short-circuited or open-circuited transmission line is wired in parallel with a line used to transfer signals from point A to point B, then it will function as a filter. The method for making stubs is similar to the method for using Lecher lines for crude frequency measurement, but it is 'working backwards'. One method recommended in the RSGB's radiocommunication handbook is to take an open-circuited length of transmission line wired in parallel with the feeder delivering signals from an aerial. By cutting the free end of the transmission line, a minimum in the strength of the signal observed at a receiver can be found. At this stage the stub filter will reject this frequency and the odd harmonics, but if the free end of the stub is shorted then the stub will become a filter rejecting the even harmonics.

Practical types of electrical transmission line

Coaxial cable
Coaxial lines confine the electromagnetic wave to the area inside the cable, between the center conductor and the shield. The transmission of energy in the line occurs totally through the dielectric inside the cable between the conductors. Coaxial lines can therefore be bent and twisted (subject to limits) without negative effects, and they can be strapped to conductive supports without inducing unwanted currents in them.
In radio-frequency applications up to a few gigahertz, the wave propagates in the transverse electric and magnetic mode (TEM), which means that the electric and magnetic fields are both perpendicular to the direction of propagation. However, above a certain frequency called the cutoff frequency, the cable behaves as a waveguide, and propagation switches to either a transverse electric (TE) or a transverse magnetic (TM) mode or a mixture of modes. This effect enables coaxial cables to be used at microwave frequencies, although they are not as efficient as the more expensive, purpose-built waveguides.
The most common use for coaxial cables is for television and other signals with bandwidth of multiple Megahertz. In the middle 20th Century they carried long distance telephone connections