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NUCLEAR REACTOR 

UNIT-II

COMPONENTS OF NUCLEAR REACTOR 

1.What are the Essential parts of nuclear reactor?

1. Nuclear fuel :The nuclear fuel used in a nuclear reactor is the enriched 92U235.T he

nuclear fuel is sealed in along ,narrow metal tubes called fuel rods . The

enriched 92U235ensures that at least one of the neutrons produced by a fission reaction

has a good chance of causing fission in another 92U235 nucleus.

2.Moderator: The neutron released by fission normally move very fast .At this high

speed , the chance of a neutron being captured by another 92U235 nucleus is very small ,

If the neutron is slowed , its chance of capture is much better . In order to slowed

down the fast fission neutrons, A moderator is used .

3. Control rods :In order to control the rate at which fission reaction occurs , control

rods of neutron - absorbing material (eg. cadmium) are used .The control rods keep the

net rate of production of neutrons to the required level by capturing the necessary

 proportion of neutrons before they initiate fission. When the control are moved upward

out of the reactor , the number of neutrons left to produce fission is increased .On the

other hand , when the control rods are lowered , the number of neutrons producing

fission is decreased .

4. Coolant : The propose of the coolant is to removed heat from the reactor core and

take it to the place of its utilization eg. steam turbine.

5. Protective shield : In a nuclear reactor ,many types of harmful radiations are

emitted .In order to prevent this radiations from reaching the persons working near the

reactor , the reactor is enclosed in thick concrete walls.

2.What is Nuclear reactor?

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A nuclear reactor is a device to initiate and control a sustained nuclear chain

reaction. Nuclear reactors are used at nuclear power plants for generating

electricity and in propulsion of ships. Heat from nuclear fission is passed to a

working fluid (water or gas), which runs through turbines. These either drive a

ship's propellers or turn electrical generators. Nuclear generated steam in principle

can be used for industrial process heat or for district heating. Some reactors are

used to produce isotopes for medical and industrial use, or for production of 

 plutonium for weapons. Some are run only for research.

3. What are the components of nuclear power plants?

•  Nuclear fuel

•  Nuclear reactor core

•  Neutron moderator 

•  Neutron poison

•  Neutron howitzer (provides steady source of neutrons to re-initiate reaction

following shutdown)

• Coolant (often the Neutron Moderator and the Coolant are the same, usually both

 purified water)

• Control rods

• Reactor vessel

• Boiler feed water pump

• Steam generators (not in BWRs)

• Steam turbine

• Electrical generator 

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• Condenser 

• Cooling tower (not always required)

• Radwaste System (a section of the plant handling radioactive waste)

• Refueling Floor 

• Spent fuel pool

•  Nuclear safety systems

o Reactor Protective System (RPS)

o Emergency Diesel Generators

o Emergency Core Cooling Systems (ECCS)

o Standby Liquid Control System (emergency boron injection, in BWRs

only)

• Essential service water system (ESWS)

• Containment building

• Control room

• Emergency Operations Facility

•  Nuclear training facility (usually contains a Control Room simulator)

4.What are the different types of nuclear reactors?

Classifications

 Nuclear Reactors are classified by several methods; a brief outline of these classification

methods is provided.

Classification by type of nuclear reaction

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 Nuclear fission. All commercial power reactors are based on nuclear fission. They

generally use uranium and its product plutonium as nuclear fuel, though a thorium fuel

cycle is also possible. Fission reactors can be divided roughly into two classes,

depending on the energy of the neutrons that sustain the fission chain reaction:

Thermal reactors use slowed or thermal neutrons. Almost all current reactors are

of this type. These contain neutron moderator materials that slow neutrons until

their  neutron temperature is thermalized , that is, until their kinetic

energy approaches the average kinetic energy of the surrounding particles.

Thermal neutrons have a far higher  cross section (probability) of fissioning

the fissile nuclei uranium-235, plutonium-239, and plutonium-241, and a

relatively lower probability of neutron capture by uranium-238 (U-238) comparedto the faster neutrons that originally result from fission, allowing use of low-

enriched uranium or even natural uranium fuel. The moderator is often also

the coolant, usually water under high pressure to increase the  boiling point. These

are surrounded by a reactor vessel, instrumentation to monitor and control the

reactor,radiation shielding, and a containment building.

Fast neutron reactors use fast neutrons to cause fission in their fuel. They do not

have a neutron moderator , and use less-moderating coolants. Maintaining a chain

reaction requires the fuel to be more highly enriched in fissile material (about 20%

or more) due to the relatively lower probability of fission versus capture by U-238.

Fast reactors have the potential to produce less transuranic waste because

all actinides are fissionable with fast neutrons,[16]  but they are more difficult to

 build and more expensive to operate. Overall, fast reactors are less common than

thermal reactors in most applications. Some early power stations were fast

reactors, as are some Russian naval propulsion units. Construction of prototypes is

continuing (see fast breeder or  generation IV reactors).

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•  Nuclear fusion. Fusion power is an experimental technology, generally

with hydrogen as fuel. While not suitable for power production, Farnsworth-Hirsch

fusors are used to produce neutron radiation.

Classification by moderator material

Used by thermal reactors:

Graphite moderated reactors

Water moderated reactors

Heavy water reactors

Light water moderated reactors (LWRs). Light water reactors use ordinary water to

moderate and cool the reactors. When at operating temperature, if the temperature of 

the water increases, its density drops, and fewer neutrons passing through it are slowed

enough to trigger further reactions. That negative feedback stabilizes the reaction rate.

Graphite and heavy water reactors tend to be more thoroughly thermalised than light

water reactors. Due to the extra thermalization, these types can use natural

uranium/unenriched fuel.

Light element moderated reactors. These reactors are moderated by lithium or 

 beryllium.

Molten salt reactors (MSRs) are moderated by a light elements such as lithium or 

 beryllium, which are constituents of the coolant/fuel matrix salts LiF and BeF2.

Liquid metal cooled reactors, such as one whose coolant is a mixture of Lead and

Bismuth, may use BeO as a moderator.

Organically moderated reactors (OMR) use  biphenyl and terphenyl as moderator 

and coolant.

Classification by coolant

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Treatment of the interior part of a VVER-1000 reactor frame on Atommash.

In thermal nuclear reactors (LWRs in specific), the coolant acts as a moderator that

must slow down the neutrons before they can be efficiently absorbed by the fuel.

Water cooled reactor. There are 104 operating reactors in the United States. Of these,

69 are pressurized water reactors (PWR), and 35 are boiling water reactors (BWR). [17]

Pressurized water reactor (PWR)

A primary characteristic of PWRs is a pressurizer, a specialized  pressure

vessel. Most commercial PWRs and naval reactors use pressurizers. During

normal operation, a pressurizer is partially filled with water, and a steam

 bubble is maintained above it by heating the water with submerged heaters.

During normal operation, the pressurizer is connected to the primary reactor 

 pressure vessel (RPV) and the pressurizer "bubble" provides an expansion

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space for changes in water volume in the reactor. This arrangement also

 provides a means of pressure control for the reactor by increasing or 

decreasing the steam pressure in the pressurizer using the pressurizer heaters.

Pressurised heavy water reactors are a subset of pressurized water reactors,

sharing the use of a pressurized, isolated heat transport loop, but using heavy

water  as coolant and moderator for the greater neutron economies it offers.

Boiling water reactor (BWR)

BWRs are characterized by boiling water around the fuel rods in the lower 

 portion of a primary reactor pressure vessel. A boiling water reactor uses 235U,

enriched as uranium dioxide, as its fuel. The fuel is assembled into rods that

are submerged in water and housed in a steel vessel. The nuclear fission

causes the water to boil, generating steam. This steam flows through pipes into

turbines. The turbines are driven by the steam, and this process generates

electricity.[18] During normal operation, pressure is controlled by the amount of 

steam flowing from the reactor pressure vessel to the turbine.

Pool-type reactor 

Liquid metal cooled reactor . Since water is a moderator, it cannot be used as a

coolant in a fast reactor. Liquid metal coolants have

includedsodium, NaK , lead, lead-bismuth eutectic, and in early reactors, mercury.

Sodium-cooled fast reactor 

Lead-cooled fast reactor 

Gas cooled reactors are cooled by a circulating inert gas, often helium in high-

temperature designs, while carbon dioxide has been used in past British and French

nuclear power plants. Nitrogen has also been used.[citation needed ] Utilization of the heat

varies, depending on the reactor. Some reactors run hot enough that the gas can

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directly power a gas turbine. Older designs usually run the gas through a heat

exchanger  to make steam for a steam turbine.

Molten Salt Reactors (MSRs) are cooled by circulating a molten salt, typically a

eutectic mixture of fluoride salts, such as FLiBe. In a typical MSR, the coolant is

also used as a matrix in which the fissile material is dissolved.

Classification by generation

Generation I reactor (early prototypes, research reactors, non-commercial power 

 producing reactors)

Generation II reactor (most current nuclear power plants 1965-1996)

Generation III reactor (evolutionary improvements of existing designs 1996-now)

Generation IV reactor  (technologies still under development unknown start date,

 possibly 2030)

Classification by phase of fuel

• Solid fueled

• Fluid fueled

Aqueous homogeneous reactor 

Molten salt reactor 

• Gas fueled (theoretical)

Classification by use

• Electricity

•  Nuclear power plants including small modular reactors

• Propulsion, see nuclear propulsion

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•  Nuclear marine propulsion

• Various proposed forms of rocket propulsion

• Other uses of heat

• Desalination

• Heat for domestic and industrial heating

• Hydrogen production for use in a hydrogen economy

• Production reactors for transmutation of elements

• Breeder reactors are capable of producing more fissile material than

they consume during the fission chain reaction (by

converting fertile U-238 to Pu-239, or Th-232 to U-233). Thus, a

uranium breeder reactor, once running, can be re-fueled

with natural or even depleted uranium, and a thorium breeder 

reactor can be re-fueled with thorium; however, an initial stock of 

fissile material is required.[21]

•

Creating various radioactive isotopes, such as americium for use in smokedetectors, and cobalt-60, molybdenum-99 and others, used for imaging and medical

treatment.

• Production of materials for nuclear weapons such as weapons-

grade  plutonium

• Providing a source of neutron radiation (for example with the pulsed Godiva

device) and positron radiation[clarification needed ] (e.g. neutron activation analysis and  potassium-

argon dating[clarification needed ])

• Research reactor : 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

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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.

What are the current technology of the nuclear reactors?

Pressurized Water Reactors (PWR)

These reactors use a pressure vessel to contain the nuclear fuel, control rods,

moderator, and coolant. They are cooled and moderated by high pressure liquid

water. The hot radioactive water that leaves the pressure vessel is looped through a

steam generator, which in turn heats a secondary (non-radioactive) loop of water to

steam that can run turbines. They are the majority of current reactors. This is

a thermal neutronreactor design, the newest of which are the VVER-1200, Advanced Pressurized Water Reactor and the European Pressurized

Reactor . United States Naval reactors are of this type.

Boiling Water Reactors (BWR)

A BWR is like a PWR without the steam generator. A boiling water reactor is

cooled and moderated by water like a PWR, but at a lower pressure, which allows

the water to boil inside the pressure vessel producing the steam that runs the

turbines. Unlike a PWR, there is no primary and secondary loop. The thermal

efficiency of these reactors can be higher, and they can be simpler, and even

 potentially more stable and safe. This is a thermal neutron reactor design, the

newest of which are the Advanced Boiling Water Reactor  and the Economic

Simplified Boiling Water Reactor .

Pressurized Heavy Water Reactor (PHWR)

A Canadian design (known as CANDU), these reactors are heavy-water -cooled

and -moderated Pressurized-Water reactors. Instead of using a single large pressure

vessel as in a PWR, the fuel is contained in hundreds of pressure tubes. These

reactors are fueled with natural uraniumand are thermal neutron reactor designs.

PHWRs can be refueled while at full power, which makes them very efficient in

their use of uranium (it allows for precise flux control in the core). CANDU

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PHWRs have been built in Canada, Argentina, China, India, Pakistan, Romania, 

and South Korea. India also operates a number of PHWRs, often termed 'CANDU-

derivatives', built after the Government of Canada halted nuclear dealings with

India following the 1974 Smiling Buddha nuclear weapon test.

Reaktor Bolshoy Moschnosti Kanalniy (High Power Channel Reactor)

(RBMK )

A Soviet design, built to produce plutonium as well as power. RBMKs are water 

cooled with a graphite moderator. RBMKs are in some respects similar to CANDU

in that they are refuelable during power operation and employ a pressure tube

design instead of a PWR-style pressure vessel. However, unlike CANDU they are

very unstable and large, making containment buildings for them expensive. A

series of critical safety flaws have also been identified with the RBMK design,

though some of these were corrected following the Chernobyl disaster . Their main

attraction is their use of light water and un-enriched uranium. As of 2010, 11

remain open, mostly due to safety improvements and help from international safety

agencies such as the DOE. Despite these safety improvements, RBMK reactors are

still considered one of the most dangerous reactor designs in use. RBMK reactors

were deployed only in the former Soviet Union.

These are generally graphite moderated and CO2 cooled. They can have a highthermal efficiency compared with PWRs due to higher operating temperatures.

There are a number of operating reactors of this design, mostly in the United

Kingdom, where the concept was developed. Older designs (i.e. Magnox stations)

are either shut down or will be in the near future. However, the AGCRs have an

anticipated life of a further 10 to 20 years. This is a thermal neutron reactor design.

Decommissioning costs can be high due to large volume of reactor core.

Define briefly about the Liquid Metal Fast Breeder Reactor (LMFBR)

This is a reactor design that is cooled by liquid metal, totally unmoderated, and

 produces more fuel than it consumes. They are said to "breed" fuel, because they

 produce fissionable fuel during operation because of neutron capture. These

reactors can function much like a PWR in terms of efficiency, and do not require

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much high pressure containment, as the liquid metal does not need to be kept at

high pressure, even at very high temperatures. BN-350 and BN-600 in USSR 

and Superphénix in France were a reactor of this type, as was Fermi-I in the United

States. TheMonju reactor in Japan suffered a sodium leak in 1995 and

was restarted in May 2010. All of them use/used liquid sodium. These reactors

arefast neutron, not thermal neutron designs. These reactors come in two types:

Lead cooled

Using lead as the liquid metal provides excellent radiation shielding, and allows

for operation at very high temperatures. Also, lead is (mostly) transparent to

neutrons, so fewer neutrons are lost in the coolant, and the coolant does not

 become radioactive. Unlike sodium, lead is mostly inert, so there is less risk of 

explosion or accident, but such large quantities of lead may be problematic from

toxicology and disposal points of view. Often a reactor of this type would use

a lead-bismuth eutectic mixture. In this case, the bismuth would present some

minor radiation problems, as it is not quite as transparent to neutrons, and can be

transmuted to a radioactive isotope more readily than lead. The Russian Alfa class

submarine uses a lead-bismuth-cooled fast reactor as its main power plant.

Sodium cooled

Most LMFBRs are of this type. The sodium is relatively easy to obtain and work 

with, and it also manages to actually prevent corrosion on the various reactor parts

immersed in it. However, sodium explodes violently when exposed to water, so

care must be taken, but such explosions would not be vastly more violent than (for 

example) a leak of superheated fluid from a SCWR or PWR. EBR-I, the first

reactor to have a core meltdown, was of this type.

Pebble Bed Reactors (PBR)

These use fuel molded into ceramic balls, and then circulate gas through the balls.

The result is an efficient, low-maintenance, very safe reactor with inexpensive,

standardized fuel. The prototype was the AVR .

Molten Salt Reactors

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These dissolve the fuels in fluoride salts, or use fluoride salts for coolant. These

have many safety features, high efficiency and a high power density suitable for 

vehicles. Notably, they have no high pressures or flammable components in the

core. The prototype was the MSRE, which also used Thorium's fuel cycle to

 produce 0.1% of the radioactive waste of standard reactors.

Aqueous Homogeneous Reactor (AHR)

These reactors use soluble nuclear salts dissolved in water and mixed with a

coolant and a neutron moderator .

List out the generation Four reactors?

Gas cooled fast reactor 

Lead cooled fast reactor 

Molten salt reactor 

Sodium-cooled fast reactor 

Supercritical water reactor 

Very high temperature reactor 

Describe briefly about pressurized water reactor

There are two major systems utilized to convert the heat generated in the fuel into

electrical power for industrial and residential use. The primary system transfers the

heat from the fuel to the steam generator,where the secondary system begins. The

steam formed in the steam generator is transferred by the secondary system to the

main turbine generator, where it is converted into electricity. After passing through

the low pressure turbine, the steam is routed to the main condenser. Cool water,

flowing through the tubes in the condenser, removes excess heat from the steam,

which allows the steam to condense. The water is then pumped back to the steam

generator for reuse.

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The primary system (also called the Reactor Coolant System) consists of the reactor 

vessel, the steam generators, the reactor coolant pumps, a pressurizer, and the connecting piping. A reactor coolant loop is a reactor coolant pump, a steam generator, and the piping

that connects these components to the reactor vessel. The primary function of the reactor 

coolant system is to transfer the heat from the fuel to the steam generators. A second

function is to contain any fission products that escape the fuel.The following drawings

show the layout of the reactor coolant systems for three pressurized water reactor vendors.

All of the systems consist of the same major components, but they are arranged in

slightly different ways. For example, Westinghouse has built plant with two, three, or four 

loops,

depending upon the power output of the plant. The Combustion Engineering plants and

the Babcock & Wilcox plants only have two steam generators, but they have four reactor 

coolant pumps.

In a typical commercial pressurized light-water reactor(1) the core inside the reactor vessel

creates heat, (2) pressurized water in the primary coolant loop carries the heat to the steam

generator, (3) inside the steam generator, heat from the steam, and (4) the steam line

directs the steam to the main turbine, causing it to turn the turbine generator, which

 produces electricity. The unused steam is exhausted in to the condenser where it

condensed into water. The resulting water is pumped out of the condenser with a series of 

 pumps, reheated and pumped back to the steam generators. The reactor's core contains fuel

assemblies that are cooled by water circulated using electrically powered pumps. These

 pumps and other operating systems in the plant receive their power from the electrical

grid. If offsite power is lost emergency cooling water is supplied by other pumps, which

can be powered by onsite diesel generators. Other safety systems, such as the containment

cooling system, also need power. Pressurized-water reactors contain between 150-200 fuel

assemblies

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PWR is the abbreviation for the Pressurized Water Reactor. These reactors were originally

designed by Westinghouse Bettis Atomic Power Laboratory for military ship applications,

then by the Westinghouse Nuclear Power Division for commercial applications. The first

commercial PWR plant in the United States was Shippingport, which operated for 

Duquesne Light until 1982.

In addition to Westinghouse, Asea Brown Boveri-Combustion Engineering (ABB-CE),

Framatome, Kraftwerk Union, Siemens, and Mitsubishi have typically built this type of 

reactor throughout the world. Babcock & Wilcox (B&W) built a PWR design power plant

 but used vertical once-through steam generators, rather than the U-tube design used by the

rest of the suppliers. Industry consolidation has occurred so that Framatome-

ANP and Westinghouse are two key remaining manufacturers. Refuelings are done withthe plant shutdown.

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The Pressurized Water Reactor (PWR) has 3 separate cooling systems. Only 1 is expected

to have radioactivity - the Reactor Coolant System.

The Reactor Coolant System, shown inside the Containment, consists of 2, 3, or 4

Cooling "Loops" connected to the Reactor, each containing a Reactor Coolant Pump,

and Steam Generator. The Reactor heats the water that passes upward past the fuel

assemblies from a temperature of about 530F to a temperature of about 590F. Boiling,

other than minor bubbles called nucleate boiling, is not allowed to occur. Pressure is

maintained by a Pressurizer (not shown) connected to the Reactor Coolant System.

Pressure is maintained at approximately 2250 pounds per square inch through a heater and

spray system in the pressurizer. The water from the Reactor is pumped to the steam

generator and passes through tubes. The Reactor Cooling System is expected to be theonly one with radioactive materials in it. Typically PWRs have 2, 3, or 4 reactor cooling

system loops inside the containment.

In a Secondary Cooling System (which include the Main Steam

System and the Condensate-Feedwater Systems), cooler water is pumped from the

Feedwater System and passes on the outside of those steam generator tubes, is heated and

converted to steam. The steam then passes through the a Main Steam Line to

the Turbine, which is connected to and turns the Generator. The steam from the Turbine

condenses in aCondenser. The condensed water is then pumped by Condensate

Pumps through Low Pressure Feedwater Heaters, then to the Feedwater Pumps, then to

High Pressure Feedwater Heaters, then to the Steam Generators.The diagram above

simplifies the process by only showing the condenser, a pump, and the steam generator.

The condenser is maintained at a vacuum using either vacuum pumps or air ejectors.

Cooling of the steam is provided by Condenser Cooling Water pumped through the

condenser by Circulating Water Pumps, which take a suction from water supplied from

the ocean, sea, lake, river, or Cooling Tower (shown). A discussion of cooling towers is

 provided in the photo section.

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What is Boiling water reactor?

In a typical commercial boiling-water reactor, (1) the core inside the reactor vessel creates

heat, (2) a steam-water mixture is produced when very pure water (reactor coolant) moves

upward through the core, absorbing heat, (3) the steam-water mixture leaves the top of the

core and enters the two stages of moisture separation where water droplets are removed

 before the steam is allowed to enter the steam line, and (4) the steam line directs the steam

to the main turbine, causing it to turn the turbine generator, which produces electricity.

The unused steam is exhausted into the condenser where it is condensed into water. The

resulting water is pumped out of the condenser with a series of pumps, reheated and

 pumped back to the reactor vessel. The reactor's core contains fuel assemblies that are

cooled by water circulated using electrically powered pumps. These pumps and other 

operating systems in the plant receive their power from the electrical grid. If offsite power 

is lost emergency cooling water is supplied by other pumps, which can be powered by

onsite diesel generators. Other safety systems, such as the containment cooling system,

also need electric power. Boiling-water reactors contain between 370-800 fuel assemblies.

The BWR reactor typically allows bulk boiling of the water in the reactor. The operating

temperature of the reactor is approximately 570F producing steam at a pressure of about

1000 pounds per square inch. Current BWR reactors have electrical outputs of 570 to 1300

MWe. As this the PWR designs, the units are about 33% efficient.

In the figure above, water is circulated through the Reactor Core picking up heat as the

water moves past the fuel assemblies. The water eventually is heated enough to convert to

steam. Steam separators in the upper part of the reactor remove water from the steam.

The steam then passes through the Main Steam Lines to the Turbine-Generators. The

steam typically goes first to a smaller High Pressure (HP) Turbine, then passes

to Moisture Separators (not shown), then to the 2 or 3 larger Low Pressure (LP)

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Turbines. In the sketch above there are 3 low pressure turbines, as is common for 1000

MWe plant. The turbines are connected to each other and to the Generator by a long shaft

(not one piece).

The Generator produces the electricity, typically at about 20,000 volts AC. This electrical

 power is then distributed to a Generator Transformer, which steps up the voltage to

either 230,000 or 345,000 volts. Then the power is distributed to a switchyard or 

substation where the power is then sent offsite.

The steam, after passing through the turbines, then condenses in the Condenser, which is

at a vacuum and is cooled by ocean, sea, lake, or river water. The condensed steam then is

 pumped to Low Pressure Feedwater Heaters (shown but not identified). The water then

 passes to the Feedwater Pumps which in turn, pump the water to the reactor and start the

cycle all over again.

The BWR is unique in that the Control Rods, used to shutdown the reactor and maintain

an uniform power distribution across the reactor, are inserted from the bottom by a high

 pressure hydraulically operated system. The BWR also has a Torus (shown above) or 

a Suppression Pool. The torus or suppression pool is used to remove heat released if an

event occurs in which large quantities of steam are released from the reactor or theReactor Recirculation System, used to circulate water through the reactor.

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Give a detailed study about the gas cooled reactors?

The Gas Cooled Reactor was one of the original designs. In the Gas Cooled Reactor 

(GCR), the moderator is graphite. Inert gas, e.g. helium or carbon dioxide, is used as the

coolant. The advantage of the design is that the coolant can be heated to higher temperatures than water. As a result, higher plant efficiency (40% or more) could be

obtained compared to the water cooled design (33-34%).

In the United States, Gulf General Atomics was the proponent of this design. Public

Service of Colorado (now Xcel Energy) built the Fort Saint Vrain facility north of 

Denver. The NRC has also written NUREG/CR-6839,Fort Saint Vrain Gas Cooled

Reactor Operational Experience that provides a history of the operation of the facility.

Currently, there is little movement toward the gas cooled design in the US or elsewhere.

In the United Kingdom, the government was the proponent that developed, constructed,

and operated a number of gas cooled reactors. The older design used carbon dioxide gas

circulating through the core at a pressure of ~1.6 MP a or 230 pounds per square inch to

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remove the heat from the fuel elements. The fuel consists of natural uranium metal clad

with an alloy of magnesium known as Magnox (thus the name for the reactor type).

The newer Advanced Gas Cooled (AGR) Reactors use a slightly enriched uranium dioxide

clad with stainless steel. Carbon dioxide is the coolant gas used.

Two key advantages of this design are:

• higher operating temperature with a higher thermal efficiency

• not susceptible to accidents of the type possible with water cooled/moderated

reactors.

The Gas Cooled Reactor or Advanced Gas Reactor cycle is illustrated in the simple sketch below:

Define reactor cooling systems in pwr?

Functions

The PWR Reactor Coolant System has three (3) major functions:

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• Transfer the heat from the reactor for the steam generator 

• Maintain the pressure within acceptable limits

• Maintain the pressure boundary

 Major Components and Simplified Sketch

The Reactor Coolant System consists of the following major components other than the

reactor:

• Hot Leg Piping between the Reactor and the Steam Generator 

• Steam Generator 

• Piping between the Steam Generator and the Reactor Coolant Pump

• Reactor Coolant Pump

• Piping between the Reactor Coolant Pump and the Reactor 

• Piping between the Hot Leg and the Pressurizer (Surge Line)

• Pressurizer 

• Piping between the Cold Leg Piping of both Reactor Coolant Loops and the

Pressurizer (not shown)

• Relief line Piping on top of the Pressurizer (not shown).

The sketch below illustrates the Reactor Coolant System's major components- Reactor 

Vessel, Steam Generators, (Reactor) Coolant Pump, and Pressurizer. Click to see a

51K detailed drawing of the RCS with parts identified.

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The diagram below illustrates the flow paths involved and relative positions of the

Reactor, Steam Generator, and Reactor Coolant Pump (not identified). This diagram also

nicely illustrates the inside of the steam generator with the U tubes, divider plate betweenthe "hot" and "cold" parts of the bottom head of the steam generator.

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Functions of Major Components

The vertical Steam Generator as shown has over 3000 tubes. The tubes are in an upside

down U shape. Reactor coolant from the hot leg comes in the bottom, passes through the

inside of the tubes. The reactor coolant water cools from about 590-600F to 520-530F.

The heat passes through the tubes to the "secondary side" where water supplied from thefeedwater system is heated from about 425F to about 510F. This water is converted to

steam. In the upper part of the steam generator are moisture-separators which remove the

water in the steam and divert it back into the lower part of the steam generator. Click the

figure to see a 43K detailed drawing of the Steam generator with parts identified.

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The Pressurizer in a Pressurized Water Reactor design maintains the pressure through

heaters mounted in the bottom and a spray nozzle mounted in the upper part of the vessel.

Also included are safety valves which lift if pressure is too high and automatic relief 

valves (also called Power Operated Relief Valves) designed to lift before the safety valves

lift. Pressure is normally maintained in a range of 2200 to 2250 pound per square inch.

This corresponds to a temperature of about 650F. Click the figure to see a 26K detailed

drawing of the Pressurizer with parts identified

The Reactor Coolant Pump circulates the water through the Reactor Coolant System.

These pumps generally pump at a rate of almost 100,000 gallons per minute. Click the

figure to see a 51K detailed drawing of the Reactor Coolant Pump with parts identified.

The Reactor is intended to produce heat in all reactor designs. Water usually is supplied

to the reactor at about mid-height, then flows downward, then turns at the bottom where it

is directed upwards, passing past the fuel assemblies, removing heat as the water passes

 by.

What is Fast breeder reactors?

FBR power stations have been liquid metal fast breeder reactors (LMFBR) cooled by

liquid sodium. These have been of one of two designs.

•  Loop type, in which the primary coolant is circulated through primary heat

exchangers outside the reactor tank (but inside the biological shield due to radioactive

sodium-24 in the primary coolant)

•  Pool type, in which the primary heat exchangers and pumps are immersed in the

reactor tank 

All current fast neutron reactor designs use liquid metal as the primary coolant, to transfer 

heat from the core to steam used to power the electricity generating turbines. FBRs have

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other experimental reactors have used a sodium- potassium alloy called NaK . Both have

the advantage that they are liquids at room temperature, which is convenient for 

experimental rigs but less important for pilot or full scale power stations.Lead and lead-

 bismuth alloy have also been used. The relative merits of lead vs sodium are

discussed here. Looking further ahead, three of the proposed generation IV reactor  types

are FBRs:[30]

• Gas-Cooled Fast Reactor (GFR) cooled by helium.

• Sodium-Cooled Fast Reactor (SFR) based on the existing Liquid Metal FBR 

(LMFBR ) and Integral Fast Reactor designs.

• Lead-Cooled Fast Reactor (LFR) based on Soviet naval propulsion units.

• FBRs usually use a mixed oxide fuel core of up to 20% plutonium dioxide (PuO2)

and at least 80% uranium dioxide (UO2). Another fuel option is metal alloys,

typically a blend of uranium, plutonium, and zirconium (used because it is

"transparent" to neutrons). Enriched uranium can also be used on its own.

• In many designs, the core is surrounded in a blanket of tubes containing non-fissile

uranium-238 which, by capturing fast neutrons from the reaction in the core, is

converted to fissile plutonium-239 (as is some of the uranium in the core), which isthen reprocessed and used as nuclear fuel. Other FBR designs rely on the geometry

of the fuel itself (which also contains uranium-238), arranged to attain sufficient

fast neutron capture. The plutonium-239 (or the fissile uranium-235) fission cross-

section is much smaller in a fast spectrum than in a thermal spectrum, as is the

ratio between the 239Pu/235U fission cross-section and the 238U absorption cross-

section. This increases the concentration of 239Pu/235U needed to sustain a chain

reaction, as well as the ratio of breeding to fission.[10]

• On the other hand, a fast reactor needs no moderator to slow down the neutrons at

all, taking advantage of the fast neutrons producing a greater number of neutrons

 per fission than slow neutrons. For this reason ordinary liquid water , being a

moderator as well as a neutron absorber, is an undesirable primary coolant for fast

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reactors. Because large amounts of water in the core are required to cool the

reactor, the yield of neutrons and therefore breeding of  239Pu are strongly affected.

Theoretical work has been done on reduced moderation water reactors, which may

have a sufficiently fast spectrum to provide a breeding ratio slightly over 1. This

would likely result in an unacceptable power derating and high costs in an liquid-

water-cooled reactor, but the supercritical water coolant of the SCWR has

sufficient heat capacity to allow adequate cooling with less water, making a fast-

spectrum water-cooled reactor a practical possibility

•

FBRs usually use a mixed oxide fuel core of up to 20%  plutonium dioxide (PuO2) and at

least 80% uranium dioxide (UO2). Another fuel option is metal alloys, typically a blend of 

uranium, plutonium, and zirconium (used because it is "transparent" to

neutrons). Enriched uranium can also be used on its own.

In many designs, the core is surrounded in a blanket of tubes containing non-fissile

uranium-238 which, by capturing fast neutrons from the reaction in the core, is converted

to fissile plutonium-239 (as is some of the uranium in the core), which is then reprocessed

and used as nuclear fuel. Other FBR designs rely on the geometry of the fuel itself (which

also contains uranium-238), arranged to attain sufficient fast neutron capture. The

 plutonium-239 (or the fissile uranium-235) fission cross-section is much smaller in a fast

spectrum than in a thermal spectrum, as is the ratio between the 239Pu/235U fission cross-

section and the 238U absorption cross-section. This increases the concentration of 239Pu/235U

needed to sustain a chain reaction, as well as the ratio of breeding to fission.[10]

On the other hand, a fast reactor needs no moderator to slow down the neutrons at all,

taking advantage of the fast neutrons producing a greater number of neutrons per fission

than slow neutrons. For this reason ordinary liquid water , being a moderator as well asa neutron absorber , is an undesirable primary coolant for fast reactors. Because large

amounts of water in the core are required to cool the reactor, the yield of neutrons and

therefore breeding of 239Pu are strongly affected. Theoretical work has been done

on reduced moderation water reactors, which may have a sufficiently fast spectrum to

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 provide a breeding ratio slightly over 1. This would likely result in an unacceptable power 

derating and high costs in an liquid-water-cooled reactor, but the supercritical water 

coolant of the SCWR has sufficient heat capacity to allow adequate cooling with less

water, making a fast-spectrum water-cooled reactor a practical possibility.

Integral Fast Reactor[edit]

One design of fast neutron reactor, specifically designed to address the waste disposal and

 plutonium issues, was the Integral Fast Reactor  (also known as an Integral Fast Breeder 

 Reactor , although the original reactor was designed to not breed a net surplus of fissile

material).[31][32]

To solve the waste disposal problem, the IFR had an on-site electrowinning fuel

reprocessing unit that recycled the uranium and all the transuranics (not just plutonium)via electroplating, leaving just short half-life fission products in the waste. Some of these

fission products could later be separated for industrial or medical uses and the rest sent to

a waste repository (where they would not have to be stored for anywhere near as long as

wastes containing long half-life transuranics). The IFR pyroprocessing system uses

molten cadmium cathodes and electrorefiners to reprocess metallic fuel directly on-site at

the reactor.[33] Such systems not only commingle all the minor actinides with both uranium

and plutonium, they are compact and self-contained, so that no plutonium-containing

material ever needs to be transported away from the site of the breeder reactor. Breeder 

reactors incorporating such technology would most likely be designed with breeding ratios

very close to 1.00, so that after an initial loading of enriched uranium and/or plutonium

fuel, the reactor would then be refueled only with small deliveries of natural uranium

metal. A quantity of natural uranium metal equivalent to a block about the size of a milk 

crate delivered once per month would be all the fuel such a 1 gigawatt reactor would need.

[34] Such self-contained breeders are currently envisioned as the final self-contained and

self-supporting ultimate goal of nuclear reactor designers.[3][10] The project was canceled in

1994, at the behest of then-United States Secretary of Energy Hazel O'Leary.[35][36]

Other fast reactors[edit]

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Another proposed fast reactor is a fast Molten Salt Reactor, one in which the molten salt's

moderating properties are insignificant. This is typically achieved by replacing the light

metal fluorides (e.g. LiF, BeF2) in the salt carrier with heavier metal chlorides (e.g., KCl,

RbCl, ZrCl4).

Several prototype FBRs have been built, ranging in electrical output from a few light

 bulbs' equivalent (EBR-I, 1951) to over 1000 MWe. As of 2006, the technology is not

economically competitive to thermal reactor technology—but India, Japan, China, South

Korea and Russia are all committing substantial research funds to further development of 

Fast Breeder reactors, anticipating that rising uranium prices will change this in the long

term. Germany, in contrast, abandoned the technology due to safety concerns. The SNR-

300 fast breeder reactor was finished after 19 years despite cost overruns summing up to a

total of 3.6 billion Euros, only to then be abandoned.[37]

As well as their thermal breeder program, India is also developing FBR technology, using

 both uranium and thorium feedstocks

Define Neutron Population growth?

The multiplication of neutrons in a reactor can be described by the effective multiplication

factor k , as discussed in Chapter 11. The introduction of one neutron produces k neutrons;

they in turn produce k 2, and so on. Such a behavior tends to be analogous to the increase in

 principal with compound interest or the exponential growth of the human population. The

fact that k can be less than, equal to, or greater than 1 results in significant differences,

however.

The total number of reactor neutrons is the sum of the geometric series 1 + k + k 2 + ….

For k < 1 this is finite, equal to 1/(1 - k ). For k > 1 the sum is infinite (i.e., neutrons

multiply indefinitely). We thus see that knowledge of the effective multiplication factor of 

any arrangement of fuel and other material is needed to assure safety. Accidental

criticality is prevented in a number of situations: (a) chemical processing of enriched

uranium or plutonium, (b) storage of fuel in arrays of containers or of fuel assemblies, and

(c) initial loading of fuel assemblies at time of startup of a reactor. A classic measurement

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involves the stepwise addition of small amounts of fuel with a neutron source present. The

thermal neutron flux without fuel f 0and with fuel f is measured at each stage. Ideally, for a

subcritical system with a nonfission source of neutrons in place, in a steady-state

condition, the multiplication factor k appears in the relation As k gets closer to 1, the

critical condition, the flux increases greatly. On the other hand, the reciprocal ratio

goes to zero as k goes to 1. Plotting the measured flux ratio as it depends on the mass of 

uranium or the number of fuel assemblies allows increasingly accurate predictions of the

 point at which criticality occurs, . Fuel additions are always intended to be less than the

amount expected to bring the system to criticality.

What is emergency core cooling systems(ECCS)?

Functions

All nuclear power plants have some form of emergency makeup water system in the event

that normal makeup is lost and a major break occurs in the reactor cooling system. These

emergency systems are called such names as - High Pressure Coolant or Safety Injection,

Low Pressure Coolant or Safety Injection, Reactor Core Injection Cooling.

There are two phases considered - (1) the Injection phase when the pumps take a suction

from a large tank and pump that water into the reactor cooling system or reactor, and (2)

the Recirculation phase when the pumps take a suction from the containment sump after 

all of the water has been pumped into the containment.

The Emergency Core Cooling Systems have 1 major function:

• Provide makeup water to cool the reactor in the event of a loss of coolant from the

reactor cooling system. This cooling is needed to remove the decay heat still in the

reactor's fuel after the reactor is shutdown.

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• Provide chemicals to the reactor and reactor cooling system to ensure the reactor 

does not produce power.

 Major Components

Water supplies (tanks), pumps, interconnecting piping

• High Pressure Pumps - pump lots of water (e.g. 700 gpm) at a pressure of about

2,000 pound per square inch.

• Low Pressure Pumps - pump lots of water (e.g. 2000 gpm) at a pressure of about

150 pound per square inch.

• Storage Tanks - the emergency makeup water has to come from somewhere;

usually about 250,000 - 300,000 gallons; can be refilled once they are empty.

These tanks may be referred to as Refueling Water Storage Tanks.

• Accumulators - used in PWRs; big storage tanks connected to the reactor cooling

system that have water pressurized with nitrogen (e.g. 1000 gallons at 750 psi)

• Containment Sump - used to keep recirculating the water through the reactor once

the storage tanks are empty.

Containment sump at a PWR. Note the grillwork sized to keep loose materials from

getting to the suction of the low pressure ECCS pumps during the recovery phase

following the postulated loss of coolant accident.

 Power Sources

Emergency makeup or cooling pumps are usually motor-driven. In some cases, steam

turbine-driven pumps are used (e.g. in the case of BWR systems HPCI (High Pressure

Coolant Injection). For the motor-driven pumps, power may be received from diesel

generators if power is lost from the normal power supply.

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