Nuclear Power
Nuclear energy is a form of energy that is obtained when atoms larger than iron are split, or atoms smaller than iron are joined, thereby causing conversion of mass to energy. Upon the occurrence of either of these events, the atoms lose some mass. The formula E = MC2 where M represents the mass and C represents the speed of light is useful in the calculation of that mass. The Manhattan Project successfully made nuclear bombs using this reaction in the 1940s, and the energy found a new use in powering of navy seal engines in the Second World War. Submarines running on nuclear energy could run for years without refuelling. To date nuclear technology has a wide range of commercial uses. Today the nuclear reactors in the world are over 400 with at least 20% of electricity in the USA being produced from this energy. Apart from the radioactive waste that nuclear energy produces, it does not pollute the environment in any way.
The splitting of a large atom into to smaller ones is referred to as fission. This process releases a significant amount of energy and results to neutron release. The neutrons released are useful in enabling the chain reaction that produces more energy. Atoms commonly used are Uranium and Plutonium. This reaction is used in commercial nuclear plants to generate electricity.
The merging of two small atoms into one is referred to as fusion. This reaction does not result to any radioactive by products and produces more energy than fission. Atoms commonly used are hydrogen and helium. Fusion is the reaction that keeps the sun burning, involving fusion of hydrogen and giving out helium as waste. Fusion is still a concept under development and does not have commercial applications.
Fission and fusion reactions take place in a nuclear reactor, which is a system designed to contain and control them. Reactors are found in electricity generation plants, aircraft carriers, submarines, and research centres. They also assist in imaging and cancer treatment as medical isotopes are produced in them.
Fuel that is composed of heavy radioactive atoms is put in a reactor vessel, and a small amount of neutrons is fed to it from a source to initiate the reaction. When the reaction commences, the atoms release more neutrons that cause more atoms to split. The process goes on and on, forming a chain reaction. Fission releases large amounts of heat energy, which is carried by the reactor to the coolant chamber where plain water is used to cool it
The main components of the reactor include:
• The core contains low enriched uranium, control systems, and structural materials. It produces all the energy.
• The coolant could be water, liquid sodium or another substance. It transfers the heat from the core to the turbine.
• The turbine generates electric power from the heat energy.
• The containment is a structure that sets apart the structure and encloses it in order to protect the external environment from nuclear reactor by products.
• Cooling towers are disposal points for the excess heat that cannot be converted into energy. They are visible from outside the containment and emit clean water vapour.
The core
The nuclear core contains thousands of fuel pins. The fuel pin is the smallest unit of the nuclear core. Although the fuel pins can take other forms, they are typically uranium oxide (UO2). A metal cube surrounds them and prevents fission products from escaping. This is referred to as cladding. A fuel assembly constitutes a bunch of fuel pins. The fact that they don’t touch serves to create some space for coolant. It is using assemblies that fuel is put into and out of the reactor. A collection of hundreds of assemblies forms a full core. They differ in nature and, therefore, have different fuels in them. The variations may be in parameters such as age and the top of the core could have different enrichments from the bottom.
It is important to understand the process of nuclear energy generation in order to fully appreciate its worth and the potential it represents. Generation of nuclear power depends heavily on the chain reaction that occurs when fission is initiated. Fission begins when a target atom absorbs an incident neutron. The atom becomes unstable and splits into two small atoms thereby releasing more neutrons. These neutrons are absorbed by other atoms thereby destabilising the atoms. Each fission of an atom releases a tremendous amount of heat. The process that sustains the nuclear reaction is, however, more complicated than has been described.
Upon splitting, an atom releases a tremendous amount of energy. A fission reaction also releases fast moving neutrons with so much energy that they travel at speeds of over 14000 km/s. A neutron moving at this speed could cause three occurrences. It is probable that the neutron will leak out of the reactor and disappear. The probability that the neutron will not leak out is referred to as the fast non-leakage probability. The neutron could also cause its own fusion process. The probability of fast fission is the fast fission factor. This option, however, is unlikely.
If the neutron does not result to either of those outcomes discussed above, it is slowed down using the moderator, which is a substance that seeps into the reactor and tends to scatter incident electrons. In most circumstances, water is used as a moderator. The neutron slows down to about 2200 m/s through a process referred to as thermalisation. The neutron bombards the moderator atoms, transferring energy to them, therefore, making them fast. The temperature of the atoms also rises, which leads to the conversion of water into steam. This steam rotates the turbine facilitating electricity generation.
The neutron slows down into the resonance region, a state in which absorption probability fluctuates highly. The neutron has to go through this region successfully into the thermo stage. The probability that the neutron will do this is known as the resonance escape probability. Thermo neutrons are those that have attained room temperature speeds. An atom may absorb thermo neutrons, or they may escape out of the reactor. Thermo non-leakage probability is the name given to the probability that the neutron will not escape. A neutron that leaks out has been lost for good and is no longer of benefit to the reaction. Any other atom that is not of the fuel such as the moderator’s may also absorb the neutron. In that case, it is also lost and is not useful in the reaction. If a fuel atom absorbs the neutron, the fusion cycle is now complete and the fuel atom splits producing new neutrons.
Types of Nuclear Reactors
Many nuclear reactors have been designed, tested and some sold commercially. The discussion below revolves around these types and prototypes and their operation.
a) Pressurised water reactors (PWRs) are commonly used in western nuclear power plants. They use water as the primary coolant, which is pumped to the core of the reactor where it is heated by the heat energy generated from atom fusion. The hot water flows to a steam generator and transfers its energy to a secondary steam generating system. The steam turns turbines, which generate electricity. In PWRs, the primary coolant loop ensures the water does not boil while still in the reactor. PWRs use light water as coolant and moderator. PWRs were originally designed to power submarines but are now used by countries such as France to generate most of its electricity.
PWRs have various advantages over other types of reactors. First, the coolant is also the moderator and the reactor cools down if the water starts boiling. The reactor has a strong negative void coefficient. Maintenance is easy as the secondary loop helps keep the reacting elements from the turbines. They are easily operated and their familiarity to many people hassled to their legalization. The reactor is not devoid of a few shortcomings. It necessitates the construction of many back up cooling systems as pressurized steam escapes quickly in case a pipe leaks. These reactors are also prone to uranium shortage due to their inability to breed new fuel.
b) Heavy Water Reactor
The CANDU reactor is the only heavy water reactor in commercial use, designed in Canadaand exported to a few countries. Enriched uranium dioxide in zirconium alloy cans is loaded into horizontal zirconium alloy tubes. The steam is cooled by heavy water at high pressure, which subsequently produces steam at the steam generator. Additional moderation is necessary in this reactor. To cater for this, immersion of zirconium alloys into a callandria (unpressurised container) containing more water is important. Inserting or withdrawing cadmium rods from the callandria assists in maintaining control. A concrete shield and containment vessel enclose the whole assembly.
These reactors have their advantages. The main one is that they do not really require much uranium enrichment. The capacity factors also remain high since the reactors can be refuelled while operating. The reactors use any type of fuel and thus are very flexible. Their main disadvantage is that they are a safety concern due to positive coolant temperature coefficients found in some variants. Tritium, which is a by-product of neutron combination with deuterium, also leaks in small quantities and poses a health risk because of its radioactivity. This reactor can also be used to produce weapons grade plutonium after a little modification.
c) Light Water Reactor
The RBMK (Reaktor Bolszoj Moszcznosti Kanalnyj) is a representative of the LWGR (Light Water Graphite Reactor). The coolant is light water, the moderator is graphite, and boiling water is produced directly from the reactor. It is a pressure tube with a single circuit reactor that generates steam and separates it in steam drums. The reactor should be shielded as the radioactive compounds contaminate the water produced. RBMK reactors are composed of graphite blocks whose great mass makes the core heavy and the power density low at 5.8 MW/m3. Outlet steam parameters are average, 280 ºC and 6,38 MPa.
The main advantage of RBMK reactors is low fuel enrichment level. It is also possible to replace the fuel tubes while the reactor is still operating. RBMK reactors are, however, very hazardous as they have a positive reactivity factor and graphite is usually at very high temperatures.
RBMK was initially perceived to be highly secure as it has low power density. However, security system lapse and design mistakes caused a disaster that invalidated that fact. The danger it poses and the fact that it can also produce plutonium for weapons production made ZSRR not sell it outside the Soviet Union.
d) Gas Cooled, Graphite Moderated Reactor.
Magnox and AGR are two examples of this category of reactors. In the Magnox reactor, magnesium alloy encases the uranium fuel. Fuel rods in Magnox cans are put in vertical channels in a graphite core. To control the heat output, it is essential to regulate the fission process. To achieve this, control rods that absorb neutrons are inserted and withdrawn from the core in the vertical channels accordingly. Carbon dioxide gas is blown past the fuel cans in order to cool them. The hot gas is used to heat water to steam in a steam generator.
Reduction of capital cost objective necessitated the model to work at high temperatures in order to increase thermal efficiency and the power density. In order to achieve this, the Magnox casing was discarded and replaced by stainless steel. The fuel used also changed to uranium dioxide. Subsequently, the proportion of U235 in the fuel increased. The rector that resulted from the changes is referred to as the Advanced Gas-cooled Reactor (AGR), which still uses graphite as the moderator and is enclosed in a concrete casing that doubles as a pressure vessel and radiation shield.
The pros of this reactor include its ability to operate at high temperatures and can process heat for water desalinisation plants, hydrogen fuel production and other uses. The reactor also extensively protects the environment against radioactivity using the containment structures. However, the system requires many emergency backup cooling systems. Large amounts of gas are also required to cool the plant as gas is a poor coolant. The high temperatures also limit the number of materials that can maintain their structure in those conditions.
e) Fast-neutron Breeder Reactor (FBR)
This is the only reactor that does not need a moderator. The reaction chain sustains itself because the fissile material is at a high concentration (20%). A U238 also surrounds the core of the fissile material and produces fissile material, upon absorption of some neutrons, to be used for more nuclear fission. A liquid metal conducts the heat to the heat exchanger that heats water to steam that drives turbines for electricity generation. FBRs are found in France, Japan and Russia.
The merits of this design start by the fact that it can breed its own fuel, ruling out the possibility of uranium shortage. The reactor is also environmentally friendly as it can burn its own waste. Using the ordinary physics concepts, the system can also shut down and cool itself down due to the thermal properties of metal, especially sodium. This enhances its safety. The reactor also has certain demerits. Sodium is very reactive with water and air raising the possibility of sodium fire occurrences. Nuclear proliferation is a risk associated with FBRs especially during burning of their wastes. The reactor also has the capability of making plutonium for weapon production. The reactor’s positive void coefficients are also a safety concern.
f) Boiling Water Reactors (BWR)
In these reactors, water is the reactor, coolant and moderator. It is directly boiled in the core, and the steam turns the turbine generator. There are two BWR categories; ABWRs and BWRs. GE originally developed BWRs. The first BWR was the Dresden Unit-1. Since then, many companies including Toshiba, Hitachi and Siemens have produced BWRs.
The boiling of the reactor coolant in BWRs produces a steam void that can contain a power rise even on addition of a power rise due to its negative-reactivity effect. Two methods, the reactor-coolant recirculation flow control and the control rod operation come in handy when trying to control the power of the reactor.
ABWR plants, on the other hand, are composed of the reactor-coolant recirculation system. The nuclear reactor is found in the main steam system. This reactor employs various safety processes such as the emergency core, reactor core isolation and containment cooling systems. There is also a myriad of other equipment such as the fuel handling and waste processing equipment and so forth.
The main advantage of this reactor is the reduction of costs by simple plumbings that constitute the structures. Load following is also easy since power levels are easily amplifiable by speeding up the jet pumps. The designs of the reactor have been optimised thereby resulting to accumulation of operating experience. The problems plaguing the reactor system include the difficulty in safety assessment and analysis difficulty due to the many transients possible from the gaseous and liquid water in the system. The system is also at risk of radioactive elements leak due to the close proximity of the primary coolant and the turbines. The system is also susceptible to uranium shortage due to its inability to breed new fuel.

 

 

 

 

 

Overview of Nuclear Energy
Nuclear energy comes from mass-to-energy conversions that occur in the splitting of atoms larger than iron or joining atoms smaller than iron. The small amount of mass that is lost in either of these events follows Einstein’s famous formula E = MC2, where M is the small amount of mass and C is the speed of light. In the 1930s and ’40s, humans discovered this energy and recognized its potential as a weapon. Technology developed in the Manhattan Project successfully used this energy in a chain reaction to create nuclear bombs. During the World War II , the newfound energy source found a home in the propulsion of the nuclear navy, providing submarines with engines that could run for over 20 years without refueling. This technology was quickly transferred to the public sector, where commercial power plants were developed and deployed.
Nuclear Energy Today
Nuclear reactors produce about 20% of the electricity in the USA. There are over 400 power reactors in the world (about 100 of these are in the USA). They produce base-load electricity 24/7 without emitting any pollutants into the atmosphere (this includes CO2). They do, however, create radioactive nuclear waste that must be stored carefully.
Fission and Fusion
• Fission is the splitting of a large atom such as Uranium or Plutonium into two smaller atoms, called fission products. Also released during such a fission are several neutrons (that enable a chain reaction) and substantial energy. This nuclear reaction was the first to be discovered. All commercial nuclear power plants use this reaction to generate electricity.
• Fusion is the combining of two small atoms such as Hydrogen or Helium to produce heavier atoms and energy. These reactions can release more energy than fission without producing radioactive byproducts. Fusion reactions occur in the sun, using Hydrogen as fuel and producing Helium as waste. This reaction has not been commercially developed and is a serious research interest worldwide, due to its promise of limitless, pollution-free, and non-proliferation features.
Energy density of various fuel sources
The amount of energy released in nuclear reactions is astounding. The natural uranium undergoes nuclear fission and thus attains very high energy density (energy stored in a unit of mass).
What is nuclear reactor?
A nuclear reactor is a system that contains and controls sustained nuclear chain reactions. Reactors are used for generating electricity, moving aircraft carriers and submarines, producing medical isotopes for imaging and cancer treatment, and for conducting research.
Fuel, made up of heavy atoms that split when they absorb neutrons, is placed into the reactor vessel (basically a large tank) along with a small neutron source. The neutrons start a chain reaction, it releases more neutrons that cause other atoms to split. Each fission releases large amounts of energy in the form of heat. The heat is carried out of the reactor by coolant, which is most commonly just plain water. The water cools the reactor and heat, boils

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Components of nuclear reactors
The control room

• Main components
• The core of the reactor contains all of the nuclear fuel and generates all of the heat. It contains low-enriched uranium (<5% U-235), control systems, and structural materials. The core can contain hundreds of thousands of individual fuel pins.
• The coolant is the material that passes through the core, transferring the heat from the fuel to a turbine. It could be water, heavy-water, liquid sodium, helium, or something else. In the US fleet of power reactors, water is the standard.
• The turbine transfers the heat from the coolant to electricity, just like in a fossil-fuel plant.
• The containment is the structure that separates the reactor from the environment. These are usually dome-shaped, made of high-density, steel-reinforced concrete. Chernobyl did not have a containment to speak of.
• Cooling towers are needed by some plants to dump the excess heat that cannot be converted to energy due to the laws of thermodynamics. These are the hyperbolic icons of nuclear energy. They emit only clean water vapor.
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Animated reactor system

This image (reproduced from the NRC) shows a nuclear reactor heating up water and spinning a generator to produce electricity. It captures the essence of the system well. The water coming into the condenser and then going right back out would be water from a river, lake, or ocean. It goes out the cooling towers. As you can see, this water does not go near the radioactivity, which is in the reactor vessel.
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The nuclear core
Fuel pins

The smallest unit of the reactor is the fuel pin. These are typically uranium-oxide (UO2), but can take on other forms, including thorium -bearing material. They are often surrounded by a metal tube (called the cladding) to keep fission products from escaping into the coolant.

Fuel assembly

Fuel assemblies are bundles of fuel pins. Fuel is put in and taken out of the reactor in assemblies. The assemblies have some structural material to keep the pins close but not touching, so that there’s room for coolant. Click here to see a 3-D blowup diagram of an assembly.
Full core

This is a full core, made up of several hundred assemblies. Some assemblies are control assemblies. Various fuel assemblies around the core have different fuel in them. They vary in enrichment and age, among other parameters. The assemblies may also vary with height, with different enrichments at the top of the core from those at the bottom
Neutron life cycle
All power-generating nuclear reactors rely on a chain of nuclear reactions to generate
heat, and therefore produce power. With recent events, the importance of understanding
nuclear power is in the spotlight. Often people ask how the reaction is sustained, or why it
cannot be stopped immediately. The answer requires understanding the underlying mechanism
of nuclear power, fission. Fission is the atomic reaction involving a target atom absorbing an
incident neutron. The new target atom becomes unstable, and splits into two different
elements, often releasing more neutrons that can be absorbed and a tremendous amount of
heat. However, not all neutrons share the same fate. How is this chain reaction sustained, even
as fuel is used? In fact, there are more steps to the neutron life cycle, and by the end of this
document, you will understand what it takes to keep nuclear reactors running.

Start with a fission reaction within the reactor. Fission reactions, as mentioned earlier,
occur as an atom absorbs a nearby neutron. The atom becomes unstable when this occurs,
often splitting into two semi-stable atoms. While the atom is splitting, it releases neutrons and
a tremendous amount of heat. For our model, let us say that fast neutrons are created by a
fission reaction. Fast neutrons contain a large amount of energy, and thus travel at an upwards
of 14,000 km/s (30 million mph). A fast neutron will now meet three fates. First, the neutron
could cause a fission reaction of its own in a process called fast fission. Although this is unlikely,
it is assigned a probability known as the fast fission factor, (given the symbol ). On the other
hand, this fast neutron could leak out of the reactor, never to be seen again. The probability of
this not occurring is known as the fast non-leakage probability (denoted). After all, once the
neutron escapes, it will not return to the reactor and is worthless to sustain the reaction. The
third possibility is important, and will be discussed in the next section

If the neutron does not create a fission reaction of its own, and does not escape, then
the moderator will slow the neutron. The moderator is a substance that permeates the reactor
(along with fuel) and tends to scatter incident neutrons, rather than absorbing them, like a
billiards ball striking another. The moderator, often water, slows the neutron down, usually to
about 2,200 m/s. The process of slowing down a neutron is known as thermalisation.
Thermalisation is a vital step for power generation. As the neutron bounces off moderator atoms,
it transfers its energy to them, making them move fast. This raises the temperature of
the moderator, allowing it to generate steam and drive a turbine. However, while this neutron
is slowing down, it has to survive the resonance region. The resonance region is a very chaotic
region where the probability of being absorbed fluctuates from very large values to very small
values. The probability that the neutron slows to thermal energies without being absorbed is ,
the resonance escape probability. The resonance region is circled in the graph of Figure 1. After
the resonance region, the neutron has a much lower energy than before. Once the neutron
reaches approximately room temperature speeds, it is known as thermal. Thermal neutrons
also have a variety of fates, which are discussed in the following sections. Figure 2, below,
summarizes the first two steps of the neutron life cycle.
Now that the neutron is of thermal energy, it may either leak out of the reactor, or be absorbed by an atom within the reactor. The probability that the neutron will not leak out is known as the thermal non-leakage probability (denoted ). Once again, if the neutron leaks out of the reactor, it will not come back and cannot sustain the reaction. Otherwise, the neutron will be absorbed. If the neutron is absorbed by something other than fuel, such as the moderator (or perhaps a structure supporting the reactor), it will not sustain the reaction, and is lost in our model. If the thermal neutron manages to survive the previous steps, the fuel will absorb it. Finally, the process comes full circle as the fuel will fission into two separate atoms, generating heat and birthing a new generation of neutrons. The probability that the neutron is absorbed by the fuel, and not the non-fuel, is , the thermal utilization factor. The entire life cycle of a neutron is summarized in the Figure below

NUCLEAR REACTOR TYPES
Many different reactor systems have been proposed and some of these have been developed to prototype and commercial scale. Six types of reactor (Magnox, AGR, PWR, BWR, CANDU and RBMK) have emerged as the designs used to produce commercial electricity around the world. A further reactor type, the so-called fast reactor, has been developed to full-scale demonstration stage. These various reactor types will now be described, together with current developments and some prototype designs.

Pressurized water reactors (PWRs) constitute the large majority of all Western nuclear power plants and are one of three types of light water reactor (LWR), the other types being boiling water reactors (BWRs) and supercritical water reactors(SCWRs). In a PWR, the primary coolant (water) is pumped under high pressure to the reactor core where it is heated by the energy generated by the fission of atoms. The heated water then flows to asteam generator where it transfers its thermal energy to a secondary system where steam is generated and flows to turbines which, in turn, spin an electric generator. In contrast to a boiling water reactor, pressure in the primary coolant loop prevents the water from boiling within the reactor. All LWRs use ordinary light water as both coolant and neutron moderator.
PWRs were originally designed to serve as nuclear propulsion for nuclear submarines and were used in the original design of the second commercial power plant at Shippingport Atomic Power Station.
PWRs currently operating in the United States are considered Generation II reactors. Russia’s VVERreactors are similar to U.S. PWRs. France operates many PWRs to generate the bulk of its electricity.

Pros:
• Strong negative void coefficient — reactor cools down if water starts bubbling because the coolant is the moderator, which is required to sustain the chain reaction
• Secondary loop keeps radioactive stuff away from turbines, making maintenance easy.
• Very much operating experience has been accumulated and the designs and procedures have been largely optimized.
Cons:
• Pressurized coolant escapes rapidly if a pipe breaks, necessitating lots of back-up cooling systems.
• Can’t breed new fuel — susceptible to “uranium shortage”.

Heavy water reactor
The only design of heavy water moderated reactor in commercial use is the CANDU, designed in Canada and subsequently exported to several countries. In the CANDU reactor, unenriched uranium dioxide is held in zirconium alloy cans loaded into horizontal zirconium alloy tubes. The fuel is cooled by pumping heavy water through the tubes (under high pressure to prevent boiling) and then to a steam generator to raise steam from ordinary water (also known as natural or light water) in the normal way. The necessary additional moderation is achieved by immersing the zirconium alloy tubes in an unpressurised container (called a callandria) containing more heavy water. Control is effected by inserting or withdrawing cadmium rods from the callandria. The whole assembly is contained inside the concrete shield and containment vessel.
Pros:
• Require very little uranium enrichment.
• Can be refueled while operating, keeping capacity factors high (as long as the fuel handling machines don’t break).
• Are very flexible, and can use any type of fuel.
Cons:
• Some variants have positive coolant temperature coefficients, leading to safety concerns.
• Neutron absorption in deuterium leads to tritium production, which is radioactive and often leaks in small quantities.
• Can theoretically be modified to produce weapons-grade plutonium slightly faster than conventional reactors could be.

Light water reactor
LWGR ( Light Water Graphite Reactor) is a group which representative construction is RBMK reactor – Reaktor Bolszoj Moszcznosti Kanalnyj. It is boiling water reactor, light water cooled and with graphite as moderator. This design was developed from army construction to pluton production. It is pressure tube and single circuit reactor. Steam is generated directly in the reactor and separated in steam drums. Water used in RBMK is radioactively contaminated. Reactor should be shielded up as BWR is. As all tube reactors, RBMK is a large construction. It has vertical graphite blocks with fuel tubes. Mass of graphite is enormous so the core is heavy and power density is very low, only 5,8 MW/m3. Parameters of steam in the outlet of steam drums are average, 280 ºC and 6,38 MPa. Advantage of RBMK reactors is low fuel enrichment level and possibility to replace fuel tubes during reactor’s operation (up to 5 replacements per day). However very high graphite temperature and positive reactivity factor makes RBMK reactors very dangerous.

RBMK net efficiency is around of 31 %. It uses enriched fuel in uranium dioxide. An enrichment level is 2,6 – 2,8 % U-235.

RBMK was a Soviet design, at the beginning assumed to be highly secure, because of very low power density in the core and possibility to control technology process separately in each tube – modular construction. As the history shown, mistakes in design and lack of elementary security systems led to one of the most serious nuclear disaster in the World.

RBMK was not sold outside Soviet Union not only because dangerous construction but also because it can produce plutonium. ZSRR did not want to sell a technology to weapon production.
Gas Cooled, Graphite Moderated
Of the six main commercial reactor types, two (Magnox and AGR) owe much to the very earliest reactor designs in that they are graphite moderated and gas cooled. Magnox reactors were built in the UK from 1956 to 1971 but have now been superseded. The Magnox reactor is named after the magnesium alloy used to encase the fuel, which is natural uranium metal. Fuel elements consisting of fuel rods encased in Magnox cans are loaded into vertical channels in a core constructed of graphite blocks. Further vertical channels contain control rods (strong neutron absorbers) which can be inserted or withdrawn from the core to adjust the rate of the fission process and, therefore, the heat output. The whole assembly is cooled by blowing carbon dioxide gas past the fuel cans, which are specially designed to enhance heat transfer. The hot gas then converts water to steam in a steam generator. Early designs used a steel pressure vessel, which was surrounded by a thick concrete radiation shield.
In later designs, a dual-purpose concrete pressure vessel and radiation shield was used.
In order to improve the cost effectiveness of this type of reactor, it was necessary to go to higher temperatures to achieve higher thermal efficiencies and higher power densities to reduce capital costs. This entailed increases in cooling gas pressure and changing from Magnox to stainless steel cladding and from uranium metal to uranium dioxide fuel. This in turn led to the need for an increase in the proportion of U235 in the fuel. The resulting design, known as the Advanced Gas-Cooled Reactor, or AGR , still uses graphite as the moderator and, as in the later Magnox designs, the steam generators and gas circulators are placed within a combined concrete pressure-vessel/radiation-shield.
Pros:
• Can operate at very high temperatures, leading to great thermal efficiency (near 50%!) and the ability to create process heat for things like oil refineries, water desalination plants, hydrogen fuel cell production, and much more.
• Each little pebble of fuel has its own containment structure, adding yet another barrier between radioactive material and the environment.
Cons:
• High temperature has a bad side too. Materials that ca
• n stay structurally sound in high temperatures and with many neutrons flying through them are hard to come by.
• If the gas stops flowing, the reactor heats up very quickly. Backup cooling systems are necessary.
• Gas is a poor coolant, necessitating large amounts of coolant for relatively small amounts of power. Therefore, these reactors must be very large to produce power at the rate of other reactors.
• Not as much operating experience.
•
Fast-neutron Breeder Reactor (FBR)

In a fast-neutron breeder reactor, there is no need for a moderator. the neutrons produced after nuclear fission remain at high energy and a chain of nuclear fission is maintained by having a high concentration of fissile material, around 20%. In addition, the core of fissile material is surrounded by a “blanket” of “fertile” material of U-238 which on capturing some of the neutrons from the nuclear fission, will “breed” fissile material to be used for more nuclear fission.
A liquid metal is a very good conductor of heat. It is used to take the heat away from the reactor core to a heat exchanger, and heats water in a closed secondary circuit to raise steam to drive a turbine-generator to produce electricity.
Several FBRs have been built to demonstrate the viability of the design, in France, Japan and Russia and a unit capacity of 1200 MW has been reached.

Pros:
• Can breed its own fuel, effectively eliminating any concerns about uranium shortages (see what is a fast reactor?)
• Can burn its own waste
• Metallic fuel and excellent thermal properties of sodium allow for passively safe operation — the reactor will shut itself down and cool decay heat without any backup-systems working (or people around), only relying on physics (gravity, natural circulation, etc.)
Cons:
• Sodium coolant is reactive with air and water. Thus, leaks in the pipes results in sodium fires. These can be engineered around (by making a pool and eliminating pipes, etc.) but are a major setback for these nice reactors.
• To fully burn waste, these require reprocessing facilities which can also be used for nuclear proliferation.
• The excess neutrons used to give the reactor its resource-utilization capabilities could clandestinely be used to make plutonium for weapons.
• Positive void coefficients are inherent to most fast reactors, especially large ones. This is a safety concern.
• Not as much operating experience has been accumulated
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Boiling Water Reactor

Boiling water reactors (BWRs) are nuclear power reactors utilizing light water as the reactor
coolant and moderator to generate electricity by directly boiling the light water in a reactor
core to make steam that is delivered to a turbine generator. There are two operating BWR
types, roughly speaking, i.e., BWRs and ABWRs (advanced boiling water reactors).

BWRs have been originally developed by GE. GE started its development in 1950s as light
water reactor type nuclear power reactors, and the Dresden Unit-1 (200,000 kWe)
commissioned in July 1960 is the first BWR nuclear power station. After that, the GE
company has supplied many BWRs, Siemens (KWU, Germany), ABB-Atom
(Switzerland/Sweden) and Toshiba and Hitachi (Japan) also supplied many BWRS. In the
following, features and types of BWRs, mainly of conventional BWRs, are explained and
those of ABWRs are addressed in the next.
For BWRs, the steam void due to reactor coolant boiling has a negative-reactivity effect,
which can suppress a power rise even if a positive reactivity is added. The reactor power can
be controlled by two methods: reactor-coolant recirculation-flow control and control rod
operation.

A BWR nuclear power plant consists of the reactor coolant recirculation system and main
steam system that compose a nuclear reactor, engineered safety features that consist of the
emergency core cooling system, reactor core isolation cooling system, containment cooling
system and boric-acid injection system, turbine and generator equipment and other systems,
such as the reactor coolant purification system, waste processing equipment, fuel handling
equipment, other auxiliary equipment, etc.

Pros:
• Simpler plumbing reduces costs
• Power levels can be increased simply by speeding up the jet pumps, giving less boiled water and more moderation. Thus, load-following is simple and easy.
• Very much operating experience has been accumulated and the designs and procedures have been largely optimized.
Cons:
• With liquid and gaseous water in the system, many weird transients are possible, making safety analysis difficult
• Primary coolant is in direct contact with turbines, so if a fuel rod had a leak, radioactive material could be placed on the turbine. This complicates maintenance as the staff must be dressed for radioactive environments.
• Can’t breed new fuel — susceptible to “uranium shortage”
• Does not typically perform well in station blackout events, as in Fukushima.