The Nuclear Reactor

Use and Issues

Prepared in recharge for Nuclear Reactor Circulam; phy

Tought by Prof. Dr. Fayza

Prepared by

Mohamed Hassan Mohamed Salem

Table of Contents

The Use of Nuclear power,......................................................................................................................................................................... 3

Issues facing Nuclear power and Nuclear reactors;.................................................................................................................................. 5

Nuclear power plant..................................................................................................................................................................................... 5

Life cycle...................................................................................................................................................................................................... 5

Conventional fuel resources......................................................................................................................................................................... 6

Reprocessing................................................................................................................................................................................................. 6

Radioactive Waste Disposal......................................................................................................................................................................... 6

Solid waste.................................................................................................................................................................................................... 7

Nuclear Weapons Proliferation................................................................................................................................................................... 8

Environmental issue...................................................................................................................................................................................... 9

Climate change............................................................................................................................................................................................. 9

Power Plant Emission.................................................................................................................................................................................. 10

Radioactive gases and effluent.................................................................................................................................................................. 10

Tritium......................................................................................................................................................................................................... 10

Uranium mining.......................................................................................................................................................................................... 11

Meltdown.................................................................................................................................................................................................... 11

What Causes of Nuclear Meltdown?......................................................................................................................................................... 12

Introduction;

As we know the Nuclear power is a plant for manage and introduce nuclear energy where the use of exothermic nuclear processes to generate useful heat and electricity. Such energy has a great power that must be carefully dealing with. All Nuclear Reactors are devices designed to maintain a chain reaction producing a steady flow of neutron generated by the nuclear fission of heavy nuclei. They are, differentiated either by their purpose or by their design features. In terms of purpose, they are either research reactors or power reactors. Research reactors are operated at universities and research centers in many countries, including somewhere no nuclear power reactors are operated. These reactors generate neutrons for multiple purposes, including producing radiopharmaceuticals for medical diagnosis and therapy, testing materials and conducting basic research. Power reactors are usually found in nuclear power plants. Dedicated to generating heat mainly for electricity production, they are operated in more than 30 countries. In the form of smaller units, they also power ships. Beyond that there is a whole class of nuclear-based, electricity-producing devices that are sometimes called atomic batteries, which are used on spacecraft and are relatively unknown to the public at large. Such as the known radioactive decay battery (sometimes called an atomic battery), which uses the radiation released by the decay of certain radioactive. Substances as a source of heat to generate electricity. Overall, there is the nuclear bomb and it provide a huge nuclear weapon, and the use of depleted Uranium

In the 2010 book Why vs. Why: Nuclear Power Barry Brook and Ian Lowe discuss and articulate the debate about nuclear power. Brook argues that there are seven reasons why people should say "yes" to nuclear power.

· Because renewable energy and energy efficiency won’t solve the energy and climate crises

· Because nuclear fuel is virtually unlimited and packs a huge energy punch

· Because new technology solves the "nuclear waste" problem

· Because nuclear power is the safest energy option

· Because advanced nuclear power will strengthen global security

· Because nuclear power's true costs are lower than either fossil fuels or renewables

· Because nuclear power can lead the "clean energy" revolution

Lowe argues that there are seven reasons why people should say "no" to nuclear power

· Because it is not a fast enough response to climate change

· Because it is too expensive

· Because the need for baseload electricity is exaggerated

· Because the problem of waste remains unresolved

· Because it will increase the risk of nuclear war

· Because there are safety concerns

· Because there are better alternatives

Nuclear power plant

Unlike fossil fuel power plants, the only substance leaving the cooling towers of nuclear power plants is water vapor and thus does not pollute the air or cause global warming. Just as many conventional thermal power stations generate electricity by harnessing the thermal energy released from burning fossil fuels, nuclear power plants convert the energy released from the nucleus of an atom via nuclear fission that takes place in a nuclear reactor. The heat is removed from the reactor core by a cooling system that uses the heat to generate steam, which drives a steam turbine connected to generator producing electricity.

Life cycle

The nuclear fuel cycle begins when uranium is mined, enriched, and manufactured into nuclear fuel, which is delivered to a nuclear power plant. After usage in the power plant, the spent fuel is delivered to a reprocessing plant or to a final repository for geological disposition. In reprocessing 95% of spent fuel can potentially be recycled to be returned to usage in a power plant.

A nuclear reactor is only part of the life cycle for nuclear power. The process starts with mining. Uranium mines are underground, open-pit, or in-situ leach mines. In any case, the uranium ore is extracted, usually converted into a stable and compact form such as yellowcake, and then transported to a processing facility. Here, the yellowcake is converted to uranium hexafluoride, which is then enriched using various techniques. At this point, the enriched uranium, containing more than the natural 0.7% U-235, is used to make rods of the proper composition and geometry for the particular reactor that the fuel is destined for. The fuel rods will spend about 3 operational cycles (typically 6 years total now) inside the reactor, generally until about 3% of their uranium has been fissioned, then they will be moved to a spent fuel pool where the short lived isotopes generated by fission can decay away. After about 5 years in a spent fuel pool the spent fuel is radioactively and thermally cool enough to handle, and it can be moved to dry storage casks or reprocessed.

The Use of Nuclear power,

Electricity

Nuclear power plants including small modular reactors.

Propulsion, see nuclear propulsion: Nuclear power for propulsion has several operating and logistic characteristics that appeal to the designers of ships for both civil and military purposes. A small amount of nuclear fuel can provide energy equivalent to millions of times its weight in coal or oil. It is quite practical to build a reactor, which will operate a vessel for several years without refueling. A sustained nuclear reaction in the reactor produces heat that is used to boil water. The resulting steam spins a turbine. The turbine shaft may be coupled through a gearbox speed reducer to the ship's propeller, or in a turbo-electric drive system may run a generator that supplies electric power to motors connected to the propellers.

o Nuclear marine propulsion; is propulsion of a ship with power provided by a nuclear reactor. Naval nuclear propulsion is propulsion that specifically refers to naval warships. Very few experimental civil nuclear ships have been built. Operation of a civil or naval ship power plant is similar to land-based nuclear power reactors.

o Various proposed forms of rocket propulsion; Spacecraft propulsion is any method used to accelerate spacecraft and artificial satellites.

Other uses of heat

o Desalination: refers to any of several processes that remove some amount of salt and other minerals from saline water. More generally, desalination may also refer to the removal of salts and minerals, as in soil desalination. Salt water is desalinated to produce fresh water suitable for human consumption or irrigation. One potential byproduct of desalination is salt.

o Heat for domestic and industrial heating

o Hydrogen production for use in a hydrogen economy

Production reactors for transmutation of elements

o 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.

o Creating various radioactive isotopes, such as americium for use in smoke detectors, and cobalt-60, molybdenum-99 and others, used for imaging and medical treatment.

o Production of materials for nuclear weapons such as weapons-grade plutonium

o Providing a source of neutron radiation (for example with the pulsed Godiva device) and (e.g. neutron activation analysis and potassium-argon dating).

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 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.

Issues facing Nuclear power and Nuclear reactors;

Nuclear power poses numerous threats to people and the environment and point to studies in the literature that question if it will ever be a sustainable energy source. These threats include health risks and environmental damage from uranium mining, processing and transport, the risk of nuclear weapons proliferation or sabotage, and the unsolved problem of radioactive nuclear waste. They also contend that reactors themselves are enormously complex machines where many things can and do go wrong, and there have been many serious nuclear accident Problems of Nuclear Reactors; concerns about the safety of nuclear fission reactors include the possibility of radiation-releasing nuclear accidents, the problems of radioactive waste disposal and the possibility of contributing to nuclear weapon proliferation. From the knowing construction of the Nuclear power plant and the fuel life cycle there is many issue we face to generate the power and to use the nuclear power plant, let’s scope it here;

Nuclear power plant

Life cycle

Conventional fuel resources

Proportions of the isotopes, uranium-238 (blue) and uranium-235 (red) found naturally, versus grades that are enriched. light water reactors require fuel enriched to (3-4%), while others such as the CANDU reactor uses natural uranium.

Uranium is a fairly common element in the Earth's crust. Uranium is approximately as common as tin or germanium in the Earth's crust, and is about 40 times more common than silver.Uranium is a constituent of most rocks, dirt, and of the oceans. The fact that uranium is so spread out is a problem because mining uranium is only economically feasible where there is a large concentration. Still, the world's present measured resources of uranium, economically recoverable at a price of 130 USD/kg, are enough to last for between 70 and 100 years. Uranium represents a higher level of assured resources than is normal for most minerals. On the basis of analogies with other metallic minerals, a doubling of price from present levels could be expected to create about a tenfold increase in measured resources, over time.

Reprocessing

Reprocessing can potentially recover up to 95% of the remaining uranium and plutonium in spent nuclear fuel, putting it into new mixed oxide fuel. This produces a reduction in long term radioactivity within the remaining waste, since this is largely short-lived fission products, and reduces its volume by over 90%. Reprocessing of civilian fuel from power reactors is currently done in Britain, France and (formerly) Russia, soon will be done in China and perhaps India, and is being done on an expanding scale in Japan. The full potential of reprocessing has not been achieved because it requires breeder reactors, which are not commercially available. France is generally cited as the most successful preprocessor, but it presently only recycles 28% (by mass) of the yearly fuel use, 7% within France and another 21% in Russia.

Radioactive Waste Disposal

The nuclear fission of uranium-235 produces large quantities of intermediate mass radioisotopes. The mass distribution of these radioisotopes peaks at about mass numbers 95 and 137, and most of them are radioactive. The most dangerous for environmental release are probably cesium and strontium because of their intermediate half-lives and propensity for re-concentration in the food chain.

When spent fuel assemblies are removed from nuclear reactors, they are transported to "swimming pool" storage facilities to dissipate the heat of decay of short-lived isotopes as well as for isolation from the environment. The long term disposal of these wastes remains a major problem. It was assumed that these wastes would be encased in glass and placed in geologic disposal sites in underground salt domes. The site at Yucca Mountain was chosen as a first site, but both technical and political problems have thus far blocked its implementation.

Solid waste

The most important waste stream from nuclear power plants is spent nuclear fuel. It is primarily composed of unconverted uranium as well as significant quantities of transuranic actinides (plutonium and curium, mostly). In addition, about 3% of it is fission products from nuclear reactions. The actinides (uranium, plutonium, and curium) are responsible for the bulk of the long-term radioactivity, whereas the fission products are responsible for the bulk of the short-term radioactivity.^[112]

High-level radioactive waste

Main article: High-level radioactive waste management

A nuclear fuel rod assembly bundle being inspected before entering a reactor.

Following interim storage in a spent fuel pool, the bundles of used fuel assemblies of a typical nuclear power station are often stored on site in the likes of the eight dry cask storage vessels pictured above.^[113] At Yankee Rowe Nuclear Power Station, which generated 44 billion kilowatt hours of electricity over its lifetime, its complete spent fuel inventory is contained within sixteen casks.^[114]

High-level radioactive waste management concerns management and disposal of highly radioactive materials created during production of nuclear power. The technical issues in accomplishing this are daunting, due to the extremely long periods radioactive wastes remain deadly to living organisms. Of particular concern are two long-lived fission products, Technetium-99 (half-life 220,000 years) and Iodine-129 (half-life 15.7 million years),^[115] which dominate spent nuclear fuel radioactivity after a few thousand years. The most troublesome transuranic elements in spent fuel are Neptunium-237 (half-life two million years) and Plutonium-239 (half-life 24,000 years)

Waste disposal

Disposal of nuclear waste is often said to be the Achilles' heel of the industry.^[134] Presently, waste is mainly stored at individual reactor sites and there are over 430 locations around the world where radioactive material continues to accumulate. Some experts suggest that centralized underground repositories which are well-managed, guarded, and monitored, would be a vast improvement.^[134] There is an "international consensus on the advisability of storing nuclear waste in deep geological repositories",^[135] with the lack of movement of nuclear waste in the 2 billion year old natural nuclear fission reactors in Oklo, Gabon being cited as "a source of essential information today."^[136][137]

As of 2009 there were no commercial scale purpose built underground repositories in operation.^{[135][138][139][140]} The Waste Isolation Pilot Plant in New Mexico has been taking nuclear waste since 1999 from production reactors, but as the name suggests is a research and development facility.

Reprocessing is not allowed in the U.S.^[142] The Obama administration has disallowed reprocessing of nuclear waste, citing nuclear proliferation concerns.^[143] In the U.S., spent nuclear fuel is currently all treated as waste

Depleted uranium

Main article: Depleted uranium

Uranium enrichment produces many tons of depleted uranium (DU) which consists of U-238 with most of the easily fissile U-235 isotope removed. U-238 is a tough metal with several commercial uses—for example, aircraft production, radiation shielding, and armor—as it has a higher density than lead. Depleted uranium is also controversially used in munitions; DU penetrators (bullets or APFSDS tips) "self sharpen", due to uranium's tendency to fracture along shear bands.^[145][146]

Nuclear Weapons Proliferation

One concern about nuclear reactors is that the fuel could be diverted for the production of nuclear weapons. While the the uranium fuel is enriched to only 3-5% and could not easily be further separated to the >90% U-235 needed to produce a bomb, the spent fuel elements contain plutonium-239. The plutonium could be separated chemically and diverted to nuclear weapons production. Security concerns about the plutonium has thus far blocked any reprocessing of fuel from nuclear power plants.

A similar concern exists for fast breeder reactors, where the breeding process produces plutonium-239 for future generations of reactors.

Environmental issue

A 2008 synthesis of 103 studies, published by Benjamin K. Sovacool, estimated that the value of CO₂ emissions for nuclear power over the lifecycle of a plant was 66.08 g/kW·h. Comparative results for various renewable power sources were 9–32 g/kW·h.^[175] A 2012 study by Yale University arrived at a different value, with the mean value, depending on which Reactor design was analyzed, ranging from 11 to 25 g/kW·h of total life cycle nuclear power CO₂ emissions.^[176]

Main articles: Environmental effects of nuclear power and Comparisons of life-cycle greenhouse gas emissions

Life cycle analysis (LCA) of carbon dioxide emissions show nuclear power as comparable to renewable energy sources. Emissions from burning fossil fuels are many times higher.^{[175][177][178]}

According to the United Nations (UNSCEAR), regular nuclear power plant operation including the nuclear fuel cycle causes radioisotope releases into the environment amounting to 0.0002 mSv (milli-Sievert) per year of public exposure as a global average.^[179] (Such is small compared to variation in natural background radiation, which averages 2.4 mSv/a globally but frequently varies between 1 mSv/a and 13 mSv/a depending on a person's location as determined by UNSCEAR).^[179] As of a 2008 report, the remaining legacy of the worst nuclear power plant accident (Chernobyl) is 0.002 mSv/a in global average exposure (a figure which was 0.04 mSv per person averaged over the entire populace of the Northern Hemisphere in the year of the accident in 1986, although far higher among the most affected local populations and recovery workers).^[179]

Climate change

Climate change causing weather extremes such as heat waves, reduced precipitation levels and droughts can have a significant impact on nuclear energy infrastructure.^[180] Seawater is corrosive and so nuclear energy supply is likely to be negatively affected by the fresh water shortage.^[180] This generic problem may become increasingly significant over time.^[180] This can force nuclear reactors to be shut down, as happened in France during the 2003 and 2006 heat waves. Nuclear power supply was severely diminished by low river ﬂow rates and droughts, which meant rivers had reached the maximum temperatures for cooling reactors.^[180] During the heat waves, 17 reactors had to limit output or shut down. 77% of French electricity is produced by nuclear power and in 2009 a similar situation created a 8GW shortage and forced the French government to import electricity.^[180] Other cases have been reported from Germany, where extreme temperatures have reduced nuclear power production 9 times due to high temperatures between 1979 and 2007.^[180] In particular:

the Unterweser nuclear power plant reduced output by 90% between June and September 2003^[180]
the Isar nuclear power plant cut production by 60% for 14 days due to excess river temperatures and low stream ﬂow in the river Isar in 2006^[180]

Similar events have happened elsewhere in Europe during those same hot summers.^[180] If global warming continues, this disruption is likely to increase.

Power Plant Emission

Radioactive gases and effluent

The Grafenrheinfeld Nuclear Power Plant. The tallest structure is the chimney that releases effluent gases.

Most commercial nuclear power plants release gaseous and liquid radiological effluents into the environment as a byproduct of the Chemical Volume Control System, which are monitored in the US by the EPA and the NRC. Civilians living within 50 miles (80 km) of a nuclear power plant typically receive about 0.1 μSv per year.^[12] For comparison, the average person living at or above sea level receives at least 260 μSv from cosmic radiation.^[12]

The total amount of radioactivity released through this method depends on the power plant, the regulatory requirements, and the plant's performance. Atmospheric dispersion models combined with pathway models are employed to accurately approximate the dose to a member of the public from the effluents emitted. Effluent monitoring is conducted continuously at the plant.

Tritium Effluent Limits^{[citation needed]}
Country	Limit (Bq/L)
Australia	76,103
Finland	30,000
WHO	10,000
Switzerland	10,000
Russia	7,700
Ontario, Canada	7,000
European Union	1001
United States	740
California Public Health Goal	14.8

Tritium

A leak of radioactive water at Vermont Yankee in 2010, along with similar incidents at more than 20 other US nuclear plants in recent years, has kindled doubts about the reliability, durability, and maintenance of aging nuclear installations in the United States.^[13]

Tritium is a radioactive isotope of hydrogen that emits a low-energy beta particle and is usually measured in becquerels (i.e. atoms decaying per second) per liter (Bq/L). Tritium can be contained in water released from a nuclear plant. The primary concern for tritium release is the presence in drinking water, in addition to biological magnification leading to tritium in crops and animals consumed for food.

Uranium mining

Uranium mining is the process of extraction of uranium ore from the ground. The worldwide production of uranium in 2009 amounted to 50,572 tonnes. Kazakhstan, Canada, and Australia are the top three producers and together account for 63% of world uranium production.^[16] A prominent use of uranium from mining is as fuel for nuclear power plants. As of 2008, known uranium ore resources that can be mined at about current costs are estimated to be sufficient to produce fuel for about a century, based on current consumption rates.^[17]

After mining uranium ores, they are normally processed by grinding the ore materials to a uniform particle size and then treating the ore to extract the uranium by chemical leaching. The milling process commonly yields dry powder-form material consisting of natural uranium, "yellowcake," which is sold on the uranium market as U₃O₈. Uranium mining can use large amounts of water — for example, the Roxby Downs mine in South Australia uses 35,000 m³ of water each day and plans to increase this to 150,000 m³ per day.^[18]

The Church Rock uranium mill spill occurred in New Mexico on July 16, 1979 when United Nuclear Corporation's Church Rock uranium mill tailings disposal pond breached its dam.^[19][20] Over 1,000 tons of solid radioactive mill waste and 93 millions of gallons of acidic, radioactive tailings solution flowed into the Puerco River, and contaminants traveled 80 miles (130 km) downstream to Navajo County, Arizona and onto the Navajo Nation.^[20] The accident released more radiation than the Three Mile Island accident that occurred four months earlier and was the largest release of radioactive material in U.S. history.^{[20][21][22][23]} Groundwater near the spill was contaminated and the Puerco rendered unusable by local residents, who were not immediately aware of the toxic danger.^[24]

Despite efforts made in cleaning up uranium sites, significant problems stemming from the legacy of uranium development still exist today on the Navajo Nation and in the states of Utah, Colorado, New Mexico, and Arizona. Hundreds of abandoned mines have not been cleaned up and present environmental and health risks in many communities.^[25] The Environmental Protection Agency estimates that there are 4000 mines with documented uranium production, and another 15,000 locations with uranium occurrences in 14 western states,^[26] most found in the Four Corners area and Wyoming.^[27] The Uranium Mill Tailings Radiation Control Act is a United States environmental law that amended the Atomic Energy Act of 1954 and gave the Environmental Protection Agency the authority to establish health and environmental standards for the stabilization, restoration, and disposal of uranium mill waste.^[28]

Meltdown

Nuclear meltdown is primary associated with the complete or partial melting of the nuclear reactor due to over heating. It result to the release of radioactive materials through leak in a nuclear plant facility or through emission of radioactive particles in the environment particularly the deadly particles of plutonium, radium and iodine in excessive dosage could be dangerous to human lives and to the environment. The past events known to history happen in Chernobyl nuclear meltdown, Long Mile Island in the USA in Russia and the recent Fukushima Daiichi Nuclear Power plant meltdown in Japan caused by massive tsunami and intensity magnitude of 9.0 earthquakes. When it comes to comparison of the Chernobyl explosion that happen in Russia the nuclear plant did not have containment structure thereby caused massive radiation and endanger the environment while the current Fukushima Daiichi nuclear power plant have containment structure which control radiation emission. Although the end result still awaiting wider scope of study and test measuring the amount of radiation emitted to its effect on the people expose and to the environment.

What Causes of Nuclear Meltdown?

The nuclear meltdown happen when the heat generated by nuclear reactor exceed the heat remove by the cooling system thereby reaching the point of melting where cooling mechanism system result in Alternate current stoppage. It may be due to system error and environmental causes which out of human control. Nuclear meltdown happens due to the following causes;

1. Loss of Coolant Accident –

It happen when there are reduce supply of allowable amount of deionized water and other substances of inert gas such as NaK known as liquid sodium which used to balance the excessive pressure of heat inside the core loss its control. It results to steam bubble that a sign of excessive heating due to loss of pressure control.

2. Loss of forced circulation Accident -

It happens when the motor system or turbines that used to distribute the cooling gas stop to operate causing failure to circulate inside the nuclear reactor core. When it stop to reach it destination as consequences over heating may occur though natural circulation by convection would keep the fuel cool so long as the depressurization subsides the heating to reach excessively.

3. Loss of pressure control Accident

It happens when the pressures of the nuclear reactor reduce below the allowable specification pressure without any method to restore it. Thereby resulting in heat transfer efficiency slowdown even when an inert gas is applied still a bubble is formed due to decay heat as the pressure which is used to cool down the nuclear reactor core exceed the requirements in the reactor design of the amount of pressure it could hold reach its limit. Whereas in the case of the boiling water reactor it posed no problem as it is design to depressurized that the emergency cooling system automatically function to take control in case of emergency so long as the gas circulator which use to control are fully functioning keeping the fuel cool including the surrounding parts of the nuclear core.

4. Uncontrolled power excursion accident –

It happen when the there is uncontrolled nuclear reactor reactivity reaches its limit and exceeded the specification as allow by the reactor unit design system. It become uncontrolled due to the alteration of the parameter duly affects the multiplication rate of chain reactor. During a prompt critical a reactor with a positive coefficient of reactivity are moderated and under circumstances caused by operator negligence. While a negative void coefficient of reactivity happens when there is a limit to heighten reactivity at any setting out of control will cause rapid shutdown from transient activity within the nuclear reactor.

5. Core Based Fire –

It happens when an air enters a graphite-moderated reactor or liquid sodium cooled reactor, which caused fire inside the core resulting in over heating. Reactor are of many types such as the water light and the gas cooled type reactor are not subject to inflammation due to the use of non reactive carbon dioxide and helium components containing fuel that could withstand high temperature without melting the reactor core.

6. Byzantine fault and Cascading Failure –

It happen due to instrumentation and control operation system failure when not dealt rapidly would result to core damage. Often it occur when the pilot operated relief valve get stuck with the failure of the operator to manually activate the cooling system of the nuclear reactors

Criticality”

Criticality and “re-criticality” have been used extensively in the media coverage. Criticality is a nuclear term that refers to the balance of neutrons in the system. “Subcritical” refers to a system where the loss rate of neutrons is greater than the production rate of neutrons and therefore the neutron population (or number of neutrons) decreases as time goes on. “Supercritical” refers to a system where the production rate of neutrons is greater than the loss rate of neutrons and therefore the neutron population increases. When the neutron population remains constant, this means there is a perfect balance between production rate and loss rate, and the nuclear system is said to be “critical.” The criticality of a system can be calculated by comparing the rate at which neutrons are produced, from fission and other sources, to the rate at which they are lost through absorption and leakage out of the reactor core. A nuclear reactor is a system that controls this criticality or balance of neutrons.

The power of a reactor is directly proportional to the neutron population. If there are more neutrons in the system, more fission will take place producing more energy. When a reactor is starting up, the neutron population is increased slowly in a controlled manner, so that more neutrons are produced than are lost, and the nuclear reactor becomes supercritical. This allows the neutron population to increase and more power to be produced. When the desired power level is achieved, the nuclear reactor is placed into a critical configuration to keep the neutron population and power constant. Finally, during shutdown, the reactor is placed in a subcritical configuration so that the neutron population and power decreases. Therefore, when a reactor is said to have “gone critical,” it actually means it is in a stable configuration producing a constant power.

A reactor is maintained critical during normal power operations. In other systems, such as a spent fuel pool, mechanisms are in place to prevent criticality. If such a system still achieves criticality, it is called “re-criticality”. Boron and other materials, which absorb neutrons, are in place to make sure that this re-criticality does not occur. The added neutron absorbers substantially increase the rate of loss of neutrons, to ensure a subcritical system.

References

· Nuclear Power Reactor Safety, E.E. Lewis, Wiley-Interscience (1977), Section 9-4, page 480 et seq

· Nuclear Reactor Engineering, Samuel Glasstone and Alexander Sesonske, Van Nostrand Reinhold Company, 3rd Edition (1981), Section 11, page 724 et seq

· Introduction to Nuclear Power, John G. Collier and Geoffrey F. Hewitt, Hemisphere Publishing Corporation, (1987), Chapter 5, page 119-147

· Environmental Radioactivity from Natural, Industrial, and Military Sources, Merril Eisenbud, Academic Press, 3rd Edition (1987), Chapter 14, page 343-389

· http://en.wikipedia.org/wiki/Nuclear_meltdown

· http://scienceray.com/technology/the-causes-of-nuclear-reactor-meltdown/#ixzz2m1h2iQZe

· http://scienceray.com/technology/the-causes-of-nuclear-reactor-meltdown/#ixzz2m1i18Yci

· http://www.nucleartourist.com/events/meltdown.htm