Loss of coolant accident
A loss-of-coolant accident (LOCA) is a mode of failure for a
nuclear reactor; if not managed effectively, the results of a LOCA could result in reactor core damage. Each nuclear plant's Emergency Core Cooling System(ECCS) exists specifically to deal with a LOCA.
Nuclear reactors generate heat internally; to remove this heat and convert it into useful electrical power, a
coolantsystem is used. If this coolant flow is reduced, or lost altogether, the nuclear reactor's emergency shutdown system is designed to stop the fission chain reaction. However, due to radioactive decaythe nuclear fuel will continue to generate a significant amount of heat. If all of the independent cooling trains of the ECCS fail to operate as designed, this heat can increase the fuel temperature to the point of damaging the reactor.
* If water is present, it may boil, bursting out of its pipes. (For this reason,
nuclear power plants are equipped with pressure-operated relief valves and backup supplies of cooling water.)
graphiteand air are present, the graphite may catch fire, spreading radioactive contamination. This situation exists only in AGRs, RBMKs, Magnoxand weapons-production reactors, which use graphite as a neutron moderator. (see Chernobyl disaster.)
* The fuel and reactor internals may melt; if the melted configuration remains critical, the molten mass will continue to generate heat, possibly melting its way down through the bottom of the reactor. Such an event is called a
nuclear meltdownand can have severe consequences. The so-called " China syndrome" would be this process taken to an extreme: the molten mass working its way down through the soil to the water table(and below) - however, current understanding and experience of nuclear fission reactions suggests that the molten mass would become too disrupted to carry on heat generation before descending very far; for example, in the Chernobyl accidentthe reactor core melted and core material was found in the basement, too widely dispersed to carry on a chain reaction (but still dangerously radioactive).
* Some reactor designs have passive safety features that prevent meltdowns from occurring in these extreme circumstances. The
Pebble Bed Reactor, for instance, can withstand extreme temperature transients in its fuel. Another example is the CANDUreactor, which has two large masses of relatively cool, low-pressure water (first is the heavy-water moderator; second is the light-water-filled shield tank) that act as heat sinks.
Under operating conditions, a reactor may passively (that is, in the absence of any control systems) increase or decrease its power output in the event of a LOCA or of voids appearing in its coolant system (by water boiling, for example). This is measured by the coolant void coefficient. Most modern
nuclear power plants have a negative void coefficient, indicating that as water turns to steam, power instantly decreases. Two exceptions are the Russian RBMKand the Canadian CANDU (in the latter case, for reasons outlined at the site [http://www.nuclearfaq.ca/cnf_sectionD.htm#s Nuclearfaq] , which also describes the safety systems designed to reliably handle this feature of the design). Boiling water reactors, on the other hand, are designed to have steam voids inside the reactor vessel.
Modern reactors are designed to prevent and withstand loss of coolant, regardless of their
void coefficient, using various techniques. Some, such as the pebble bed reactor, passively shut down the chain reaction when coolant is lost; others have extensive safety systems to rapidly shut down the chain reaction, and may have extensive passive safety systems (such as a large thermal heat sink around the reactor core, passively-activated backup cooling/condensing systems, or a passively cooled containment structure) that mitigate the risk of further damage.
The Three Final Defenses
A great deal of work goes into the prevention of a serious core event. If such an event was to occur, three different physical processes are expected to increase the time between the start of the accident and the time when a large release of radioactivity could occur. These three factors would provide additional time to the plant operators in order to mitigate the result of the event:
#The time required for the water to boil away (coolant, moderator). Assuming that at the moment that the accident occurs the reactor will be
SCRAMed (immediate and full insertion of all control rods), so reducing the thermal power input and further delaying the boiling.
#The time required for the fuel to melt. After the water has boiled, then the time required for the fuel to reach its melting point will be dictated by the heat input due to decay of fission products, the heat capacity of the fuel and the melting point of the fuel.
#The time required for the molten fuel to breach the primary pressure boundary. The time required for the molten metal of the core to breach the primary pressure boundary (in
light water reactorsthis is the pressure vessel; in CANDUand RBMKreactors this is the array of pressurized fuel channels) will depend on temperatures and boundary materials. Whether or not the fuel remains critical in the conditions inside the damaged core or beyond will play a significant role.
Pressurized water reactor
Nuclear fuel response to reactor accidents
Nuclear safety in the U.S.
Wikimedia Foundation. 2010.
Look at other dictionaries:
Loss of pressure control accident — Most commercial types of nuclear reactor use a pressure vessel to maintain pressure in the reactor plant. This is necessary in a pressurized water reactor to prevent boiling in the core, which could lead to a nuclear meltdown. This is also… … Wikipedia
Three Mile Island accident — The Three Mile Island accident of 1979 was the most significant accident in the history of the American commercial nuclear power generating industry. It resulted in the release of a significant amount of radioactivity, an estimated 43,000 curies… … Wikipedia
Behavior of nuclear fuel during a reactor accident — This page is devoted to a discussion of how uranium dioxide nuclear fuel behaves during both normal nuclear reactor operation and under reactor accident conditions such as overheating. Work in this area is often very expensive to conduct, and so… … Wikipedia
LOCA — Loss Of Coolant Accident (An incident involving the loss of primary coolant in a nuclear reactor) Contributor: CASI … NASA Acronyms
LOCA — Loss Of Coolant Accident (Academic & Science » Physics) * Laguna Outreach Community Arts (Community) … Abbreviations dictionary
Nuclear meltdown — Three of the reactors at Fukushima I overheated, causing core meltdowns. This was compounded by hydrogen gas explosions and the venting of contaminated steam which released large amounts of radioactive material into the air. … Wikipedia
Nuclear and radiation accidents — This article is about nuclear and radiation accidents in general. For a list of military nuclear accidents, see List of military nuclear accidents. For a list of civilian nuclear accidents, see List of civilian nuclear accidents. For a discussion … Wikipedia
Boiling water reactor — A boiling water reactor (BWR) is a type of nuclear reactor developed by the General Electric in the mid 1950s.Fact|date=April 2008 The BWR is characterized by two phase fluid flow (water and steam) in the upper part of the reactor core. See… … Wikipedia
Nuclear safety in the United States — Nuclear safety in the U.S. is governed by federal regulations and continues to be studied by the Nuclear Regulatory Commission (NRC). The safety of nuclear plants and materials controlled by the U.S. government for research and weapons production … Wikipedia
Void coefficient — In nuclear engineering, the void coefficient (more properly called void coefficient of reactivity ) is a number that can be used to estimate how much the reactivity of a nuclear reactor changes as voids (steam bubbles) form in the reactor… … Wikipedia