![LWR Meltdown](https://upload.wikimedia.org/wikipedia/commons/7/70/LWR_Meltdown.gif)
A loss-of-coolant accident (LOCA) is a mode of failure for a
nuclear reactor
A nuclear reactor is a device used to initiate and control a fission nuclear chain reaction or nuclear fusion reactions. Nuclear reactors are used at nuclear power plants for electricity generation and in nuclear marine propulsion. Heat fr ...
; if not managed effectively, the results of a LOCA could result in reactor core damage. Each nuclear plant's
emergency core cooling system
:''This article covers the technical aspects of active nuclear safety systems in the United States. For a general approach to nuclear safety, see nuclear safety.''
The three primary objectives of nuclear reactor safety systems as defined by the ...
(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
coolant
A coolant is a substance, typically liquid, that is used to reduce or regulate the temperature of a system. An ideal coolant has high thermal capacity, low viscosity, is low-cost, non-toxic, chemically inert and neither causes nor promotes corrosi ...
system 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 decay, the nuclear fuel will continue to generate a significant amount of heat. The
decay heat
Decay heat is the heat released as a result of radioactive decay. This heat is produced as an effect of radiation on materials: the energy of the alpha, beta or gamma radiation is converted into the thermal movement of atoms.
Decay heat occur ...
produced by a reactor shutdown from full power is initially equivalent to about 5 to 6% of the thermal rating of the reactor.
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 plant
A nuclear power plant (NPP) is a thermal power station in which the heat source is a nuclear reactor. As is typical of thermal power stations, heat is used to generate steam that drives a steam turbine connected to a electric generator, generato ...
s are equipped with pressure-operated
relief valve
A relief valve or pressure relief valve (PRV) is a type of safety valve used to control or limit the pressure in a system; pressure might otherwise build up and create a process upset, instrument or equipment failure, or fire. The pressure is re ...
s and backup supplies of cooling water.
*If
graphite
Graphite () is a crystalline form of the element carbon. It consists of stacked layers of graphene. Graphite occurs naturally and is the most stable form of carbon under standard conditions. Synthetic and natural graphite are consumed on large ...
and air are present, the graphite may catch
fire
Fire is the rapid oxidation of a material (the fuel) in the exothermic chemical process of combustion, releasing heat, light, and various reaction Product (chemistry), products.
At a certain point in the combustion reaction, called the ignition ...
, spreading
radioactive contamination
Radioactive contamination, also called radiological pollution, is the deposition of, or presence of radioactive substances on surfaces or within solids, liquids, or gases (including the human body), where their presence is unintended or undesirab ...
. This situation exists only in
AGRs,
RBMK
The RBMK (russian: реактор большой мощности канальный, РБМК; ''reaktor bolshoy moshchnosti kanalnyy'', "high-power channel-type reactor") is a class of graphite-moderated nuclear power reactor designed and buil ...
s,
Magnox
Magnox is a type of nuclear power/production reactor that was designed to run on natural uranium with graphite as the moderator and carbon dioxide gas as the heat exchange coolant. It belongs to the wider class of gas-cooled reactors. The n ...
and weapons-production reactors, which use graphite as a
neutron moderator
In nuclear engineering, a neutron moderator is a medium that reduces the speed of fast neutrons, ideally without capturing any, leaving them as thermal neutrons with only minimal (thermal) kinetic energy. These thermal neutrons are immensely mo ...
(see
Chernobyl disaster
The Chernobyl disaster was a nuclear accident that occurred on 26 April 1986 at the No. 4 reactor in the Chernobyl Nuclear Power Plant, near the city of Pripyat in the north of the Ukrainian SSR in the Soviet Union. It is one of only two nuc ...
and
Windscale fire
The Windscale fire of 10 October 1957 was the worst nuclear accident in the United Kingdom's history, and one of the worst in the world, ranked in severity at level 5 out of a possible 7 on the International Nuclear Event Scale. The fire was in ...
).
*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 meltdown
A nuclear meltdown (core meltdown, core melt accident, meltdown or partial core melt) is a severe nuclear reactor accident that results in core damage from overheating. The term ''nuclear meltdown'' is not officially defined by the Internation ...
and 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
The water table is the upper surface of the zone of saturation. The zone of saturation is where the pores and fractures of the ground are saturated with water. It can also be simply explained as the depth below which the ground is saturated.
T ...
(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 disaster
The Chernobyl disaster was a nuclear accident that occurred on 26 April 1986 at the No. 4 reactor in the Chernobyl Nuclear Power Plant, near the city of Pripyat in the north of the Ukrainian SSR in the Soviet Union. It is one of only two nuc ...
the 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
Automotive safety is the study and practice of design, construction, equipment and regulation to minimize the occurrence and consequences of traffic collisions involving motor vehicles. Road traffic safety more broadly includes roadway design ...
features that prevent meltdowns from occurring in these extreme circumstances. The
Pebble Bed Reactor
The pebble-bed reactor (PBR) is a design for a graphite-moderated, gas-cooled nuclear reactor. It is a type of very-high-temperature reactor (VHTR), one of the six classes of nuclear reactors in the Generation IV initiative.
The basic desi ...
, for instance, can withstand extreme temperature transients in its fuel. Another example is the
CANDU reactor, 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. Another example is the
Hydrogen Moderated Self-regulating Nuclear Power Module The hydrogen-moderated self-regulating nuclear power module (HPM), also referred to as the compact self-regulating transportable reactor (ComStar), is a type of nuclear power reactor using hydride as a neutron moderator. The design is inherently s ...
, in which the chemical decomposition of the
uranium hydride
Uranium hydride, also called uranium trihydride (UH3), is an inorganic compound and a hydride of uranium.
Properties
Uranium hydride is a highly toxic, brownish grey to brownish black pyrophoric powder or brittle solid. Its density at 20 ° ...
fuel halts the fission reaction by removing the hydrogen moderator.
The same principle is used in
TRIGA
TRIGA (Training, Research, Isotopes, General Atomics) is a class of nuclear research reactor designed and manufactured by General Atomics. The design team for TRIGA, which included Edward Teller, was led by the physicist Freeman Dyson.
Design
...
research reactors.
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 plant
A nuclear power plant (NPP) is a thermal power station in which the heat source is a nuclear reactor. As is typical of thermal power stations, heat is used to generate steam that drives a steam turbine connected to a electric generator, generato ...
s have a negative void coefficient, indicating that as water turns to steam, power instantly decreases. Two exceptions are the Soviet
RBMK
The RBMK (russian: реактор большой мощности канальный, РБМК; ''reaktor bolshoy moshchnosti kanalnyy'', "high-power channel-type reactor") is a class of graphite-moderated nuclear power reactor designed and buil ...
and the Canadian
CANDU.
Boiling water reactor
A boiling water reactor (BWR) is a type of light water nuclear reactor used for the generation of electrical power. It is a design different from a Soviet graphite-moderated RBMK. It is the second most common type of electricity-generating nu ...
s, 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
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 (typically steam bubbles) form in the reactor ...
, using various techniques. Some, such as the
pebble bed reactor
The pebble-bed reactor (PBR) is a design for a graphite-moderated, gas-cooled nuclear reactor. It is a type of very-high-temperature reactor (VHTR), one of the six classes of nuclear reactors in the Generation IV initiative.
The basic desi ...
, passively slow down the chain reaction when coolant is lost; others have extensive
safety systems
Safety is the state of being "safe", the condition of being protected from harm or other danger. Safety can also refer to the control of recognized hazards in order to achieve an acceptable level of risk.
Meanings
There are two slightly dif ...
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.
Progression after loss-of-coolant
A great deal of work goes into the prevention of a serious core event. If such an event were 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
SCRAM
A scram or SCRAM is an emergency shutdown of a nuclear reactor effected by immediately terminating the fission reaction. It is also the name that is given to the manually operated kill switch that initiates the shutdown. In commercial reactor ...
ed (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 reactors
The light-water reactor (LWR) is a type of thermal-neutron reactor that uses normal water, as opposed to heavy water, as both its coolant and neutron moderator; furthermore a solid form of fissile elements is used as fuel. Thermal-neutron reac ...
this is the pressure vessel; in
CANDU and
RBMK
The RBMK (russian: реактор большой мощности канальный, РБМК; ''reaktor bolshoy moshchnosti kanalnyy'', "high-power channel-type reactor") is a class of graphite-moderated nuclear power reactor designed and buil ...
reactors this is the array of pressurized fuel channels; in
PHWR
A pressurized heavy-water reactor (PHWR) is a nuclear reactor that uses heavy water ( deuterium oxide D2O) as its coolant and neutron moderator. PHWRs frequently use natural uranium as fuel, but sometimes also use very low enriched uranium. T ...
reactors like
Atucha I, it will be a double barrier of channels and the pressure vessel) 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.
Fukushima Daiichi nuclear disaster
The Fukushima Daiichi nuclear disaster in 2011 occurred due to a loss-of-coolant accident. The circuits that provided electrical power to the coolant pumps failed causing a loss-of-core-cooling that was critical for the removal of residual decay heat which is produced even after active reactors are shut down and nuclear fission has ceased. The loss of reactor core cooling led to three nuclear meltdowns, three hydrogen explosions and the release of radioactive contamination.
The hydrogen explosions can be directly attributed to the oxidation of zirconium by steam in the fuel claddings as a result of the loss-of-coolant.
Fuel Claddings
Most reactors use a
zirconium alloy
Zirconium alloys are solid solutions of zirconium or other metals, a common subgroup having the trade mark Zircaloy. Zirconium has very low absorption cross-section of thermal neutrons, high hardness, ductility and corrosion resistance. One of the ...
as the material for fuel rod claddings due to its corrosion-resistance and low neutron absorption cross-section. However, one major drawback of zirconium alloys is that, when overheated, they oxidize and produce a runaway exothermic reaction with water (steam) that leads to the production of hydrogen:
Zr + 2H2O -> ZrO2 + 2H2. Such reactions are what led to the hydrogen explosions in the Fukushima Daiichi nuclear disaster.
Rupture Behavior
The residual decay heat causes rapid increase in temperature and internal pressure of the fuel cladding which leads to plastic deformation and subsequent bursting. During a loss-of-coolant accident, zirconium-based fuel claddings undergo high temperature oxidation, phase transformation, and creep deformation simultaneously. These mechanisms have been extensively studied by researchers using burst criterion models. In one study, researchers developed a burst criterion for Zircaloy-4 fuel claddings and determined that the effect of the steam environment on failure of the claddings is negligible at low temperatures. However, as the burst temperature increases, rapid oxidation of Zircaloy-4 claddings occurs leading to a sharp decrease in its ductility. In fact, at higher temperatures the burst strain pretty much drops to zero signifying that the oxidized cladding becomes so brittle locally that it is predicted to fail without any further deformation or straining.
The amount of oxygen picked up by the zirconium alloy depends on the exposure time to steam (H
2O) before rupture. For rapid ruptures due to high heating rates and internal pressures, there is negligible oxidation. However, oxidation plays an important role in fracture for low heating rates and low initial internal pressures.
Oxidation Resistance Coatings
The zirconium alloy substrates can be coated to improve their oxidation resistance. In one study, researchers coated a Zirlo substrate with Ti
2AlC MAX phase using a hybrid arc/magnetron sputtering technique followed by an annealing treatment. They subsequently investigated the mechanical properties and oxidation resistance in pure steam conditions at 1000 °C, 1100 °C, and 1200 °C under different oxidation times. Results showed that coating the Zirlo substrate with Ti
2AlC caused in increase in hardness and elastic modulus compared to the bare substrate. Additionally, the high-temperature oxidation resistance was significantly improved. The benefits of Ti
2AlC over other coating materials are that it has excellent stability under neutron irradiation, a lower thermal expansion coefficient, better thermal shock resistance, and higher temperature oxidation resistance. ''Table 1'' provides a good indication of the improved mechanical properties as a result of the coating and improved resistance to plastic deformation.
Another recent study evaluated Cr and FeCrAl coatings (deposited on Zircaloy-4 using atmospheric plasma spraying technology) under simulated loss-of-coolant conditions.
The Cr coating displayed superior oxidation resistance. The formation of a compact Cr
2O
3 layer on the Cr-coating acted as an oxygen diffusion barrier that protected the Zr substrate from oxidation whereas the FeCrAl coating degraded due to inter-diffusion between the coating and the Zr substrate at high temperature thereby allowing Zr to still oxidize.
See also
*
LOFT (LOCA)
*
Containment building
A containment building is a reinforced steel, concrete or lead structure enclosing a nuclear reactor. It is designed, in any emergency, to contain the escape of radioactive steam or gas to a maximum pressure in the range of . The containment i ...
*
Nuclear power
Nuclear power is the use of nuclear reactions to produce electricity. Nuclear power can be obtained from nuclear fission, nuclear decay and nuclear fusion reactions. Presently, the vast majority of electricity from nuclear power is produced b ...
*
Pressurized water reactor
A pressurized water reactor (PWR) is a type of light-water nuclear reactor. PWRs constitute the large majority of the world's nuclear power plants (with notable exceptions being the UK, Japan and Canada). In a PWR, the primary coolant (water) i ...
*
Nuclear fuel response to reactor accidents
*
Nuclear accidents in the United States
The United States Government Accountability Office reported more than 150 incidents from 2001 to 2006 of nuclear plants not performing within acceptable safety guidelines. According to a 2010 survey of energy accidents, there have been at least 5 ...
*
Nuclear safety in the U.S.
Nuclear safety in the United States is governed by federal regulations issued by the Nuclear Regulatory Commission (NRC). The NRC regulates all nuclear plants and materials in the United States except for nuclear plants and materials controlled by ...
*
Nuclear meltdown
A nuclear meltdown (core meltdown, core melt accident, meltdown or partial core melt) is a severe nuclear reactor accident that results in core damage from overheating. The term ''nuclear meltdown'' is not officially defined by the Internation ...
*
Lucens reactor
The Lucens reactor was a 6 MW experimental nuclear power reactor built next to Lucens, Vaud, Switzerland. After its connection to the electrical grid on 29 January 1968, the reactor only operated for a few months before it suffered an acc ...
References
{{Authority control
Civilian nuclear power accidents
Nuclear reactor safety