Nuclear meltdown

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"Meltdown" redirects here. For other uses, see Meltdown (disambiguation).

A nuclear meltdown occurs when the core of a nuclear reactor ceases to be properly controlled and cooled due to failure of control or safety systems, and fuel assemblies (containing the uranium or plutonium reactor fuel and highly radioactive fission products) inside the reactor begin to overheat and melt. A meltdown is considered a serious nuclear accident because of the probability that a nuclear meltdown will defeat one or more reactor containment systems and potentially release highly radioactive fission products to the environment.

Several nuclear meltdowns of differing severity have occurred throughout the history of both civilian and military nuclear reactor operations. All nuclear meltdowns are characterized by severe damage to the nuclear reactor in which it occurs. In some cases this has required extensive repairs or decommissioning of a nuclear reactor and in more severe cases it has required civilian evacuations.

1 Causes
2 Sequence of events
3 Effects
4 Reactor design
5 Other theoretical consequences of a nuclear meltdown
6 Meltdowns
7 See also
8 Reference
9 External links

[edit] Causes

In nuclear reactors, the fuel assemblies in the core can melt as a result of a loss of pressure control accident, loss of coolant accident, uncontrolled power excursion accident, or by a fire around the fuel assemblies.

In a loss of pressure control accident the pressure of the confined coolant falls below specification without the means to restore it. In some cases this may reduce the heat transfer efficiency (when using an inert gas as a coolant) and in others may form an insulating 'bubble' of steam surrounding the fuel assemblies (for pressurized water reactors). In the latter case, due to localized heatup of the steam 'bubble' due to decay heat, the pressure required to collapse the steam 'bubble' may exceed reactor design specifications until the reactor has had time to cool down.

In a loss of coolant accident either the physical loss of coolant (which is typically deioinized water, an inert gas, or a liquid sodium) or the loss of a method to ensure a sufficient flow rate of the coolant occurs. A loss of coolant accident and a loss of pressure control accidents are closely related in some reactors. In a pressurized water reactor a loss of coolant accident can also cause a steam 'bubble' to form in the core due to excessive heating of stalled coolant or by the subsequent loss of pressure control accident caused by a rapid loss of coolant.

In an uncontrolled power excursion accident a sudden power spike in the reactor exceeds reactor design specifications due to a sudden increase in reactor reactivity. An uncontrolled power excursion occurs due to significantly altering a parameter that affects the exponential rate of a nuclear chain reaction (examples include ejecting a control rod or significantly altering the nuclear characteristics of the moderator, such as by rapid cooling). In extreme cases the reactor may proceed to a condition known as prompt critical.

In certain reactor designs it is possible for hydrogen or graphite to ignite inside the reactor core. A fire inside the reactor may be caused by failure to carefully control the amount of hydrogen in the coolant, an air addition to certain types of nuclear reactors, the uncontrolled heating of the coolant or moderator of the reactor by the types of nuclear accidents listed above, or by an external source. Fires can be a much more severe casuality for nuclear reactors that are moderated with graphite because without taking proper precautions Wigner energy may accumulate (for example, during the Windscale fire).

In all of these cases, a reactor meltdown occurs when the fuel assemblies are heated beyond their melting point (2070 °F for uranium). In some cases (such as the Chernobyl accident) this may be almost instantanous and in others it could take hours or more (Three Mile Island accident). A nuclear reactor does not have to remain critical for a nuclear meltdown to occur since decay heat or fires can continue to heat the reactor fuel assemblies long after the reactor has shut down.

[edit] Sequence of events

What happens when a reactor core melts is the subject of conjecture and some actual experience (see below).

Before the core of a nuclear reactor can melt, a number of events/failures must already have happened. Once the core melts, it will almost certainly destroy the fuel bundles and internal structures of the reactor vessel (although it may not penetrate the reactor vessel). (Note that the core at Three Mile Island did melt nearly completely but stayed within the reactor vessel.) If the melt drops into a pool of water (for example, coolant or moderator), a steam explosion called a Fuel-Coolant Interaction (FCI) is likely. If air is available, any exposed flammable substances will probably burn fiercely, but the liquid nature of the molten core poses special problems.

In the worst case scenario, the above-ground containment would fail at an early stage, (due to say an FCI within the reactor vessel, ejecting part of the vessel as a missile - this is the 'alpha-mode' failure of the 1975 Rasmussen (WASH-1400) study), or there could be a large hydrogen explosion or other over-pressure event. Such an event could scatter urania-aerosol and volatile fission-products directly into the atmosphere. However, these events are considered essentially incredible in modern 'large-dry' containments. (The WASH-1400 report has been supplanted by the 1991 NUREG-1150 study.)

It seems to be an open question to what extent a molten mass can melt through a structure. The molten reactor core could penetrate the reactor vessel and the containment structure and burn down (core-concrete interaction) to ground water (this has not happened at any meltdown to date). It is possible that, as in the Chernobyl accident, the molten mass might mix with any material it melts, diluting itself down to a non-critical state. (Note that the molten core of Chernobyl flowed in a channel created by the structure of its reactor building, e.g., stairways and froze in place before core-concrete interaction. In the basement of the reactor at Chernobyl, a large "elephant's foot" of congealed core material was found.) Furthermore, the time delay and the lack of a direct path to the atmosphere would work to significantly ameliorate the radiological release. Any steam-explosions/FCI which occurred would probably work mainly to increase cooling of the core-debris. However, the ground water itself would likely be severely contaminated, and its flow could carry the contamination far afield.

In the best case scenario, the reactor vessel would hold the molten material (as at Three Mile Island), limiting most of the damage to the reactor itself. However the Three Mile Island example also illustrates the difficulty in predicting such behavior: the reactor vessel was not built, and not expected, to sustain the temperatures it experienced when it underwent its meltdown, but because some of the melted material collected at the bottom of the vessel and cooled early on in the accident, it created a resistant shell against further pressure and heat. Such a possibility was not predicted by the engineers who designed the reactor and would not necessarily occur under duplicate conditions, but was largely seen as instrumental in the preservation of the vessel integrity.

[edit] Effects

The effects of a nuclear meltdown depend on the safety features designed into a reactor. A modern reactor is designed both to make a meltdown highly unlikely, and to contain one should it occur. In the future passively safe or inherently safe designs will make the possibility exceedingly unlikely.

In a modern reactor, a nuclear meltdown, whether partial or total, should be contained inside the reactor's containment structure. Thus (assuming that no other major disasters occur) while the meltdown will severely damage the reactor itself, possibly contaminating the whole structure with highly radioactive material, a meltdown alone will generally not lead to significant radiation release or danger to the public. The effects are therefore primarily economic (see [1]).

In practice, however, a nuclear meltdown is often part of a larger chain of disasters (although there have been so few meltdowns in the history of nuclear power that there is not a large pool of statistical information from which to draw a credible conclusion as to what "often" happens in such circumstances). For example, in the Chernobyl accident, by the time the core melted, there had already been a large steam explosion and graphite fire and major release of radioactive contamination (as with almost all Soviet reactors, there was no containment structure at Chernobyl).

[edit] Reactor design

Although pressurized water reactors are more susceptible to nuclear meltdown in the absence of active safety measures, this is not a universal feature of civilian nuclear reactors. Much of the research in civilian nuclear reactors is for designs with passive safety features that would be much less susceptible to meltdown, even if all emergency systems failed. For example, pebble bed reactors are designed so that complete loss of coolant for an indefinite period does not result in the reactor overheating. The General Electric ESBWR and Westinghouse AP1000 have passively-activated safety systems.

Fast breeder reactors are more susceptible to meltdown than other reactor types, due to the larger quantity of fissile material and the higher neutron flux inside the reactor core, which makes it more difficult to control the reaction. In addition, the liquid sodium coolant is highly corrosive and very difficult to manage.

[edit] Other theoretical consequences of a nuclear meltdown

If the reactor core becomes too hot, it might melt through the reactor vessel (although this has not happened to date) and the floor of the reactor chamber and descend until it becomes diluted by surrounding material and cooled enough to no longer melt through the material underneath, or until it hits groundwater. Note that a thermonuclear explosion does not happen in a nuclear meltdown due to the low fissibility of the radioactive components. However, a steam explosion may occur, if it hits water.

The geometry and presence of the coolant has a twin role, and both cools the reactor as well as slowing down emitted neutrons. The latter role is crucial to maintaining the chain-reaction, and so even without coolant the molten core is designed to be unable to form an uncontrolled critical mass (a recriticality). However, the molten reactor core will continue generating enough heat through unmoderated radioactive decay ('decay heat') to maintain or even increase its temperature. One possibility is that a large steam explosion could occur when the molten mass encountered water (in the lower plenum of the reactor vessel or on the floor of the room the reactor is in). Also, if the melt were to go through the floor of the reactor building the result would depend on the substance underneath.

[edit] Meltdowns

A number of Russian nuclear submarines have experienced nuclear meltdowns. The only known large scale nuclear meltdowns at civilian nuclear power plants were in the Chernobyl accident at Chernobyl Nuclear Power Plant, Ukraine, in 1986, and Three Mile Island, Pennsylvania, USA, in 1979, although there have been several partial core meltdowns, including accidents at:

NRX, Ontario, Canada, in 1952
EBR-I, Idaho, USA, in 1955
Windscale, Sellafield, England, in 1957
Santa Susana Field Laboratory, Simi Hills, California, in 1959
Enrico Fermi Nuclear Generating Station, Michigan, USA, in 1966
Chapelcross, Dumfries and Galloway, Scotland, in 1967

Not all of these were caused by a loss of coolant and in several cases (the Chernobyl accident and the Windscale fire, for example) the meltdown was not the most severe problem.

The Three Mile Island accident was caused by a loss of coolant, but "despite melting of about one-third of the fuel, the reactor vessel itself maintained its integrity and contained the damaged fuel". [2]