A nuclear meltdown is a term for a severe nuclear reactor accident. This can occur when a nuclear power plant system or component failure causes the reactor core no longer to be properly controlled and cooled to the extent that the sealed nuclear fuel assemblies – which contain the uranium or plutonium and highly radioactive fission products – begin to overheat and melt. A meltdown is considered very serious because of the possibility that the reactor containment will be defeated, thus releasing the core's highly radioactive and toxic elements into the atmosphere and environment. From an engineering perspective, a meltdown is likely to cause serious damage to the reactor, and possibly total destruction.
Several nuclear meltdowns of differing severity have occurred, from localized core damage to complete destruction of the reactor core. In some cases this has required extensive repairs or decommissioning of a nuclear reactor. In the most extreme cases, such as the Chernobyl disaster, deaths have resulted and the near-permanent civilian evacuation of a large area was required.
A nuclear explosion does not result from a nuclear meltdown because, by design, the geometry and composition of the reactor core do not permit the special conditions necessary for a nuclear explosion. However, the conditions that cause a meltdown may cause a non-nuclear explosion. For example, several power excursion accidents have caused coolant to rapidly overpressurize, resulting in a steam explosion.
In some reactor types, the fuel assemblies in the core can melt as a result of a loss of pressure control accident, a loss of coolant accident (LOCA), an uncontrolled power excursion, or any other event that might start a fire around the fuel assemblies.
A nuclear reactor does not have to remain critical for a nuclear meltdown to occur because fires or decay heat can continue to heat the reactor fuel assemblies long after the reactor has shut down.
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 nearly half of the core at Three Mile Island melted but the molten debris [called "melt"] still 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 was the 'alpha-mode' failure of the 1975 Rasmussen (WASH-1400) study), or there could be a large hydrogen explosion or some 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 was replaced by better-based newer studies, and now the Nuclear Regulatory Commission has disavowed them all and is preparing the over-arching State-of-the-Art Reactor Consequence Analyses [SOARCA] study - see the Disclaimer in NUREG-1150.)
It has not been determined to what extent a molten mass can melt through a structure (although that was tested in the Loss-of-Fluid-Test Reactor desribed in ). The molten reactor core could penetrate the reactor vessel and the containment structure and burn down (via a melt-concrete interaction) to groundwater (this has not happened at any meltdown to date: see China Syndrome). A water moderated reactor would go non-critical as soon as the water boiled away (with a fast reactor it is possible that the molten mass might mix with any material it melts, diluting itself down to a non-critical state). In the Chernobyl accident, the fuel became non-critical when it melted and flowed away from the graphite moderator - however, it took considerable time to cool. If hot uranium dioxide is combined with iron(II) oxide a eutectic is formed which may cause the fuel to become more mobile than it would otherwise be.
It should be noted that the molten core of Chernobyl (that part that didn't vaporize in the fire) flowed in a channel created by the structure of its reactor building (e.g., walls and stairways) and froze in place before a core-concrete interaction could happen. 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 (such as a containment building is designed to provide) 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, if the basemat of the reactor building were penetrated the groundwater 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. The American Nuclear Society has said "despite melting of about one-third of the fuel, the reactor vessel itself maintained its integrity and contained the damaged fuel". However the Three Mile Island example also illustrates the difficulty in predicting such behavior: the reactor vessel was not built for, and not expected to remain intact with, the temperatures it experienced when it the core melted, but possibly 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 reactor vessel's integrity. (However, it should be noted that the reactor vessel was inside a containment building, as in all non-Soviet nuclear plants, so a failure of the reactor vessel would not automatically mean that radioactive material would be released into the environment.)
All non-Soviet nuclear power plants are designed with Emergency Core Cooling Systems, some active and some passive and automatic. CANDU reactors, for example, are designed with at least one, and generally two, large low-temperature and low-pressure water reservoirs around its fuel/coolant channels. The first is the bulk heavy-water moderator (a separate system from the coolant), and the second is the light-water-filled shield tank. It has been shown that even under severe loss-of-coolant conditions these backup heat sinks are sufficient to prevent either the fuel meltdown in the first place (using the moderator heat sink), or the breaching of the core vessel should the moderator eventually boil off (using the shield tank heat sink). [Allen et al.]
These three factors would provide additional time to the plant operators in order to mitigate the result of the event:
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.
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).
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. The CANDU reactor has two low-temperature and low-pressure water systems surrounding the fuel (i.e. moderator and shield tank) that act as back-up heat sinks and preclude meltdowns and core-breaching scenarios [Allen et al].
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.
Accidental fires are widely acknowledged to be risk factors that can contribute to a nuclear meltdown. It is for this reason that circuit integrity measures are used for the electrical wiring that runs between control rooms and reactors. Ideally, a reactor is equipped with two "shutdown trains" or two sets of wires so that if one should fail, the other can be used to shut down the reactor. This common procedure became the subject of controversy during the Thermo-Lag scandal, when whistleblower Gerald W. Brown notified the NRC that the fire testing used to qualify Thermo-Lag was inadequate, meaning the fire-resistance rating thought to exist was in fact much lower, which meant that the majority of NRC licensees did not have operable protection of its safe shutdown wiring. Similar criticisms were leveled by US Congressman Ed Markey at the use of combustible silicone foam as firestops. The problem did not occur in German plants as operators must follow not just the directives of their federal regulators but are also required to follow the local building code, which makes product certification mandatory. Bounding in US and Canadian plants is not based on product certification. The Canadian disclosures by Gerald W. Brown revealed that Canadian plants also used unbounded silicone foam and Elastaseal based on indefensible test reports. The safe shutdown trains, typically consisting of wiring inside of cable trays used single-sided "fireproofing", consisting of sheet metal and proprietary intumescent sheets, for three dimensional cable trays. The disclosures were made public by the Canadian Broadcasting Corporation's "The National" program, which caused the proceedings of the Select Committee on Ontario Hydro Nuclear Affairs to take place. Still, to this date, neither the NRC, nor the Canadian Nuclear Safety Commission require product certification, which is mandatory for civilian construction.
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. This type of nuclear meltdown is known as a China Syndrome. Note that a nuclear explosion does not happen in a nuclear meltdown due to the low fissility 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.
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 disaster at Chernobyl Nuclear Power Plant, Ukraine, in 1986, and the Three Mile Island accident at Three Mile Island, Pennsylvania, USA, in 1979, although there have been partial core meltdowns at:
Not all of these were caused by a loss of coolant and in several cases (the Chernobyl disaster and the Windscale fire, for example) the meltdown was not the most severe problem.