| Nuclear Meltdown |
Article Index for Nuclear |
Website Links For Nuclear |
Information AboutNuclear Meltdown |
| CATEGORIES ABOUT NUCLEAR MELTDOWN | |
| nuclear safety | |
| nuclear accidents | |
|
A nuclear meltdown occurs when the core of a Nuclear Reactor melts, and is generally considered a serious Nuclear Accident . The reactor core may melt through the floor of the reactor chamber and drop down. It continues down until it melts enough surrounding material that it is diluted and cooled to a temperature no longer hot enough to melt through the material underneath, or until it hits Groundwater . Note that a Thermonuclear Explosion does not happen in a nuclear meltdown, although a Steam Explosion may occur, if it hits water. As the geometry and presence of water is crucial to maintaining the chain-reaction, the molten core cannot form an uncontrolled Critical Mass (a recriticality). However, the molten Reactor Core will continue generating enough heat through radioactive decay (' Decay Heat ') to maintain or even increase its temperature. One possibility is that a large steam explosion occurs when the molten mass encounters water (in the lower plenum or in the reactor cavity) after melting, it depends on the substance underneath (e.g. a huge explosion if it hits a methane filled cavern). CAUSES In Pressurized Water Reactor s, Boiling Water Reactor s, RBMK s, and Breeder Reactor s, the core can melt as a result of a Loss Of Coolant Accident (in which all emergency core cooling systems have failed). A similar circumstance is created should the steam generator secondary dry-out together with emergency system failure. A rapid loss of water from the reactor system naturally stops the chain reaction. Borated water is injected by the emergency systems and thus in the large-break accidents, control rod insertion is not needed to stop the Fission reaction. Smaller breaks do need the Control Rod s to fully insert because water stays in the core and the highly borated water from the emergency systems is naturally delayed. However, Radioactive decay of the fission products in the fuel ceramic will continue to generate heat. This heat (7% decreasing exponentially to 3% of full power) can cause the reactor core to melt within an hour after coolant flow is stopped. Other sources of heat may be present in a nuclear reactor core. If the reactor vessel has been breached and air enters the reactor, core material such as Graphite or Zirconium may burn, sharply heating the core (as in the Chernobyl Accident and others). If the reactor contains graphite (such as at Chernobyl) and appropriate care is not taken, Wigner Energy may accumulate, to be suddenly released (as occurred in the Windscale Fire ). 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). 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. 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. 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 Exceedingly Unlikely , and to contain one should it occur. In a modern reactor, a nuclear meltdown, whether partial or total, will be contained inside the reactor's containment structure. Thus (in the unlikely event that no other 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 {Link without Title} ). In practice, however, a nuclear meltdown is often part of a larger chain of disasters. 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). 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 Feature s that would be much less susceptible to meltdown, even if all emergency systems failed. For example, Pebble Bed Reactor s 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. MELTDOWNS A number of Russia n Nuclear Submarine s have experienced nuclear meltdowns. The only known large scale nuclear meltdowns at civilian nuclear power plants were in the Chernobyl Accident at Chernobyl , Ukraine , in 1986 , and Three Mile Island , Pennsylvania , USA , in 1979 although there have been several partial core meltdowns, including accidents at:
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". {Link without Title} SEE ALSO
REFERENCE
EXTERNAL LINKS
|
|
|