| The mightiest|
of them all
Core meltdown (commonly known as a meltdown) is an accident scenario in nuclear reactors, and is one of the possible modes of failure for light water reactors, during which the reactor pile turns into a pile of reactor.
What a meltdown is
A meltdown happens when insufficient cooling of the core causes the fuel elements to melt from the heat of nuclear reactions. During normal operation of a LWR, meltdown is not possible, because overheating of the core causes water to boil, which removes the moderator and the chain reaction automatically slows down, reducing the power. However, in a reactor that has been operating for some time, completely removing water from the core does not reduce power to zero. Some heat is still generated by the decay of fission products. This heat is sufficient to melt the fuel elements, release radioactive fission products into the cooling water, and cause the zirconium cladding to react with water, generating explosive hydrogen.
What a meltdown is not
- Big explosion. A meltdown is not an explosion of any kind. The reaction of overheated fuel cladding (zirconium) with water generates hydrogen, which can explode if not vented properly. This hydrogen is what caused explosions in Fukushima. Hydrogen explosions have nothing to do with nuclear explosions and are millions of times less powerful.
- Chernobyl disaster. Chernobyl was caused by a steam explosion and unintended nuclear excursion, and the radiation was spread over large areas by a graphite fire. Light water reactor cores do not contain graphite.
- An event that makes large areas of land inhospitable. This would only happen if the molten fuel was dispersed in the environment, which is independent of a meltdown.
Meltdowns in modern and future reactors
Modern reactor designs have several mechanisms that prevent a meltdown from occurring. This includes having multiple independent cooling systems, each capable of keeping the reactor core submerged on its own; not having any welds in the pressure vessel below the top of the core; elaborate instrumentation and control equipment; and "core catchers" under the pressure vessel - concrete structures that are designed to capture and redirect the flow of corium (the lava-like molten core material) when the meltdown does happen.
The maximum core damage frequency (which means any damage of the fuel elements, not an all-out meltdown) of currently operating Generation II reactors is lower than once per 10000 years. Generation III designs, of which a few are already deployed and will make up the majority of reactors built over the next decade, reduce the probability even further. However, since these numbers are usually provided by reactor manufacturers, they should be taken with a grain of salt.
|Reactor||CDF/plant year ||MTBCDE|
|BWR/4 (Gen II)||1 × 10–5||10 000 years|
|BWR/6 (Gen II)||1 × 10–6||100 000 years|
|ABWR||2 × 10–7||5 000 000 years|
|ESBWR||3 × 10–8||33 000 000 years|
|AP1000||2.4 × 10–7||4 100 000 years|
|EPR||5.3 × 10–7||1 900 000 years|
CDF means "core damage frequency", MTBCDE means "mean time between core damage events". The total operating experience of the world's nuclear power plants is 14719 reactor years (as of January 2012). This does not include research or experimental reactors.
Core meltdown is a physical impossibility in some future designs of reactors. This includes the Liquid Fluoride Thorium Reactor (LFTR), where the fuel is in liquid form, and the Integral Fast Reactor (IFR), where fission stops due to thermal expansion of the fuel and decay heat is removed by convection of sodium. Passive shutdown of the IFR was successfully tested in 1986 using the EBR-II prototype at Argonne National Laboratory.
- ↑ These numbers for internal events only, which roughly means "core damage not caused by natural disasters". CDF for all events is considered to be less than twice that of internal events, but the impact of natural disasters is considerably harder to model.
- ↑ The current number is available from the main page of World Nuclear Association.