Decay heat

The decay of the short-lived radioisotopes such as iodine-131 created in fission continues at high power for a time after shut down.

[2] Because radioisotopes of all half-life lengths are present in nuclear waste, enough decay heat continues to be produced in spent fuel rods to require them to spend a minimum of one year, and more typically 10 to 20 years, in a spent fuel pool of water before being further processed.

[1] If no cooling system is working to remove the decay heat from a crippled and newly shut down reactor, the decay heat may cause the core of the reactor to reach unsafe temperatures within a few hours or days, depending upon the type of core.

These extreme temperatures can lead to minor fuel damage (e.g. a few fuel particle failures (0.1 to 0.5%) in a graphite-moderated, gas-cooled design[3]) or even major core structural damage (meltdown) in a light water reactor[4] or liquid metal fast reactor.

Radioactive isotopes of uranium, thorium and potassium are the primary contributors to this decay heat, and this radioactive decay is the primary source of heat from which geothermal energy derives.

For example, the light curves of Type Ia supernovae are widely thought to be powered by the heating provided by radioactive products from the decay of nickel and cobalt into iron (Type Ia light curve).

[8] An approximation for the decay heat curve valid from 10 seconds to 100 days after shutdown is where

[9] For an approach with a more direct physical basis, some models use the fundamental concept of radioactive decay.

The removal of the decay heat is a significant reactor safety concern, especially shortly after normal shutdown or following a loss-of-coolant accident.

Failure to remove decay heat may cause the reactor core temperature to rise to dangerous levels and has caused nuclear accidents, including the nuclear accidents at Three Mile Island and Fukushima I.

The failure of ESWS circulating pumps was one of the factors that endangered safety during the 1999 Blayais Nuclear Power Plant flood.

[12] Hence effective active or passive cooling for spent nuclear fuel is required for a number of years.