Decay heat

Decay heat is the heat released as a result of radioactive decay. This heat is produced as an effect of radiation on materials: the energy of the alpha, beta or gamma radiation is converted into the thermal movement of atoms.

Decay heat occurs naturally from decay of long-lived radioisotopes that are primordially present from the Earth's formation.

In nuclear reactor engineering, decay heat continues to be generated after the reactor has been shut down (see SCRAM and nuclear chain reactions) and power generation has been suspended. The decay of the short-lived radioisotopes such as iodine-131 created in fission continues at high power for a time after shut down.^[1] The major source of heat production in a newly shut down reactor is due to the beta decay of new radioactive elements recently produced from fission fragments in the fission process.

Quantitatively, at the moment of reactor shutdown, decay heat from these radioactive sources is still 6.5% of the previous core power if the reactor has had a long and steady power history. About 1 hour after shutdown, the decay heat will be about 1.5% of the previous core power. After a day, the decay heat falls to 0.4%, and after a week, it will be only 0.2%.^[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. However, the heat produced during this time is still only a small fraction (less than 10%) of the heat produced in the first week after shutdown.^[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. Chemical species released from the damaged core material may lead to further explosive reactions (steam or hydrogen) which may further damage the reactor.^[5]