Decay heat

Decay heat is the

Decay heat occurs naturally from decay of long-lived

In nuclear reactor engineering, decay heat continues to be generated after the reactor has been shut down (see

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]

Natural occurrence

Naturally occurring decay heat is a significant input to Earth's internal heat budget. 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.^[6]

Decay heat has significant importance in astrophysical phenomena. For example, the light curves of

where ${\displaystyle \tau }$ is the time since reactor startup, ${\displaystyle P(\tau )}$ is the power at time ${\displaystyle \tau }$, ${\displaystyle P_{0}}$ is the reactor power before shutdown, and ${\displaystyle \tau _{s}}$ is the time of reactor shutdown measured from the time of startup (in seconds), so that ${\displaystyle (\tau -\tau _{s})}$ is the elapsed time since shutdown.[9]

For an approach with a more direct physical basis, some models use the fundamental concept of radioactive decay. Used nuclear fuel contains a large number of different isotopes that contribute to decay heat, which are all subject to the radioactive decay law, so some models consider decay heat to be a sum of exponential functions with different decay constants and initial contribution to the heat rate.^[10] A more accurate model would consider the effects of precursors, since many isotopes follow several steps in their radioactive decay chain, and the decay of daughter products will have a greater effect longer after shutdown.

${\displaystyle {\frac {P}{P_{0}}}=\sum _{i=1}^{11}P_{i}e^{-\lambda t}.}$

The

• Decay energy
• Spent fuel pool
• Dry cask storage
• Radioisotope thermoelectric generator

References