Decay heat is a type of energy that is released in a nuclear reactor both before and (most importantly) after it shuts down. When large atoms like Uranium or Plutonium fission, the vast majority of the energy released comes from the two halves (called fission products) flying out at extreme speeds and slowing down in their material, heating it up. That’s called the kinetic energy of the fission products. But, once slowed down, the two fission products that remain are often radioactive, meaning they decay with time, producing a little bit more heat. When the control rods enter a nuclear core to shut down the chain reaction, all fission stop and the fission products stop flying around releasing the majority of the heat, but the fission products remain radioactive and will produce heat no matter what -- there is absolutely no way to stop them.

As physics would have it, the decay heat power level is usually about 6-7% of the full power of the reactor immediately after a shutdown. Then it decays exponentially such that it’s at 1% within a day and continues to drop. The problem is, in a 3 large GWt plant, 1% of full power is still 30 million Watts.

The Safety Issue

Since there’s effectively no way to immediately shut a nuclear reactor all the way down, the cooling systems must operate in some fashion after a shutdown or else the fuel will heat up above its melting point and...melt, possibly releasing radioactive nuclides into the environment. This is what happened at Fukushima. Emergency diesel generators as well as some passive systems are typically relied upon to provide this cooling. In some advanced designs such as sodium-cooled fast reactors, the large vat of liquid metal allows natural circulation to provide all the decay heat removal without any generators or pumps. Liquid fueled reactors such as the LFTR are sometimes proposed to have online fisson product removal, so the plant truly can shut all the way down immediately. Advanced light water reactors have large pools of water high up where gravity can provide cooling for a long time if the generators fail.

In risk assessments, loss of decay heat removal accidents are usually the highest-risk scenario to release radiation to the public. Innovations in getting rid of decay heat are therefore welcome. Discuss any ideas you may have in the forum!.

Explore!

Find out how much energy comes out in which form down at the good old National Nuclear Data Center. Click the link, then click a fissionable element like U or Pu. Then click a good isotope like U-235 or Pu-239. Then click the little link that says "Interpreted" right next to (n,fis.ene.release). Hours of fun.