# Decay energy of Reactor Waste

• lordentropy
In summary, the decay energy of fission products taken out of a Light Water Reactor (LWR) is about 5% of the nominal power of the reactor immediately after shutdown, dropping to less than 1% after 2 hours. This heat production decreases over time and after 30 years, a ton of spent fuel will only produce about 500 W of heat. Vitrification of the waste reduces the volume and heat production is similar to that of a desktop PC. There are various standards for calculating decay heat, with the NRC Branch Technical Position ASB 9-2 being easily accessible. However, the low heat production makes it difficult to find practical applications for it.

#### lordentropy

Assuming a Light Water Reactor, what would be the decay energy of the fission products taken out of a reactor (the reactor's nuclear waste) and it's thermal yield?

What I am getting at is, there is all this talk of safe storage of nuclear waste...but that nuclear waste is still emitting heat and instead of just storing it, I want to see if I can get it to do some work...thus how much power (electricity) can someone get out of the waste and depending on that power...depends on the applications it can be used for.

so is their anyone out there that can help me out...I am just a beginner at nuclear physics, and need a little bit of extra help
Thanks!

lordentropy said:
Assuming a Light Water Reactor, what would be the decay energy of the fission products taken out of a reactor (the reactor's nuclear waste) and it's thermal yield?

When a reactor is shut down, in the few seconds after the shutdown, the radioactive decay still gives about 5% of the nominal power of the reactor, and this drops to less than 1% after 2 hours or so. It drops pretty fast in the beginning, and then levels off to almost constant, low power production.

Note that during normal working, the radioactive decay also contributes to the power of the reactor. It is only at shutdown that heat gets lost.

A ton of used fuel will give rise, I think, to about 2 KW of heat production after 4 years, and to 500 W after 30 years. If one vitrifies the waste in 1 ton of spend fuel, that takes about 280 kg of material (the uranium and plutonium have been removed, only the actual radioactive waste is mixed with glass). Concerning volume, you can say that the heat production of this vitrified waste per unit of volume is more or less the same as that of a desktop PC. There's not much you can do with that... except maybe making some lukewarm water.

There are many "standards" for calculating the decay heat. ANS 5-1 in its various revisions is used in many safety analyses, but the current versions require a fair amount of knowledge on the specifics of the core design and operation. The NRC Branch Technical Position ASB 9-2 is a formulation easily applied in a spreadsheet type calculation, and it is easily available from the NRC website. Look for SRP 9.2.5 (Ultimate Heat Sink) Rev 2 in NUREG-0800. Beware a typo in the equation 2; the uncertainty term should multiply both terms on the R.H.S.

As Vanesch says above, you're not likely to make much use of the decay heat in any real-world application. But have fun learning more about decay heat. If you're doing searches, the term "residual heat" is also often used.