Visual examination of rods at AREVA's MELOX, MOX Fuel Manufacturing Facility

Here’s another great question on recycling that was submitted by Max Epstein:

I have heard the argument (including by at least one professor with a related PhD) that the benefits of reprocessing in terms of reducing volume of waste overstate the real benefits, because the reprocessed fuel comes out hotter (or ends up hotter after being run back through a reactor). Since the real constraint on storage capacity of any geologic site would be heat load, not volume of the casks, then the heat issue would be a problem if true. But obviously many experts do not seem to agree with this, which I presume means they do not agree that reprocessing and reusing fuel leads the eventual waste to be hotter. If you could help clear this up it would be much appreciated.

Answer by Gilles Clement, Vice-President of Recycling Technologies, and Dr. Alan Hanson, Executive Vice President of Technology and Used-Fuel Management:

Max,

You raise a very good point about the properties involved in used fuel storage. Heat load is indeed one of the most important elements to consider when accommodating spent fuel storage. In an underground repository, the heat load from waste packages emplaced in the galleries must be calculated in order to limit the strain on the repository structure. Such constraint may lead to leave significant open space between adjacent containers to reduce the linear heat load, which is a waste of repository capacity. .

The other point for storage consideration is the radioactive decay of the spent fuel materials and when to place these materials into a storage facility. There are two “heat load peaks” to consider, one due to the “short life” radionuclides and the other one due to the “long life” radionuclides. The short life radionuclide sees its radioactivity significantly diminished shortly, along with heat output, within decades. Waiting to store the spent fuel until this point greatly reduces the heat load to the repository. This same peak for long life atoms takes place much later, and it is practically ineffectual to wait.

When we process used fuel, we separate waste material from actually recyclable material. The waste material is composed of fission products and remaining minor actinides which are vitrified. Such packages generate much less long-term heat because the “major” actinides i.e: plutonium and uranium have been removed for recycling. Consequently vitrified waste packages can be stored much closer, thus maximizing the use of the very expensive repository space.

What you make reference to is probably used MOX fuel. MOX fuel is made from recycled plutonium after being unloaded from a reactor and recovered in a reprocessing plant. And you are right, this fuel comes out from the reactor hotter than the standard uranium used fuel. Used MOX fuel can itself be recycled another time, but there is a limit in the current generation of reactors (called thermal neutron reactors). If used MOX fuel is disposed directly in a repository, then the heat load constraint would inappropriately consume the repository volume. However, used MOX fuel should not be directly bound for storage in an underground repository. Used MOX fuel still contains a great amount of recoverable energy when it is recycled in a “fast neutron” technology reactor. It is a resource for the next generation of reactors which will make the best use of this energy content. Additionally, fast neutron reactors will “burn” the long-lived actinides further improving the optimization of repository volume, a very precious and costly asset.