Spent Nuclear Fuel

This is a reprint of an article written for SAM group. The first article was an overview of nuclear power.

The last time I wrote here, I filled up my space with an explanation of the basics of nuclear power technology. I left with a cliffhanger on what to do with spent fuel.  I was not ignoring or lightly treating the question. It deserves a column all its own for more complete discussion. Trying to tack it on to the end of that column would have been a disservice to all.

Before we can discuss options for spent fuel, we must have a common basis of understanding. So back to the reactor for a moment. All fission reactors work essentially the same way, they split a larger atom (usually Uranium) into two smaller atoms and two or three extra neutrons. The amount of uranium loaded into a core at the start of a typical two year cycle is about 50 metric tons. Of that, less than 2.5 metric tons is U-235 (the isotope that splits most easily), the rest is U-238. Typically, this fuel would stay in reactor six years or so and generate over 21,000,000 MWhr of electricity. When the fuel is removed, there is still about 48 metric tons of uranium, but only about 0.5 metric tons of U-234. However, there is now about 0.5 metric tons of plutonium that was generated when the U-238 picked up a neutron and lost an electron. So of the initial 50 metric tons of uranium, at the end of six years of operation we still have about 48.5 metric tons of material that could still be used in a reactor (uranium and plutonium). Only 1.5 metric tons are the fission remnants (called actinides).

To put this on an annual basis, each year, each reactor in the US generates, on average, 24.25 metric tons of reusable material and 0.75 metric tons of actinides. For a total spent fuel mass of about 25 metric tons. With 104 operating reactors, that means about 2600 metric tons of material is generated, with about 2522 metric tons of reusable material and about 78 metric tons of true waste material. If all 2600 metric tons was stored in one place, it would require about 3200 square feet of storage 10 feet deep – an area less than the end zone of a football field. If we consider only the material that cannot be reused, this space drops to an area about 96 square feet.

All of the spent material from every operating reactor in the US for their entire operating life can be contained on one football field to a depth of about 17 feet. Of this, only one end zone is needed for the waste material.

So what do we do with the stuff? I’m going to stay out of the politics of this and stick with technical options that are available today. A blue ribbon panel appointed by President Obama and Secretary of Energy Steve Chu are beginning to process of considering these options, singly or in combination, to make a recommendation.

1. Reprocess the material and extract the usable fuel components – uranium and plutonium. This is being actively pursued in France and Japan and several other countries around the world. The US has not pursued this option for fear of proliferation risk, but could at some point in the future.
2. Store the material in a central repository with the idea of being able to retrieve it at some point in the future if option 1) is ever exercised. This was the Yucca Mountain repository. Part of the reason it was selected as the site to consider for long term storage was that it provided the option of retrieval.
3. Store the waste in a permanent non-retrievable depository. There is a salt mine in New Mexico that is doing just that with some government materials. However, once the material becomes encased in salt retrieval is impractical. There are a number of additional options in this category.
4. Store the waste in situ at each reactor site until option 1) and 3) are fully up and operational. This has become the de facto solution on the ground. Most of the spent fuel is being stored in deep pools of water. These pools allow continued cooling of the material as decay of radioactive elements occurs. At most sites in the US, additional dry cask storage has been constructed to store some of the oldest fuel. These dry case storage yards are well within the plant’s security boundaries and present no additional radiation risk to the public or to the workers at the nuclear site.

Considering that many industries take the stance of dump and run with the toxic wastes that they generate, the nuclear industry has worked hard to keep all of the toxic waste materials from its industry contained and controlled. All of these options require manufacturing, construction and engineering design work. As the nuclear industry continues to move toward an expanded energy role, all kinds of industries will be needed to provide support. SAM group can help you figure out what you need to do to get started.