The Hyperion Power Module continues to generate discussion. Many blog posts on the topic are mere repetitions of the same stories, right down to the misquotes about cost per unit energy.
There is a good article posted on Physicsworld.com with answers to some of the questions that people have asked about the HPM. I highly recommend going and reading that article and taking a look through the comments being generated. For those of you who are interested in small reactors, even if you are interested in using different concepts than the Hyperion, it is good to get a feel for the kinds of questions that will be posed and the kinds of cheering that you might receive.
One claim made by Hyperion on their web site and repeated in several of the articles that I have found in recent weeks on the topic is the following:
“The waste produced after five years of operation is approximately the size of a softball and is a good candidate for fuel recycling.” http://www.hyperionpowergeneration.com/
The claim seemed a bit exaggerated to me. I am fascinated by fission and impressed with the energy density, but producing 70 MW of thermal energy for five years with material the size of a softball is not possible. To a reasonable approximation, the volume of the byproducts must be at least as large as the initial fission fuel – and will actually be a bit larger because of the gaseous fission products.
Here are my computations of the minimum waste volume:
Uranium hydride (UH3) has a density of about 10.5 gm/cc.
Fission produces about 1 MW-day for every gram U fissioned. There is almost a 1-1 relationship between gms of uranium hydride and grams of uranium since the mass of the molecule is mostly U.
Five years (1825 days) of operation at 70 MWth thus requires about 128 kilograms of UH3. That requires a volume of about 11,700 cc.
According to the international softball rules, the standard softball has a circumference of 30.5 cm.
Using the old standby formulas from geometry, that gives a softball volume of 479 cc. Using old fashioned division, that results in a waste volume of about 25 softballs, not one. That is still an impressively small waste volume for a power plant producing enough heat to power a city of 20,000 residents, but it is quite a bit different from a single softball.
There is a prize for anyone who can point out any major errors in the above assumptions or computations. I still have a few Adams Atomic Engines polo shirts and would be willing to part with one to the first commenter with a correction. This is what the polo shirt logo looks like: