Hyperion Power Generation (HPG) company has announced the commercial development of their Hyperion Power Module.  While there are numerous reports on the internet, it is more useful for curious and tech savvy folk to read the patent application (US 20040062340) for a detailed description of the device. While the idea has been knocking around for 50 years, it took the inventor, Dr. Otis G. Peterson, to work out the control issues for a safe, self regulating system.

The reactor uses the hydride of a fissile actinide like U-235 (as UH3 powder) at ~5% enrichment in U-238 to serve as a self-moderating nuclear pile. The marvels of chemistry, namely chemical equilibrium, play a large role here because the hydrogen content (as hydride) varies as a function of temperature. An increase in temperature of the UH3 leads to loss of hydrogen from the U to another hydrogen storing metal. Loss of hydrogen moderator leads to loss of reactivity and a downturn in heat generation. But the downturn in heat generation favors the return of hydrogen (as H2) to the uranium to make hydride. This causes the reactivity of the system to increase, so the rate of fission and heat generation rises as a result.

The system eventually reaches a steady state temperature where the rates of hydrogen gain and loss from uranium become equal and the rate of heat evolution reaches a steady output.

According to Table 1 of the appln, at 5 MW thermal the U-235 critical mass is 30 kg and at 50 MW thermal it is 215 kg. The table also discloses that at a loading of 30 kg U-235 the energy content is 78 MW years and at a loading of 215 kg U-235 the energy content is 540 MW years.

Of course, this is a patent and not a peer reviewed publication. But it was developed at Los Alamos so one would suppose it should have some credibility. The patent suggests that the reactor would be buried underground while in service. It is unclear if that is for shielding or security, or both.