IEEE spectrum has an article about GTI’s work on a nuclear voltaic cell consisting, basically, of a semiconductor and an amount of radioactive material. They are using liquid semiconductors. Liquids don’t suffer any structural damage, so Tsang’s nuclear battery could run on much more powerful radiation than a beta cell, and therefore generate more electricity. The batteries are currently about 1% efficient. With submarine power plants in mind, DARPA wanted GTI to run full speed toward proving that a reactor of the 100- to 1000-kilowatt scale could be built. GTI decided not to work on scaling up yet.
Fuel assemblies cool in a water pond at the French nuclear complex at La Hague. The blue light is generated by Cherenkov radiation, which arises from a particle’s traveling through a medium faster than the speed of light in that medium