Every once in a while, something reminds me of old reactors. This, for example. But, more than that, there is something called a pebble-bed reactor that has sounded to me like it's a descendent of a couple of reactors that I was close to back in the day.
I check the Web from time to time, and I haven't been able to find much on them. Until today! One of them was the Ultra High Temperature Reactor Experiment (UHTREX), and the other was its antecedent, the Rover space propellant reactor. Wikipedia now has a reasonably accurate article on UHTREX, and I see that the Japanese are attributing the ancestry as I thought.
I couldn't find as much about Rover. There was a brief flurry of attention to it a few years back, for a Moon-to-Mars run, but little of that gave proper credit to the history.
When I was in the reactor division, one of my responsibilities was collating the quarterly progress reports. The higher-ups seldom sent them on, however, despite my and others' nagging, so the lack of Web information on UHTREX didn't surprise me. I told them that was what would happen!
And the stories that didn't make it into that little Rover reminiscence I linked! Like the time they fired up a Rover reactor to destruction at the Nevada Test site. The reactor was relatively compact, a bomb, you might say. And it made a little mushroom cloud when it disintegrated. Shortly after the atmospheric test ban treaty. There was some scrambling to explain that one.
The Rover fuel elements were quite elegant: pyrolytic (very hard) graphite, about three feet long, hexagonal cross section a centimeter or two across, and holes down the length. Even back in the eighties, there was an unloaded (no uranium) one in a conference room that we used for a pointer.