Wikipedia
UHTREX
The Ultra-High Temperature Reactor Experiment (UHTREX) was an experimental gas-cooled nuclear reactor run at Los Alamos Scientific Laboratory between 1959 and 1971 as part of research into reducing the cost of nuclear power. Its purpose was to test and compare the advantages of using a simple fuel against the disadvantages of a contaminated cooling loop. It first achieved full power in 1969.
The experiment was a spin-off from the technology developed by the earlier ROVER project.