TRIGA

The TRIGA (Training, Research, Isotopes, General Atomics) was born in the late 1950s in San Diego, from a team led by Freeman Dyson under Edward Teller's safe-reactor group. The brief was a reactor that could not be damaged even by the clumsiest student error. The Mark I prototype started up on May 3, 1958, and 66 have been installed since, half in the United States and the other half in 23 more countries, at universities, hospitals and institutes.

The secret is the fuel: uranium zirconium hydride (UZrH), in which the moderator, the hydrogen of the hydride, is mixed with the uranium inside the same element. When the fuel temperature rises, the neutrons moderated in it gain energy and the hydride becomes a worse moderator; the fission probability drops and neutron leakage increases. That gives a large, prompt negative temperature coefficient that acts in milliseconds, with no rods and no operator. That is why a TRIGA can pulse: a rod is ejected at once, power climbs to 22,000 MW for a few milliseconds and the fuel itself kills the spike. In steady state they run from 0.1 to 16 MW. They were born with highly enriched uranium; since 1978 the RERTR program has converted them to under 20%.