When reactor no. 4 at Chernobyl blew up on 26th April 1986 there was a second reactor failure about a week later, this time in West Germany on May 4th. A fuel “pebble” in the pebble-bed THTR-300 reactor got stuck and broke up, releasing noticeable amounts of radioactivity into the environment. The THTR-300 operators tried to cover this up, hoping no-one would notice it in the much larger cloud of radioactive fallout from Chernobyl but isotopic analysis revealed another reactor of a different type had gone bonk.

Lots of countries have put a great deal of effort into detecting and analysing very very small amounts of fission products, driven by nuclear weapons development and the desire to know what the Other Side is up to.