[Prof_Sai]

This is one of the designs I was advocating. Sheesh.

Originally Posted by Wikipedia

Following the experience with AVR, a full scale power station (the thorium high-temperature reactor or THTR-300 rated at 300 MW) was constructed, dedicated to using thorium as the fuel. THTR-300 suffered a number of technical difficulties, and owing to these and political events in Germany, was closed after only four years of operation. One cause for the closing was an accident on 4 May 1986 with a limited release of the radioactive inventory into the environment. Although the radiological impact of this accident remained small, it is of major relevance for PBR history. The release of radioactive dust was caused by a human error during a blockage of pebbles in a pipe. Trying to restart the pebbles' movement by increasing gas flow led to stirring up of dust, always present in PBRs, which was then released, radioactive and unfiltered, into the environment due to an erroneously open valve.
In spite of the limited amount of radioactivity released (0.1 GBq 60Co, 137Cs, 233Pa), the THTR management tried to hide the accident, possibly because this accident pointed to some specific problems with pebble-bed reactors, mostly pebble flow and radioactive dust. The management might have thought that the emission would not be detectable due to the Chernobyl fallout happening at the same time. They continued to blame the Chernobyl fallout for all of the contamination found in the surroundings, until the presence of Pa-233 in the vicinity was detected. 233Pa is not formed in uranium reactors, such as Chernobyl, but only in thorium reactors (and also by natural spontaneous fissions with thorium nearby).[citation needed] Thus, step by step, the THTR management report lost all credibility.[citation needed] The radioactivity in the vicinity of the THTR-300 was finally found to result 25% from Chernobyl and 75% from THTR-300. The handling of this minor accident severely damaged the credibility of the German pebble-bed community, and pebble-bed reactors lost a lot of support in Germany.[18]