Editing Pebble-bed reactor (section)

==== Thorium high-temperature reactor ====
{{Main|THTR-300}}

Following the experience with the AVR, Germany constructed a full scale power station (the thorium high-temperature reactor or [[THTR-300]] rated at 300 MW), using thorium as the fuel. THTR-300 suffered technical difficulties, and owing to these and political events in Germany, was closed after four years of operation. An incident on 4 May 1986, only a few days after the Chernobyl disaster, allowed a release of part of the radioactive inventory into the environment. Although the radiological impact was small, it had a disproportionate impact. The release was caused by a human error during a blockage of pebbles in a pipe. Trying to restart the pebbles' movement by increasing gas flow stirred up dust, always present in PBRs, which was then released, unfiltered, into the environment due to an erroneously open valve.{{Citation needed|date=January 2021}}

In spite of the limited amount of radioactivity released (0.1 GBq [[Cobalt|{{chem|60|Co}}]], [[Caesium|{{chem|137|Cs}}]], [[Protactinium|{{chem|233|Pa}}]]), a commission of inquiry was appointed. 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, which lost support in Germany.<ref>Der Spiegel (German news magazine), no. 24 (1986) p. 28–30</ref>

The overly complex design of the reactor, which is contrary to the general concept of self-moderated thorium reactors designed in the U.S., also suffered from the unplanned high destruction rate of pebbles during the test series and the resulting higher contamination of the containment structure. Pebble debris and graphite dust blocked some of the coolant channels in the bottom reflector, as was discovered during fuel removal after final shut-down. A failure of insulation required frequent reactor shut-downs for inspection, because the insulation could not be repaired. Metallic components in the hot gas duct failed in September 1988, probably due to thermal fatigue induced by unexpected hot gas currents.<ref>R. Baeumer, THTR-300 Erfahrungen mit einer fortschrittlichen Technologie, Atomwirtschaft, May 1989, p. 226.</ref> This failure led to a long shut-down for inspections. In August, 1989, the THTR company almost went bankrupt, but was rescued by the government. The unexpected high costs of THTR operation and the accident ended interest in THTR reactors. The government decided to terminate the THTR operation at the end of September, 1989. This particular reactor was built despite criticism at the design phase. Most of those design critiques by German physicists, and by American physicists at the National Laboratory level, went ignored until shutdown. Nearly every problem encountered by the THTR 300 reactor was predicted by the physicists who criticized it as "overly complex".{{Citation needed|reason=What physicists?|date=October 2015}}