The history of carbon-moderated reactors is old and mixed. The world's first nuclear reactor was the graphite moderated reactor developed by Enrico Fermi in Chicago during World War II. Reactors for plutonium production have been primarily graphite moderated, because the conversion ratio is high, and it is relatively easy to change fuel elements frequently and avoid a large buildup of 240Pu. Graphite-moderated reactors can be either water cooled, as in the Hanford plutonium production reactors and the Chernobyl-type RBMK reactors, or gas cooled, as in the British CO2-cooled reactors and in the helium-cooled Fort St. Vrain reactor in the United States. The few graphite moderated power reactors in the United States have been shut down, but some are operating in the United Kingdom, Russia, and Lithuania (see Section 8.1.4).
The safety claims for the HTGRs might seem to fly in the face of the fact that the only reactor accidents that have resulted in major releases of
12 The Very-High-Temperature Reactor (VHTR) is designed to reach 850° C to 1000°C (see Section 16.6.2 and Section 20.3.2).
activity have been in graphite-moderated reactors, namely the Windscale and Chernobyl accidents. However, it is argued that what happened at these plants has no relevance to the planned HTGRs.
♦ Windscale. The HTGRs will run at higher temperature than did Windscale and there will be no buildup of stored energy in the crystal lattice (the so-called Wigner energy) because the graphite will be continually annealed. The temperature for annealing is about 350°C [35, p. 441], well below the normal graphite temperature in an HTGR.
♦ Chernobyl. In addition to other major design differences, the use of a helium coolant in the HTGR (rather than water, as at Chernobyl) means that loss of the coolant cannot give a positive feedback. This follows from the fact that helium has a negligible absorption cross section for neutrons and therefore, unlike the water at Chernobyl, cannot be a poison.
It might also be noted that the only significant electricity-producing HTGR in the United States, a 330-MWe prototype unit at Fort St. Vrain in Colorado, had an unusually trouble-plagued life after going into operation in 1979.13 It was shut down in 1989 by the operating utility because it was not economical to continue to run it. The difficulties were primarily with the cooling system, and it is believed that these difficulties can be avoided in a next-generation helium-cooled reactor.
In light of the above-described history, one might imagine that the nuclear industry would shy away from further attempts to develop HTGRs. However, the arguments that the Windscale and Chernobyl experiences are not relevant to future HTGRs appear to be convincing, and there are strong believers in the HTGR as a very safe reactor for the future.
Part of this confidence is based on experience with a series of prototype pebble bed HTGRs built in West Germany. The first of these was the AVR reactor, which was put into operation in 1967 to test the HTGR concepts. It was a small reactor, only 40 MWt and 15 MWe. A 300-MWe pebble bed HTGR, the THTR-300, was put into operation in 1987. These reactors have provided experience on the behavior of HTGR fuel. However, further development work on the German HTGR systems was halted in early 1991, due to lack of commercial interest . Interest was later revived by the South African company, Eskom, which has been actively considering building and marketing PBMR reactors. In addition, exploratory initiatives have recently been undertaken in Asia with the construction of small HTGRs in Japan and China: a 30-MWt prototype high-temperature test reactor (HTTR) that started up in Japan in 1998  and the 10-MWt HTR-10 pebble bed reactor that started up in China in December 2000 .
13 In addition, a much smaller HTGR—the 40-MWe Peach Bottom 1 reactor— operated from 1967 to 1974.
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