Introduction

15.32. Gas-cooled reactors, such as the HTGR, are quite different from LWRs. The moderator is graphite, a solid, while the coolant, normally pressurized helium, is chemically inert and does not change phase. The graphite moderator remains stable up to very high temperatures, and has a high heat capacity, characteristics that permit the reactor to accommodate accidental transients safely. However, since power densities tend to be much lower than those for LWRs, the cores are relatively large, a disad­vantage. By using the modular concept, in which several units of modest power rating and easily manageable core size collectively generate the desired electrical capacity, the major safety advantages of the HTGR con­cept can be practically realized.

15.33. Many commercial reactors cooled with carbon dioxide were built in the United Kingdom and in France starting in the 1950s. The early ones were fueled with metallic natural uranium clad with a magnesium- aluminum alloy called Magnox. Later plants, designated as advanced gas — cooled reactors (AGRs), used low-enriched uranium oxide clad with stain­

less steel as fuel and carbon dioxide as coolant. The AGRs are able to achieve high coolant temperatures, which provides a thermodynamic ef­ficiency of over 40 percent. Newer AGRs built in the United Kingdom featured the use of a prestressed concrete reactor vessel (PCRV) for con­taining the whole primary system, consisting of the core, steam generators, and circulators [4].

15.34. While the AGR was being developed, attention was also given in various countries, including the United States, to the concept for an all­ceramic, helium-cooled, graphite-moderated, reactor using the thorium fuel cycle. This became the basis for the HTGR, the U. S. development of which was led by General Atomics. Other development efforts have been active in other countries, particularly in Germany. A 40-MW(el) demon­stration plant operated successfully at Peach Bottom, Pennsylvania from 1967 to 1974 using a prismatic core concept (§15.41). This was superseded by a 330-MW(el) commercial-size demonstration plant at Fort St. Vrain, Colorado, which operated from 1979 to 1989. During this period, numerous mechanical problems were encountered, all of which were resolved or well understood prior to shutdown. Meanwhile, in Germany, a 300-MW(el) plant, the THTR-300, demonstrated the “pebble-bed” core concept from 1987 to 1989. During the early 1970s, a number of orders were placed by U. S. utilities for large HTGRs. However, as a result of economic pressures existing at the time, these orders were canceled.