12.5.1 Introduction

This section of Chapter 12 looks at the design of the high-temperature gas — cooled reactor (either HTR or HTGR), which was perceived to be a potential successor to the advanced gas-cooled reactor. The HTR is significantly different to the AGR and the Magnox designs as is discussed in more detail below but the fundamental difference is that the HTR was designed to operate at higher temperatures than the AGR resulting in further increased thermal efficiency (in the region of 50%).

The HTGR design was first proposed in 1947. The prototype Peach Bottom reactor in the United States was the first HTGR to produce electricity from 1966 through 1974. Fort St Vrain in the States was one example of this design that operated as an HTGR from 1979 to 1989 though this is now decommissioned and no further HTGRs have been built there since.

Small scale HTGRs have also been built in Germany (the AVR and THTR-300, which was actually a commercial design), and currently exist in Japan and China. Two full-scale HTGRs, each with 100-195 MW of electrical output are under construction in China. More recently, this reactor design type has been substantially updated and is now proposed in a form known as the very high-temperature reactor in the United States.

The pebble bed modular reactor (PBMR) is a particular design of HTGR, which was under development by a South African company. The project was for the construction of a demonstration power plant at Koeberg, near Cape Town, although this has now been postponed indefinitely.

Commercial HTGRs have never built in the United Kingdom. There was a prototype, the Dragon reactor, built at the United Kingdom Atomic Energy Authority (UKAEA) site at Winfrith as a test bed for fuel and other materials. Operation of Dragon started in 1965 with a power output of 20 MW but it was shut down in 1976.

Development of the HTGR in the United Kingdom was terminated following a comprehensive review by the government, in the 1970s, of which thermal reactor strategy should be adopted for the future. As such, there is no detailed design information available. Consequently, the sections below describe the basic design principles of the HTGR.