The concept of the high-temperature gas-cooled reactor with a prismatic core construction was first seriously investigated at the Atomic Energy Research Establish­ment, Harwell, early in 1956. At about the same time work started in Western Germany on the so-called pebble-bed version of the system in which the core consisted of a randomly packed bed of spherical fuel elements. The Harwell work resulted eventually in the construction of the 20-megawatt reactor experiment of the OECD High Temperature Reactor Project (DRAGON) at Winfrith, England; the reactor reaching its nominal power in April 1966. Also resulting indirectly from the Harwell concept, the Peach Bottom Reactor of the Philadelphia Electric Company, developed and designed by General Atomic of San Diego, achieved full power operation in May 1967. These two experimental high-temperature reactors were quickly followed by a third when in February 1968, the AVR pebble-bed reactor, built by the Brown Boveri/Krupp Reaktorbau GmbH, for Arbeitsgemeinschaft Versuchsreaktor utility group, came into operation at the Kernforschungsanlage, Jiilich.

Over the subsequent years, these three reactors have been operated with consider­able success, demonstrating in a convincing manner the feasibility of the high- temperature reactor and confirming the favourable characteristics claimed for it. Now a much larger version of the prismatic core HTR has been brought into operation at Fort Saint Vrain near Denver, Colorado. This plant, like the Peach Bottom Reactor, is a product of the General Atomic Company and it is intended to generate 300 megawatts of electric power on the network of the Public Service Company of Colorado. A large pebble-bed reactor of similar power output, the Thorium Hoch-Temperatur Reaktor (THTR), is under construction at Schmehausen in West Germany.

The special features of the high-temperature reactor are its use of helium as a coolant and the manner in which graphite is employed not only as a moderator for the neutrons, but also as the structural material of the reactor assembly, troublesome metal cladding and tubing being entirely eliminated from the core. Fissile and fertile materials are present as oxide or carbide kernels of the coated particles. These particles, up to about 1 mm in diameter with pyrocarbon and silicon carbide coating layers to contain and prevent the escape of fission products, are incorporated into the graphite structure of the fuel elements by consolidating them in a matrix of graphite powder and resin moulded under pressure and then carburized at high temperature to form robust fuel bodies.

The excellent characteristics of the HTR results from this combination of helium, graphite and coated particle fuel. Very high temperatures in excess of 800°C for the mixed core outlet gas and even above 1000°C for hot channels have been sustained over

extended periods of operation in the experimental reactors. This makes it possible not only to run the reactor in conjunction with the most advanced steam power plant, but also to contemplate the direct use of the helium in gas turbines and to consider the application of the very hot helium to carry out important industrial processes, giving the HTR great development potential.

A further merit of the HTR stems from the refractory nature of the graphite and its enormous thermal capacity coupled with negative reactivity temperature coefficients which are readily achievable. This makes the reactor particularly insensitive to power excursion and loss of coolant accidents. The HTR is consequently a very safe type of reactor. It is also clean and accessible, a consequence of the remarkable degree of fission product retention of the consolidated coated particle fuel and the absence of activated material arising from the corrosion and erosion of metal cladding and other primary circuit materials that can be a source of trouble in reactors having liquid coolants.

The physics of the high-temperature reactor is of special interest and significance because of its versatility with regard to fuel cycles. Coated particle fuels can use any of the fissile materials and either uranium-238 or thorium as fertile material. According to the level of fissile enrichment of the fuel kernels, the size and volume loading of the coated particles, the ratios of carbon to fissile and fertile atoms and the degree of heterogeneity of the core are variable to an extent that is impossible in other reactors. Absence of metal cladding greatly improves the neutron economy in comparison with other types of reactor, a factor which helps in the achievement of high conversion ratios. Furthermore, it has been demonstrated beyond all doubt that coated particle fuel can sustain a degree of burn-up which is quite unmatched in any other reactor system. The result of all these factors is a reactor which can be readily adapted to the changing economics of the uranium market. In the same reactor, for example, one can change from a low enriched uranium cycle, with a very high burn-up, to a thorium cycle, possibly sacrificing burn-up for a high conversion ratio to match changing cir­cumstances with respect to the cost and availability of uranium ore, separative work and reprocessing capacity.

This book is concerned with the physics aspects of the high-temperature reactor, a subject on which there has so far been the lack of a systematic and comprehensive text despite the very considerable volume of reports that have been written on the various aspects of the subject. Several years ago the Dragon Project decided to sponsor such a work with the co-operation of the Commission of the European Communities (Directorate-General for industrial and Technological Affairs) and invited Dr. Massimo to undertake the authorship.

Dr. Massimo is well qualified to write on the subject having been a member of the Dragon Project Staff from 1959 to 1963 during which time he was involved in the early physics assessments of the 20-MW Reactor Experiment. He was also attached by the Dragon Project for a short period to the General Atomic Company where he participated in the work on the Peach Bottom Reactor. Subsequently, as a member of the staff of Brown Boveri/Krupp, he was engaged in the design study of the THTR reactor.

In his book, the author describes the state of the art, at the time of writing, in the field of HTR reactor physics. Following a survey of the basic theory of modern reactor physics, the book covers the methods and computer codes employed in high —

temperature reactor calculations by the Dragon Project, and by the various groups working in the subject in European and American national research centres and in industry.

High-temperature reactor physics, having reached a state of maturity, is nevertheless still evolving. Any book, such as this, therefore, cannot maintain indefinitely an up-to — the-minute presentation of the subject. However, the book is aimed primarily at the students of HTR physics who are preparing to enter the field as well as technologists of other disciplines who are working on the system and in this role it should remain a useful and relevant text book for many years.

Winfrith, Dorset

July 1975

L. R. Shepherd

Chief Executive, OECD High Temperature Reactor Project (DRAGON)