Methods for the prediction and achievement of a low circuit activity

While there are about 60,000 fuel pins in the core of an LWR power station, in a comparable HTR there would be over 1010 coated particles. This five orders of magnitude disparity in the number of basic fission product retaining units has led to important differences in the approach to the prediction and achievement of a low circuit activity. With the HTR, the very large number of units requires statistical treatment of the failure mechanisms involved in the escape of key fission products from the particles and in the assessment of the distribution in the core of burnup, dose and temperature. The methods created to deal with these factors have proved to be very powerful, leading to a situation in which the distribution of fission products within the core and the probability of experiencing a given release are better known for the HTR than for the metal clad fuel of other thermal systems. This advantage has been exploited and reinforced in the UK studies, first by the development of the technology of particle design (a combination of irradiation testing and theoretical analysis) and second by refining the study of fission product movement. This is a continuing process, but already it has usefully illuminated several aspects of design and operation, ranging from the specification and quality control of the fuel itself to the philosophy of outer containments. This paper is intended to summarise this progress, the account being mainly concerned with normal operation. (author)