Accident and self-regulation studies of pulsed fast reactors

Abstract

A previously developed theory of pulsed fast reactor kinetics, inclusive of feedback effects, is reduced to a simple form and a standard method is applied to obtain a numerical description of pulsed excursions. This method is embodied in a fortran computer programme which executes the arithmetic for any desired number of cases and presents all physical variables of interest as a function of the time during a pulse. The main qualitative ideas underlying the theory are illustrated by numerical examples, in particular, the competition for dominance between a prompt positive Doppler and delayed axial thermal fuel expansion effect and the excitation of elastic waves in the fuel slugs are both brought out clearly.

It is shown that, in accident conditions, a fast pulsed reactor similar to the proposed Euratom system sora possesses a strong net negative feedback which makes the system self-regulating with respect to the fuel temperature, but that such feedback does not appreciably raise the reactivity pulse amplitudes responsible for serious damage. The effect of drifts in the base reactivity is discussed and the reaction of the safety shutdown system is considered. It is concluded that systems similar to the sora reactor have to be designed so that all possible drifts are far below 2 % Δk/sec.

In connexion with the general question of the choice of the fuel material for a pulsed fast reactor, it is shown that, from the safety viewpoint, the value of the coefficient of thermal expansion of the fuel can be ignored. Thus, all other things being equal, a ceramic is as good as a metallic fuel.