Quelques experiences relatives a la protection des piles a graphite refroidies au gaz et comparaison avec le calcul

Abstract

Experiments designed to yield information on lateral shielding and in particular on the sources of capture-gamma rays in graphite reactors were carried out in NAIADE I, which used as source a natural-uranium plate emitting 4 × 107 n cm−2 s−1. The experimental results obtained in different graphite, iron and concrete mock-ups were used to work out methods of calculation applicable to reactors. These methods were checked by means of measurements made in the lateral shielding of the EDF-1 reactor. In order to assess the damage liable to occur in pressure vessels, we related in a graphite-reactor spectrum the changes occurring in the mechanical characteristics of steel to the integrated flux measured by means of phosphorus detectors; the attenuation in the graphite of the flux responsible for the damage and the flux measured with the phosphorus detectors were compared. In reactors fitted with exchangers where the inside of the pressure vessel generally has to be accessible during shut-down, the activation of structural materials brought about by neutrons from perforated reflectors is a particularly important problem. A number of preliminary investigations have been carried out in the MARIUS assembly. These have mainly involved the study of fast-neutron sources inside the core. This work is due to be continued in a sub-critical mock-up being built at Fontenay-aux-Roses.