Experience acquired using the neutron absorbent 3He under transient conditions of fission power

To meet an increasing interest for fuel irradiations in BR2 reactor under transient conditions of fission power, the isotope 3He of helium gas is utilized as a variable neutron absorbent in various irradiation rigs. Fast and slow 3He pressure variations are carried out between 38 bar and 1 bar giving rise to fission power transients in the ratio of 1 to 1.75 for a 3He screen thickness of 2 mm. The following aspects are dealt with in this paper; the slow 3He pressure control by means of a metal bellow pressuriser; the fast 3He pressure control using high and low pressure tanks; the circulation and purification system using titanium sponge as a tritium absorber; the tritium release monitoring system. Observations are also reported with respect to the 3He neutron absorbing efficiency, the tritium leakrate, the tritium purification efficiency, and the tritium decontamination of 3He circuits. (Auth.)