The measurement of the effective delayed-neutron fraction in the fast critical assembly BFS with uranium-plutonium metal fuel

Precise measurements of the effective delayed neutron fraction βeff in zero power critical assemblies close in composition to fast power reactor geometries have been of interest for some time. Measurements with uncertainties of ≤5% would allow appreciable refinement of the absolute scale of reactivity. In the BFS-facility for example, discrepancies in excess of 10% between experimental and calculated values of the central coefficients of reactivity of fuel are not infrequent. In the framework of franco-soviet collaboration, measurements of βeff (EROS experiment) were performed in Spring '89 in the BFS facility. Two different techniques were used. The first, based on the well known methods of determination of the californium source pseudo-reactivity, was carried out by the soviet team. The second technique, based on reactor noise measurements was carried out by the French team. This technique was first used during the BALZAC program in the MASURCA critical facility and, since the method is still under developement, the associated results must accordingly be considered as preliminary. All the measurements were performed on the one-zone assembly BFS 55-LA close in composition to the plutonium breeder with metallic fuel and sodium coolant