Absolute method for measuring β_eff and l in source driven subcritical facilities

The renewed interest in Accelerator Driven Systems (ADS) world-wide, has given a new increased effort in several related research domains. One of the most important aspects which has to be addressed is the safe operation of future ADS. Therefore an expert knowledge with regard to the stability and control of reactor power levels in ADS has to be obtained. The dynamic behaviour of the neutron population in a reactor core is determined by its dynamic parameters, among which the effective delayed neutron fraction β_eff and the mean neutron lifetime l are the most important ones. In this paper, an absolute measurement technique for the simultaneous determination of these parameters in a subcritical facility loaded with MOX-fuel and driven by an external/intrinsic source is presented. This method is based on the combination of two well-known techniques: the Rossi-alpha method for reactor noise fluctuation analysis at stationary subcritical level and the Prompt Jump analysis of the time evolution after source withdrawal. The combination of both techniques, called RAPJA (Rossi-Alpha & Prompt Jump Analysis), allows to determine the reactivity together with the dynamic parameters β_eff and l without relying on code calculations to provide absolute information on additional parameters. It is shown that the RAPJA-technique is applicable for the determination of the dynamic parameters in a subcritical facility loaded with MOX-fuel.