Analytical investigation and experimental application of the source modulation technique to measure ρ/β_eff

In recent years great interest has been displayed, worldwide, for Accelerator Driven Sub-critical reactors (ADS) to incinerate the minor actinides generated by existing energy-producing reactors. In sub-critical systems, the effective neutron multiplication factor is lower than 1.0 and the neutrons otherwise required to maintain the chain reaction can be put to other uses, in particular, the destruction of nuclear wastes such as minor actinides (MA). One of the major advantages of such ADS systems is that it can be operated with very high MA content without jeopardizing the overall safety due to a small effective delayed neutron fraction, a small Doppler temperature coefficient and possibly also a large void coefficient depending on the chosen coolant. This enhanced safety, however, requires at all time a sufficient sub-criticality margin. Reliable reactivity monitoring techniques are hence required to achieve this goal. The MUSE-4 program is a series of low power experiments carried out at the CEA-Cadarache MASURCA facility to investigate the various methods leading to the measurement of the reactivity level and associated kinetic parameters such as the effective delayed neutron fraction. The aim of this paper is to present the results obtained with a method which directly gives the ratio, for a sub-critical assembly, between the reactivity ρ and the effective delayed neutron fraction β_eff. By combining these results to those obtained with the k_p-method for the prompt neutron multiplication coefficient, we have access to the parameters which govern the prompt and the slow kinetics of a sub-critical assembly. These parameters can be obtained without reference to any calibration measurement in critical configuration. It opens the way to the control of larger sub-critical demonstrators that are operating with fuels which cannot be used in critical reactor, and, thanks to sub-criticality, which are characterized by a deterministic safety.