Neutron multiplication kinetics

The MUSE-4 experimental program took place at the CEA Cadarache, at the MASURCA reactor facility between 2000 and 2004. It aimed at understanding sub-critical assembly physics and in particular the control of neutron multiplication kinetics. The LPSC (Reactor physics group and accelerator division) developped for this purpose an intense pulsed neutron generator called GENEPI, which was coupled to the MASURCA reactor at the beginning of 2000. Years 2003 and 2004 were devoted to studies of the behavior of the generator-reactor coupled system at various sub-criticality levels. Experiment interpretation showed it was possible to measure:

  • the prompt multiplication factor of a sub-critical assembly with a fast neutron spectrum,
  • the delay neutron fraction, which, combined with the prompt multiplcation factor, gives access to the sub-criticality level of the system. 

An original method was developped for each of these measurements. The prompt multiplication factor was obtained from a complete modeling of the distribution of time intervals between two consecutive fissions. This detailed distribution filled the gap where classical reactor physics models failed a reproducing sub-critical system, espcially due to the fact they are based on the first momentum of this distribution. The delayed neutron fraction was obtained by a time modulation of the pulsed beam frequency delivered by the GENEPI accelerator. A cycle of high and low pulse frequency phases allowed the measurement of this fraction. The analysis of the combined results given by these two methodologies validated them over a sub-criticality range compatible with the needs of a power ADS operation (0.995 > k eff > 0.96). Adaptation of these methods to a continuous source driven system was also investigated. It lead to envisage brief beam interruptions into the continous beam driving the reactor which wouln't affect the thermal behavior of the system but would allow the access to the time neutron multiplication distribution.

Neutron spectrometry

Methods developped for reactor kinetics use widely stochastic modeling for neutron multiplication inside the core. It relies on a good knowledge of neutron energy spectrum.  We have therefore investigated neutron spectrometry measurements thanks to development of small proportional counters filled with a low pressure of Helium 3 gas. For the first time we measured neutron energy distribution inside a fast reactor core and compared it with numerical simulations. The very good agreement seen in this figure is a validation, a posteriori, of the nuclear data library used for the calculation.

Exprimental and simulated neutron
energy distributions
in MASURCA reactor (MUSE-4)