Neutronic study of the sub-critical approach phase of the PERLE experiment in the EOLE zero-power reactor.

Subcriticality in nuclear systems is the state in which the neutron population is unable to sustain a fission chain reaction. For nuclear safety, it is important to develop calculation codes that can reproduce the behavior of subcritical systems. In France, the reference Monte Carlo code is TRIPOLI-4 developed at CEA (Commissariat à l’énegie atomique et aux énergies alternatives) Saclay. First, the convergence study of the estimation of the amplification factor of the system, defined as the ratio between fission neutrons and those produced by the external source, by TRIPOLI-4 code was carried out on a simplified geometry by analyzing the α-Pareto coefficient, the Kolmogorov-Smirnov distance and the trend of the mean value of the amplification factor. From these calculations, it was found that a certain combination of these three parameters is necessary to obtain reliable results. Subsequently, the PERLE experiment, carried out in 2008 at the CEA Cadarache, was considered as a case-study for the first step towards the validation of the TRIPOLI-4 code in subcritical conditions. Lacking information on the correct location of the experimental detector, several detection points were considered in TRIPOLI-4. Finally, the neutron fluxes obtained in TRIPOLI-4 were compared with the experimental values, finding a good match for some of the detection points, and paving the way towards the validation of the code with the data from the PERLE experiment.