Approach to Criticality in the BR2 Reactor: MCNPX vs. Point Kinetics Method

The behavior of the sub-critical core in the Belgian MTR BR2 during approach to criticality by withdrawal of the control rods is studied. The neutron source needed for the sub-critical counting is provided by the delayed neutrons from long-lived fission product decays in the burnt fuel assemblies and from the photo neutrons, released in (,n) – reactions on beryllium matrix. The three-dimensional Monte Carlo code MCNPX is used for evaluation of the keff values which are further introduced into the reactor kinetic equations to estimate the changes of the relative neutron density level in the sub-critical core. The calculations are performed and compared for two neutron source types: fission source (an eigenvalue calculation with neutrons starting from fuel assemblies) and external source, distributed in few burnt fuel assemblies around the core center. A simple method using MCNPX and point kinetics is proposed in support of the experimental technique for prediction of the counts in the detector (fission chamber) during critical approach by removal of the control rods.