Application of the modified neutron source multiplication method to the prototype FBR Monju

The Modified Neutron Source Method (MNSM) is applied to the prototype FBR Monju. This static method to estimate the reactivity in sub-critical conditions has already given good results on commercial pressurized water reactors. The MNSM consists both in the extraction of the fundamental mode seen by a detector to avoid the effect of higher modes near sources, and the correction of flux distortion effects due to control rod movement between different reactor states. Among Monju’s particularities that have a big influence on the MNSM factors are: the presence of two californium sources near the core and the position of the detector, which is located far from the core outside of the reactor vessel. The importance of spontaneous fission and (α, n) reactions, which have increased during the shutdown period of 15 years, will also be discussed. In order to evaluate the detector count rate, a propagation calculation has been conducted from the reactor vessel to the external detector. For two sub-critical states, an estimation of the reactivity has been made and compared to experimental data obtained in the restart experiments at Monju (2010). Results indicate a good agreement between the MNSM reactivity and the reactivity measured with other methods. The reactivity dependence of the correction to apply to the point kinetic equation is discussed.

► The modified neutron source multiplication is applied to the prototype FBR Monju.
► A flux extrapolation method is implemented to determine the flux in the detectors.
► Neutron sources due to spontaneous fissions, etc. in the fuel are determined.
► The reactivity calculated with MSNM is different from the measured reactivity.
► The MSNM yields a 6% change of the reactivity worth of the central control rod.