Investigation of the Fission Matrix methodology using the Monte Carlo code Serpent 2 and application to the CROCUS reactor

The Laboratory ofReactor Physics and System Behaviour (LRS) at EPFL is finalising the preparation of a detection system for dynamic studies on the CROCUS experimental reactor. In order to model the temporal behaviour of the reactor within the framework of these experiments, it is necessary to study a new numerical method, based on fission matrices. The objective of the internship is to investigate the fission matrix method recently implemented in the Monte Carlo calculation code Serpent 2, then to develop analysis tools and finally to apply them on the CROCUS reactor. The study is limited for the moment to static cases. The methodology is first studied on a simple system (Pressurised Water Reactor fuel rod), the results of which can be compared with previous studies and with classical criticality calculations. In order to be able to analyse the results quantitatively, a novel method for estimating uncertainties in the fission matrix spectrum was developed and compared to a reference method for estimating uncertainties in Monte Carlo codes. The analysis tools were coded in Python, using the Numpy and Matplotlib libraries. The difference obtained between the eigenvalue of the fundamental mode of the fission matrix on the one hand, and the multiplication factor estimated by criticality calculations on the other hand, is of the order of 10−5. The convergence of the fission matrix spectrum with the number of simulated neutrons, as well as with the number of cells used for the spatial discretization, has been shown on the PWR rod. The developed uncertainty estimation method overestimates the standard deviation for the fundamental mode by about 30%, but is found to be 100 times faster than the reference method. This method is therefore a fast way to estimate the uncertainties on the fission matrix spectrum. Finally, the application of this methodology on the CROCUS reactor has made it possible to calculate several hundred of its eigenvectors in three dimensions, as well as their associated uncertainties, something impossible with the usual criticality calculations. The analysis and uncertainty estimation tools developed during this internship make it possible to obtain the eigenvalues and eigenvectors, as well as their associated uncertainties, of a fission matrix calculated with the Serpent 2 code. This study must be continued by taking into account the time dependence of the fission matrices, in order to be able to model dynamic experiments.