Pressurised water reactor neutron flux fluctuations with increased amplitude. Final report

Approximately since 2001, various German pressurized water reactors (KWU types Vor-Konvoi and Konvoi) have exhibited increased neutron flux fluctuation amplitudes. The high fluctuation level causes an undesirably high rate of actions of the reactor-power limitation system. Therefore, this reactor physics phenomenon is of relevance for reactor safety. Furthermore, the phenomenon is not entirely understood. Consequently, it is a matter of ongoing research of universities, research centers, operators and (fuel assembly) vendors. Currently, the fluctuation amplitudes are reported to be as low as before 2001, again. The work presented here contributes to the mathematical and physical understanding of the observed cycle-by-cycle growth of the neutron flux fluctuation amplitudes. Starting with the analysis of measured neutron flux data being on the authors' disposal and with literature research, a general model hypothesis covering relevant aspects of the neutron flux fluctuation phenomenology was developed. One specific realization of the hypothesis was numerically investigated predominantly using the system code DYN3D and reduced order models. The impact of local geometry changes in the reactor core on the homogenized interaction cross sections, which are relevant for neutron kinetics calculations, was illuminated with the Monte Carlo program Serpent2. The investigations performed indicate that coolant-flow driven time-dependent deflections of a great part of the fuel-assembly collective significantly contribute to the measured neutron flux fluctuations. Variations of the mechanical fuel-assembly parameters, especially of the natural frequency of the fundamental mode and of the damping, lead to an increase or a decrease of the fluctuation amplitudes. For an ultimate clarification of the mentioned increase of fluctuation amplitudes and of connected questions concerning measured fluctuation correlations, corresponding research activities have to be continued.