Analysis of the neutron albedo’s influence on TIP deviations in Forsmark 1

Sammanfattning: In the operation of a nuclear reactor, detailed calculations are performed to predict the power distribution in the core and to ensure the maintenance of a number of safety margins. Repeatedly during a power cycle, the predictions are verified in so-called TIP measurements, and the consequences of possible deviations are investigated. During the latest power cycles until today, the Forsmark 1 reactor has experienced large deviations in TIP measurements at the edge of the reactor core that have caused reasons to question the limits of the thermal margins needed to ensure fuel safety. These deviations have especially been present in the first half of cycle 34, which is why only data from this period is investigated in this report. It is expected that the deviations between predictions and measurements occur due to approximate calculations of the neutron albedo, i.e. the model for how neutrons leaving the core's periphery are reflected by materials surrounding the core. The objective of this work was to investigate whether the albedo model in the nuclear reactor core calculation programme POLCA7 may be adjusted to reduce deviations. If so, an additional objective would be to give details on the identified adjustments and, if possible, propose a better albedo model that in the future could be implemented on-line at Forsmark 1. Two analyses are presented, power deviation analysis and CPR (Critical PowerRatio) margin analysis. The presented investigations are based on existing albedo model commands ("cards") in the POLCA7 code, which have been used to introduce alternative albedo models in the calculations. Comparisons between updated and original albedo model cards were done with pre-written analysis scripts in MATLAB that calculated and plotted power deviation and thermal margins. This approach resulted in one set of proposed albedo parameters with respect to deviations in the power distribution and another set of proposed albedo parameters with respect to CPR margin. The two sets are similar, but not identical. More research in the subject is necessary since the two proposed parameter sets do not coincide and since there still is some deviation between measurements and predictions when applying these adjustments. In conclusion, the implemented albedo models give improved agreement between simulated and measured data. The results indicate that better predictions for future cycles may be obtained if these model improvements are introduced into future calculations. However, additional studies are required in order to draw conclusions that apply to the general case.