Simulation of neutronic-thermal hydraulic oscillations observed during stability tests at German boiling water reactors

Full text of publication follows: Neutronic-thermal hydraulic oscillations, generated by feedback mechanisms, are a well known phenomenon in the operation of boiling water reactors. They can occur as core-wide power oscillations (in-phase) or as regional oscillations (out-of-phase). Despite the progresses that have been made concerning their simulation, the computational determination of the stability threshold in the power-flow map still suffers from inaccuracies. Therefore the performance of one or more elaborate stability tests during the cycle often is inevitable to determine the stability threshold. In this work, the decay ratio of neutronic-thermal hydraulic oscillations was calculated. At a value less than unity the decay ratio indicates the region of stability. The calculations were carried out with the SIMULATE-3K code by Studsvik Scandpower. This code simulates the coupled behaviour of the thermal hydraulics and the three dimensional reactor kinetics. For two different BWR plants, the properties of the pressure vessel internals, especially the characteristics of the pump system, were modelled carefully. The results of the calculations are compared to experimental results of stability measurements from different cycles, which were performed in the two plants in the range of lowest stability, i. e. between minimum pump speed and natural circulation (several pumps tripped). It is shown that the stability behaviour during the cycle can be correlated with the axial and radial power density profile due to control rod maneuvering and fuel burn-up. Furthermore it is shown that the insertion of 'shallow rods' has a strong influence on power density profiles especially in the range of lowest stability and therefore also a strong influence on the decay ratios. The comparison of calculated and measured decay ratios shows the applicability of the SIMULATE-3K code to calculations of decay ratios and the stability threshold with a high degree of accuracy if the physical properties of the plant are modelled in sufficient detail. In addition, the applied method is able to distinguish between in-phase and out-of phase oscillations and to determine their respective dominance regions in the power-flow map. (authors)