BWR stability investigation at Forsmark 1

Abstract

Sponsored by the Swedish Nuclear Power Inspectorate, a series of noise measurements have been conducted at Forsmark 1 and 2 (reactors of ABB-ATOM design) during start-up operation after the planned outage in summer '87. The main purpose was to investigate BWR stability problems, i.e. resonant power oscillations of 0.5 Hz which tend to arise at high power and low core-flow conditions. The analysis was performed to estimate the noise source which gives rise to the oscillation, to evaluate the measure of stability, i.e. the decay ratio (Dr) as well as to investigate other safety-related problems. The result indicates that the oscillation is due to the dynamic coupling between the neutron kinetics and thermal hydraulics via void reactivity feedback. The Dr ranged between values of 0.7 and 1, instead of the 0.6 expected by a code calculation. These high values imply that the core cannot suppress oscillations fast enough and a small perturbation can cause scram. Furthermore it was found that the local neutron flux in the entire core oscillates in phase with varying strength where any connection to the consequences of different fuels (8 × 8, 9 × 9) being present simultaneously cannot be excluded. This paper elucidates the importance of an on-line BWR stability surveillance system with functions like stability condition monitoring and control system diagnosis.