Analysis of fuel rod behaviour during limiting RIA in RBMK plants

Abstract

Abstract Typically, reactivity initiated accidents for nuclear power plants with RBMK type reactors are classified as design basis accidents. However, for assessment of the real probability of these events, such kind of incidents should be considered more seriously and the same acceptance criteria as for operational occurrences should be applied to them. The spurious withdrawal of control rods, as limiting reactivity initiated accident (RIA), was analysed and results of the study are presented in this paper. The power distribution in fuel channels located around the affected control rods was calculated using the QUABOX/CUBBOX-HYCA code. Evaluating possible uncertainties it was found that linear power, i. e. heat generated per unit length of coolant channel, exceeds the acceptance criterion by a factor of 1.21. This paper presents the discussion how such a peak of power influences the behaviour of RBMK-1500 reactor fuel rods. For this purpose, the FEMAXI-6 code was applied. Results of the analysis show, that after increasing and subsequently decreasing of the linear power during an accident most parameters (pressure inside fuel rods, radial displacements, fuel clad gaps, etc.) return to their initial values and no phenomena occurred that would preclude the resumption of normal operation after termination of such an event.