Power Transient Following a Loss of Low Pressure Service Water Open System in Darlington NGS A

Abstract

This paper discusses a reactor physics analysis of the power transient following a postulated loss of the low pressure service water open system. It presents the power transients based on two simplified bounding thermalhydraulic scenarios. In this accident, the loss of moderator circulation causes the moderator to heat up and swell. After about 6.5 minutes, if the setback on moderator high outlet temperature and the SDS1 trip on moderator high level are not credited, the calandria rupture disks burst. The subsequent depressurization causes the moderator to flash. Continuous generation of steam sustains a liquid-vapour mixture at the top of the vessel. This mixture starts to collapse at 300 seconds because of the decrease in vapour generated. After an additional 20 seconds, the mixture has collapsed to a single-phase liquid filling half the calandria. Since the distribution of vapour in the moderator during this accident is not easily computed to a high degree of certainty, two bounding scenarios are defined in the reactor physics analysis. In the first scenario, the moderator is assumed to be separated into a single-phase liquid covered by vapour throughout the transient. In the second scenario, the moderator is assumed to be a homogeneous mixture of vapour and liquid which collapses into a single-phase liquid at the end of the transient. In the first case, the loss of reactivity due to moderator level reduction can easily be compensated by the reactor regulating system. As the moderator level goes down, the reactor power can remain constant while a bottom-to-top power tilt develops. In the second case, because moderator voiding causes a strong negative reactivity, reactor power drops as soon as the moderator inventory decreases. However, after the moderator has collapsed, reactivity may become positive if the reactor regulating system has attempted to compensate for the loss of moderator. In that case the reactor power may rise again.