Simulations of Power Transients in a Loss of the Liquid-Zone-Control-System Pumps in CANDU 6 Reactors

Abstract

A loss of Class IV Power to the zone control system was one of the commissioning tests conducted at low power on Wolsong 2. It resulted in an increase in power of about 25%. The overpower was attributed to the relatively slow closing time of the valves in the discharge line. No trips or step backs occurred because of the low power at which the test was carried out. The occurrence of this transient led to the analysis of similar postulated events at higher power levels to gain an appreciation of the behaviour of the reactor regulating system under loss of the zone-control system conditions and of the sensitivity of the resulting transients to the closure time of the isolating valves in the discharge line. It is predicted that rapid valve closing (in less than one second) would result in a power increase of about 3-4%. The power is eventually limited by the Mechanical Control Absorbers driving in as a result of the power increase. A two second delay in the closing of the discharge valves would result in a -1 25% overpower transient before the MCA,under drive control, are able to turn the power transient around. If the initial reactor power were at or close to full power, a step back on high zone powers at 108% would arrest the power increase, causing the power to rapidly reduce to a low level. The analysis has concluded that a closing time of less than one second is desirable since it precludes relying on step back to avoid a trip in the event of a loss of power to the light water zone control water supply.