Effectiveness of Setback in Liquid Zone Control Pump Failure

Abstract

A liquid zone control pump (LZC) system failure occurred at Cernavoda 1 on 2001 Jan. 05. In that event, Shutdown System 2 (SDS-2) tripped. This paper presents an analysis of the event, to determine whether the SDS-2 trip was necessary to prevent fuel dryout, or whether the setback on low pressure in the water- supply header of the liquid zone controllers would have sufficed. In the incident the pumps were restarted within seconds, but in the simulation the pumps were purposely not restarted, in order to see whether the control-system response in the absence of the pump restart and the SDS-2 trip would have prevented dryout. The Reactor Regulating System (RRS) actions were modelled using the *CERBRRS module of RFSP-IST 3-00-05HP. In the analysis, modifications were made to *CERBRRS to disable the RRS control of the zone controllers while retaining the remaining functionality of the RRS. Cernavoda 1 Operations provided the critical channel power (CCP) map, which, together with the channel powers (CP) calculated by *CERBRRS, was used to determine the onset of dryout by means of the critical power ratio CPR = CCP/CP. The results of the simulation show that the setback by itself provides adequate margin to dryout, and that a reactor trip is not needed to protect the fuel.