Investigations on the inadvertent power increase in a PHWR as ASSET experience

Investigations were carried out using the ASSET methodology to find out the root cause of an incident involving inadvertent increase in reactor power in the Unit 1 of Narora Atomic Power Station (NAPS) in India. NAPS is a twin Unit, 220 MWe PHWR based power station. On December 4, 1992, when NPAS Unit 1 was operating at 130 MWe, the reactor power increased steadily on its own and touched 147 MWe, over a period of 14 minutes. The set (demand) power of the triplicated reactor regulating system had increased on its own and in turn has made the reactor to operated at higher power. The power was brought down to 120 MWe by manual intervention. Since adequate system related data during the incident was not available, laboratory studies were carried out using computer simulations for the various process disturbances which could affect the reactor regulating system, for establishing the causes of the event. 4 figs