Publications
A dynamic assessment of an interfacing system loss of coolant accident
Accident scenarios in nuclear power plants that bypass containment have the potential for large and early releases of radionuclides. They are typically guarded against using means such as redundant valves arranged in series and interlocks for systems that interface with the high pressure reactor coolant system. Some of these preventative arrangements rely on active systems that may fail in unique ways with the introduction of digital instrumentation and control. A hypothetical scenario in a pressurized water reactor plant is examined in which the digital controllers for the residual heat removal system intake valves are subjected to a common cause failure. This failure may cause simultaneous unintended valve opening while the reactor is at power, which has the potential to overpressurize and damage piping in the residual heat removal system and cause a leak of primary system water past containment into the auxiliary building (interfacing system loss of coolant accident). If the controllers are in a persistent fault condition, plant personnel will have to traverse the potentially contaminated auxiliary building to override at least one controller and close its associated valve. A dynamic case is assembled and run using the ADAPT dynamic event tree driver and the MELCOR severe accident analysis code in which uncertainties in the progression of the accident as well as mitigating operator actions are explored for an interfacing systems loss of coolant accident initiator. The results are assessed using recently-developed tools to gain insight into the likely outcomes and key events.