Transient Heat Transfer Analysis to Evaluate the Voiding of the Containment Emergency Cooling Units of PWR (Pressurized Water Reactor) Nuclear Plant Following a Pipe Break Accident

The U.S. Nuclear Regulatory Commission (NRC) issued the Generic Letter GL-96-06 requiring the utilities to evaluate in their Nuclear plants the operability of the Containment Emergency Cooling Units (ECUs) when the Component Cooling Water (CCW) pumps restart after a hypothetical Loss Of Coolant Accident (LOCA) or Main Steam Line Break (MSLB) accompanied by the Loss Of Offsite Power (LOOP). Normally, the ECUs are at high elevation and are susceptible to boiling when their cooling coils are exposed to the high temperature steam following a MSLB or LOCA. In a closed loop system, after the onset of boiling, the voiding is governed by the amount of water that can flow from the ECUs to the surge tank. When the CCW pumps restart, water hammer will occur due to the filling of the void. EPRI (Electric Power Research Institute) along with a number of utilities undertook an experimental research study that proposed a methodology to mitigate the water hammer by accounting for the cushioning effects of hot steam and released air. However, the EPRI study did not provide any guidance for evaluating the occurrence of boiling and the size of the void when the CCW pumps restart. Some of the utilities have used cfd based computer programs that require setting up complex models of the ECU cooling coils, which is very time consuming and costly. This paper presents a methodology based on first principles to evaluate the onset of boiling and the growth of steam void in the ECU coils of a typical PWR closed loop CCW system with a surge tank. It includes the effects of system parameters and accounts for condensation in the presence of air, natural convection, boiling heat transfer, etc. The system parameters have a very significant effect on the results of the analysis. The predictions of the calculations were compared with the predictions using the GOTHIC computer code and were found to be in reasonably good agreement. The methodology is shown to be very cost effective and reliable because it avoids the complexity of modeling the entire cooler and is based on first principles. After the water in the ECU coils reaches the saturation temperature, the void formation is dependent on the paths available from both the inlet and the outlet sides of the ECUs to the surge tank. The procedure for calculating the drainage is also illustrated. Various fixes for avoiding/mitigating the water hammer are also discussed.