Analysis of Three-Mile Island Unit 2 and Chernobyl Accidents
Three Mile Island- Unit 2
Root Cause
A loss of de-mineralized water from a blocked feed line trigged a main coolant pump shutdown and the startup of emergency feedwater pumps as well as the insertion of reactor control rods. At this point, transfer of heat from the reactor core through the steam generators was reduced and core coolant pressure began to rise. Valves on the emergency feedwater system had been left closed after maintenance operations several days prior, though records showed that they were supposedly open, and so the system was ineffective. During the main pump shutdown, pressure in the reactor coolant loop increased to a point where the pressure relief valve atop the pressurizer (PORV) opened. The pressure relief valve on the main coolant loop remained open after coolant pressure dropped. The indicator light on the operator control panel indicated that no power was applied to the valve solenoid. This resulted in the operators assuming that the valve was closed.
As pressure in the primary coolant loop fell, a high-pressure injection system automatically came online, injecting borated coolant into the primary coolant loop. With the relief valve in the open position, primary coolant was flowing into the relief valve drain tank and the pressurizer was filling with coolant. Eventually the drain tank filled and primary coolant began flowing into the reactor building sump. To avoid pressurizer “lockup”, operators throttled back the high-pressure injection system.
Even when the emergency feedwater valves were opened, the primary coolant loop pressure was reduced to the point where saturation of the core coolant was reached. Partially-saturated conditions in the primary coolant loop caused the primary coolant pumps to go offline due to excessive vibration. At the same time, primary coolant continued to flow through the relief valve.
As coolant saturation was reached, the coolant boiled...