Responding to these alarms, an operator verified that the nuclear service water supply valve was open and that the flow was within normal range. After the trip and while the engine was coasting to a stop, an explosion occurred in the crankcase. Upon examining the engine, the licensee determined that a thermostatic valve in the jacket cooling water system had failed resulting in the engine overheating, which resulted in significant engine damage. When the jacket water temperature is low, the AMOT valve, which works similarly to the thermostat in an automobile engine, diverts diesel jacket cooling flow from the heat exchanger, which is cooled by nuclear service water. The AMOT valve modulates open to control the temperature of the diesel jacket water to about 165 F as the engine reaches operating temperature.
Heat is transferred through the heat exchanger to the plant's nuclear service water system.
The licensee's investigation revealed that the AMOT thermostat valve had malfunctioned and caused the engine to overheat. During the operability test, the AMOT valve only partially opened from the bypass position, thus allowing the jacket and lube oil temperatures to rise above normal operating temperatures. At these elevated temperatures, the oil emitted an increased amount of vapors. The elevated temperatures also affected the clearances between the moving parts and reduced the oil's lubricating qualities. The heat generated by the friction between the piston and liner ignited the oil vapors.