After operating the EDG for 15 minutes at full load, operators observed that the engine cooling water and lubricating oil temperatures were increasing abnormally. The engine oil level was checked and found to be within normal range. Responding to these alarms, an operator verified that the nuclear service water supply valve was open and that the flow was within normal range. Upon examining the engine, the licensee determined that a thermostatic valve in the jacket cooling water system had failed, overheating the engine and causing 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.
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 opened only partially from the bypass position, thus allowing the jacket and lubricating 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, causing the rapid pressure increase and the explosion in the crankcase.