THE SECRETARY CORRESPONDENCE CONTROL TICKET Date

Power reactor oriented thermal-hydraulic research, from the safety viewpoint, has been dominated over the past thirty years by accident considerations. The postulated loss of coolant accident, and severe accident concerns, have been the focus of world-wide research programs whose cost is undoubtedly measured in excess of a billion dollars. Various actual accidents, notably Three Mile Island and Chernobyl, have provided impetus to this desire to know more. There are, however, additional thermal-hydraulic challenges, as evidenced by operational occurrences, of a less dramatic nature that should also be the subject of research. A systematic review of incidents revealed the following: 4 Thermal fatigue continues causes failures in piping systems interconnected to the main piping of reactor cooling systems 46 There have been a number of events around the world where pressurized water reactors have experienced a loss of decay heat removal while the reactor vessel is operating at reduced water level. In some cases vortexing at the inlet of the line connecting to the decay heat removal pump suction has contributed to the loss of forced circulation, and the evolution of dissolved gases has played a role; °. In several instances there have been reports of power oscillations in boiling water reactors, contrary to generally accepted design criteria. ° In at least one event there has been a loss of coolant event through a tortuous interconnecting pipeline from a pressurized water reactor at intermediate pressure and temperature condition. Restoration of cooling was somewhat fortuitous. 40o Water hammer events, some with damage to systems and components, have continued to be