The analysis and evaluation of recent operational experience from the Fort St. Vrain HTGR

The U.S. Nuclear Regulatory Commission's Office of Analysis and Evaluation of Operational Data has established an extensive program for screening, analyzing, and evaluating the operational experience data from all commercial nuclear power plants in the United States. This program is designed to provide feedback from field experience with actual operating events to the NRC's continuing efforts to assure the public's health and safety. Oak Ridge National Laboratory provides technical assistance to AEOD to evaluate the operating experience for Fort St. Vrain. Water ingress events continued to be a frequent problem caused most often by electrical/control system upsets. The most recent such event was in June 1983, when the circulator upset led to a moisture ingress large enough to cause icing of chillers in the helium purification train but apparently not large enough to be detected as a problem from available analytical monitors. As a result, the reactor was exposed to several hours of undiagnosed levels of ''high'' moisture, loss of purified helium flow to control rod mechanisms and finally a reactor scram in which 6 of 37 control rod pairs failed to insert automatically. Evidence has also been uncovered that high moisture has caused the transport of volatile chlorides throughout the reactor resulting in corrosion of stainless steel control rod cables and possibly hold down bolts used on a helium circulator closure. Moisture has also caused severe leaching of B2O3 contaminant from the reserve shutdown materials, precluding the complete dumping of material during a surveillance test