|ENS 48912||10 April 2013 20:40:00||On April 10, 2013, at 0952 (PDT) it was discovered that an access door to the reactor building stack was not fully closed. The door was subsequently closed by operations. On March 16, 2013, the reactor building exhaust flow rate took a step change decrease. Initially the reduction in flow was thought to be a reactor building ventilation damper issue. On April 3, 2013, after walkdown of the reactor building dampers and verification of proper system lineup, the reactor building exhaust flow rate monitor was declared inoperable and a substitute value was used for the exhaust flow rate in accordance with station procedures.
Had an actual event involving an offsite release occurred during the time period from March 16, 2013, to April 3, 2013, an inaccurate reactor building exhaust flow rate might have been used to calculate offsite dose. This would only impact dose calculations at lower doses and potentially delay declaration of an Unusual Event. This is being reported as a major loss of assessment capability. At higher dose rates, the reactor building ventilation isolates and Standby Gas Treatment is operated. Standby Gas Treatment flow rate measurement would be unaffected. The intermediate and high range radiation monitors for the reactor building effluent remained fully functional and would have provided an accurate measure of activity concentration.
Upon closure of the reactor building stack door, the reactor building exhaust flow rate returned to normal and emergency preparedness assessment capability was restored.
The licensee has notified the NRC Resident Inspector.|