At 13.02 hours on May 14, 1995 an instrumented liquid relief valve on the primary coolant system opened spuriously due to a failed diaphragm, discharging coolant to the bleed condenser. This resulted in a reactor trip by shutdown system 1 on low pressurizer level. When the bleed condenser filled, its two relief valves opened, discharging approximately 25Mg of reactor coolant onto the containment floor. A station emergency - category 1 was declared and station accounting was initiated. A coolant recovery pump was placed in service and contaiment manually buttoned-up. Reactor building containment pressure was reduced by manually opening auxiliary pressure regulating valves to the vacuum building. By 15:58, in-pant survey indicated that there was no particulate detectd, no stack alarms and no elevated radiation field. At 16:07 the situation was reduced to a unit alert. Off-site surveys confirmed that there was no increased radiological release to the environment. Tests confirmed that there were no fuel failures as a result of this incident. The unit has been cooled down and placed into a guaranteed shutdown state. Investigation into the cause of the liquid relief valve diaphragm failure has commenced.

Using INES Manual Table II, this incident is treated at INES level 1, on degradation of defence-in-depth. The initiator ("expected" frequency) was "reactor coolant leakage" that would no prevent a controlled reactor shutdown and cooldown". Safety function availability was "within operational limits and conditions".

Location: BRUCE-5
Event date: Sun, 14-05-1995
Nuclear event report
Legenda & explanation