IE Information No. 86-78: Scram solenoid pilot valve (SSPV) rebuild kit problems

On June 14, 1986, Vermont Yankee nuclear power plant reported that one control rod failed to scram and five others hesitated a few seconds before scramming during a single-rod scram time test. During the outage that preceded the scram time test, all of the scram solenoid pilot valves had been rebuilt with replacement kits supplied by General Electric Company (GE). The reactor had been taken critical to perform shutdown margin tests using the in-sequence critical method before the testing that identified the problem with the scram solenoid pilot valves. Three types of problems were identified in the scram solenoid valves (SSPVs) which operated six control rods. In the one SSPV associated with the failure to scram, the core spring of the SSPV was separated from the core assembly. On another SSPV, the diaphragm was installed backwards on the exhaust side of the solenoid valve. On the remaining four SSPVs, an incorrect core assembly, provided with the kits, was installed in the valve. The latter two types of problems were associated with delayed scram initiation. Subsequent inspection of the remaining SSPVs revealed two other types of discrepancies.These were (1) out-of-round inside diameter of the solenoid base subassembly and (2) a deformed spring. Although these two discrepancies did not cause abnormal scram performance in this case, they could have had an adverse effect on scram performance