Junk Plant LaSalle SRVs: Two Failed SRVS Failed Tech Spec Test For Unknown Reasons

This is the first time I have ever seen a failed test on SRVs disclosed as unknown. Are these guys testing the NRC with getting away with disclosing a tech spec test as failing for unknown reason? I see they did the same thing last cycle. Is this what you expect out of a good nuclear plant, not understanding why a safety equipment failed a test? They normally term this as corrosion bonding. It gets me to wonder what evidence they use to determine corrosion bonding in all other SRVs?

LaSalle County Station, Unit 2 Renewed Facility Operating License No. NPF-18 NRC Docket No.
Subject: Licensee Event Report 2017-004-02, Two Main Steam Safety Relief Valves Failed lnservice Lift Inspection Pressure Test 

During the February 2017 Unit 2 refueling outage, two main steam safety relief valves (SRV) did not pass Technical Specification (TS) Surveillance Requirement 3.4.4.1 and lnservice Testing (IST) Program lift pressure requirements. Both SRVs (2B21 ·F013C and 2B21·F013L) lifted below their expected lift pressures. On February 16, 2017, SRV 2B21-F013C was required to lift within plus or minus three percent of 1175 psi (i.e., 1175 psi plus or minus 35.2 psi), but actually lifted at 1131 psi. On February 17, 2017 SRV 2B21-F013L was required to lift within plus or minus three percent of 1195 psi (i.e., 1195 psi plus or minus 35.8 psi), but actually lifted at 1130 psi. 

Disassembly and inspection of valves 2B21-F013C and 2B21-F013L were performed at NWS Technologies to determine the cause for the failures. However, the cause for 2B21-F013C and 2B21-F013L to fail their SRV set pressure tests was found to be indeterminate.

LER 37 4-2015-002-01: During the February 2015 Unit 2 refueling outage L2R15, two main steam safety relief valves (SRV) did not pass TS Surveillance Requirement 3.4.4.1 and lnservice Testing Program lift pressure requirements. Both SRVs lifted below their expected lift pressures. SRV 2821-F013S was required to lift within plus or minus three percent of 1150 psi (i.e., 1150 psi plus or minus 34.5 psi) and actually lifted at 1099 psi. SRV 2821-F013M was required to lift within plus or minus three percent of 1195 psi (i.e., 1195 psi plus or minus 35.8 psi) and actually lifted at 1145 psi. A failure analysis was conducted by a vendor testing laboratory, but the cause for the valves lifting below their set-point was indeterminate.