A self-revealing finding was identified because the test requirements section of the station service transformer (
SST) design change (
DC) was not comprehensive in that it did not test that the isolated phase bus ducting terminal boxes were constructed to prevent water intrusion into the boxes. This was discovered during a significant rainfall event partially caused by
Hurricane Matthew, which filled up the A
SST terminal box with water and eventually shorted the A phase of the main generator causing a Unit 2 main generator,
main turbine, and subsequent
reactor trip on October 9, 2016. As corrective action, sealant was applied to the
SST terminal boxes on all seams and bolt holes; and weep holes with drain assemblies were installed on each box. This issue was documented in the licensees
CAP as CR 1049987. The inspectors reviewed
Inspection Manual Chapter (IMC) 0612, Appendix B, Issue Screening, dated September 7, 2012, and determined the
PD was more than minor because it was associated with the design control attribute of the
Initiating Events Cornerstone, and it adversely affected the cornerstone objective to limit the likelihood of events that upset plant stability and challenge critical safety functions during shutdown as well as power operations. Using
IMC 0609.04, Initial Characterization of Findings, Table 2, dated October 7, 2016, the finding was determined to affect the
Initiating Events Cornerstone. The inspectors screened the finding using Manual Chapter 0609, Appendix A,
SDP for Findings at-Power, dated June 19, 2012, and determined that it screened as Green because although the deficiency did cause a
reactor trip, it did not cause a loss of mitigation equipment relied upon to transition the plant from the onset of the trip to a stable shutdown condition. This finding has a cross-cutting aspect in the Operating Experience aspect of the Problem Identification and Resolution area, P.5, because the licensee did not evaluate and implement relevant external operating experience.
