10 CFR Part 50, Appendix B, Criterion III, Design Control, requires, in part, that design control measures shall provide for verifying or checking the adequacy of design, such as by the performance of design reviews, by the use of alternate or simplified calculational methods, or by the performance of a suitable testing program. In addition, NRC letter dated June 2, 1977, required all licensees to verify existing plant design or propose modifications to ensure onsite emergency power systems met certain criteria including Staff Position 1. Staff Position 1(c)(2) stated the DGV protection time delay duration, shall minimize the effect of short duration disturbances from reducing the availability of the offsite power source(s). Staff Position 1(c)(3) stated the DGV protection time delay shall be established such that the allowable time duration of a DGV condition at all distribution system levels shall not result in failure of safety systems or components. Contrary to the above, prior to October 9, 2014, Exelon did not adequately evaluate the sequencing of Unit 2 safety-related loads and associated
transient voltages to the Unit 2 Class 1E accident initiated motors and
MOVs on the-safety related buses and
MCCs during the initiation of a design basis loss of coolant event, subsequent unit trip, and resulting sag of the
115kV grid. Specifically, Exelon did not ensure the chosen DGV protection time delay duration: (1) minimized the effect of short duration disturbances from reducing the availability of offsite power sources; and, (2) maintained voltage requirements for safety-related loads to ensure that failures of safety-related systems would not occur. Exelon did not identify the resulting voltage
transients, minimum
4kV motor and
600V MOV starting voltages, associated motor actuator output
torque, and control circuit voltages to safety-related
MOV motors. Exelon identified these deficiencies as a result of their review of NRC
RIS 2011-12R1 and contracted in September 2014 to have the electrical calculations revised. Initial results, using the grid operator specified grid sag of 3.5 percent following a unit trip, indicated that the
transient 4KV safety bus voltage would be too low to reset the DGV relays. This would result in the unintended disconnection of offsite power and transfer to the
EDG. Immediate actions included reevaluation of the postulated grid sag and
transient 4 kV bus voltages. Exelons subsequent assessment concluded that grid stability had improved and offsite power remained
operable. The team reviewed Exelons assessment and immediate actions and found them to be reasonable. Exelon entered this issue into the corrective action program as IRs 2393336 and 2392930. The team determined that the finding was of very low safety significance (Green) in accordance with
IMC 0609, Attachment 4, Phase 1 - Initial Screening and Characterization of Findings,
Mitigating Systems, because it was a design deficiency confirmed not to result in loss of operability or functionality.
