The team identified a finding of very low safety significance, in that Exelon failed to use appropriate inputs in design calculations as required by Exelon Procedure
CC-AA-102 - Design Input and Configuration Change Impact Screening. The requirements of the procedure include ensuring performance requirements are the maximum or minimum numerical values of specific design parameters, specifically, the Maximum time to automatically initiate a system action. The team determined the response speed used by Exelon for the automatic load tap changer (
LTC) controller and mechanism for the stations startup transformers, in the calculation to determine offsite power availability, was non-conservative. This assumption resulted in the grid voltage limit, used to assess technical specification offsite power supply operability, to be nonconservative. In response, Exelon performed preliminary calculations with revised
LTC times, which showed that the offsite grid remained
operable at the specified voltage limits. Exelon entered the issue into the corrective action program to re-perform the calculation and raise the allowed offsite grid voltage level. This finding was more than minor because it is associated with the
Mitigating Systems Cornerstone and affected the cornerstone objective of ensuring the availability, reliability and capability of systems that respond to
initiating events to prevent undesirable consequences. The team determined the finding was of very low safety significance (Green) because it was design deficiency that did not result in a loss of offsite power operability. Because the licensee had recently performed this calculation with the non-conservative inputs, the finding has a cross-cutting aspect in the area of Human Performance - Resources. (
IMC 0305, aspect H.2.(c)) (1R21.2.1.4)