Title
10 CFR Part 50, Appendix B, Criterion III, Design Control, requires, in part, that for those systems, structures, and components to which this appendix applies, Design control measures shall provide for verifying or checking the adequacy of design.Contrary to the above, between September 2003, and December 19, 2017, the licensee failed to verify or check the adequacy of design of quality-related components associated with the Division 1 and 2
emergency diesel generator 125 Vdc control power circuits. Specifically, in 2003, the licensee modified the design of the control power circuit through Part Evaluation (
PE) 4222806 and replaced 24 original light bulb lamp assemblies with a different style of light bulb and a
carbon film dropping resistor (vs. the original wire-wound design). This change created an unrecognized vulnerability that left the affected portions of the circuit with dropping resistors that provided insufficient protection from shorting due to indication light bulb failures. As a result, on December 19, 2017, the licensee declared both
emergency diesel generators inoperable due to the design vulnerability.Significance/Severity Level: The finding created a design vulnerability in the
emergency diesel generator control power circuits, and resulted in the Division 1 and 2
emergency diesel generators being declared
inoperable at the
time of discovery. Although the
emergency diesel generators were declared
inoperable, subsequent licensee analysis determined that the system retained its function, and maintained a reasonable expectation of operability while the design deficiencies existed. Accordingly, the inspectors assessed the significance of this finding in accordance with
Inspection Manual Chapter 0609, Appendix A, The
Significance Determination Process (
SDP) for Findings at Power, dated June 19, 2012, and determined this finding was of very low safety significance (Green) because it was a deficiency affecting the design or qualification of a mitigating structure, system, or component (
SSC), but the
SSC maintained its operability. Corrective Action Reference(s):Immediate corrective actions included compensatory measures to remove light bulbs from the vulnerable lamp assemblies in order to eliminate the shorting hazard. This issue was entered into the licensees corrective action program as Condition Report
CR-CNS-2017-07513, and the licensee initiated a root cause evaluation
