05000455/FIN-2013005-04
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Finding | |
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| Title | Analytical Basis for PTL Curves Not Maintained Consistent With the Unit 2 Vessel Head Stud Configuration |
| Description | The inspectors identified a finding of very low safety significance and an associated NCV of TS 5.6.6, ?Reactor Coolant System Pressure and Temperature Limits Report,? when licensee personnel failed to maintain the analytical basis for deriving the Pressure Temperature Limit (PTL) curves consistent with the Unit 2 vessel head stud configuration. Specifically, the analytical model used in WCAP-16143, ?Reactor Vessel Closure Head/Vessel Flange Requirements Evaluation for Byron/Braidwood Units 1 and 2,? was based on the original closure head configuration and did not represent the modified closure head configuration (53 head studs) applicable to the Unit 2 reactor vessel. The licensee entered this issue into their CAP as IR 01578276, ?Byron PTLR [Pressure Temperature Limits Report] Outside of NRC Approved Method.? The performance deficiency was determined to be more than minor because it was associated with the Design Control attribute of the Barrier Integrity cornerstone and
adversely affected the cornerstone objective of providing reasonable assurance that physical design barriers (fuel cladding, reactor coolant system, and containment) protect the public from radionuclide releases caused by accident or events. The inspectors also answered ?Yes? to the more-than-minor screening question, ?If left uncorrected, would the performance deficiency have the potential to lead to a more significant safety concern?? Specifically, if left uncorrected, continued operation without a correct stress analysis to support the approved PTL curves could have allowed the reactor to operate at a pressure and temperature that increased the chance for a brittle fracture of the vessel under a pressurized thermal shock (PTS) event. The inspectors performed a Phase 1 SDP screening and selected the box under the Reactor Coolant System Boundary (e.g. PTS issues), which required a detailed risk evaluation. An NRC Region III senior reactor analyst performed a detailed risk evaluation of this finding. A potential increase in the probability for vessel failure would exist if the plant was operated in the unacceptable pressure-temperature regions and a PTS event occurred. Based on the licensee and supporting vendor assessments, which concluded that no substantial increase in vessel stresses will occur due to operation with 53 head studs, the driving force for crack propagation (e.g. K1) remained essentially unchanged. However, to bound the risk evaluation, it was assumed that the initiating event frequency for a reactor vessel failure increased by 10 percent. From the Byron Standardized Plant Analysis Risk (SPAR) model, the initiating event frequency for reactor vessel failure from any cause was 1E- 7/year. Core damage was expected to occur if reactor vessel failure occured. The exposure time for the finding was the maximum of 1 year. Thus, a bounding risk assessment yielded a delta risk of 1E-8/year. Therefore, based on the detailed risk evaluation, this finding was of very low safety significance (Green). This finding had a cross-cutting aspect in the Decision-Making component of the Human Performance cross-cutting area because the licensee did not use conservative assumptions in decision-making and adopt a requirement to demonstrate that a proposed action was safe in order to proceed. In this case, the licensee made a non-conservative assumption that the 10 CFR 50.59 process could be applied to authorize a change in the WCAP-16143 analysis, and therefore did not seek prior NRC approval |
| Site: | Byron  |
|---|---|
| Report | IR 05000455/2013005 Section 1R18 |
| Date counted | Dec 31, 2013 (2013Q4) |
| Type: | NCV: Green |
| cornerstone | Barrier Integrity |
| Identified by: | NRC identified |
| Inspection Procedure: | IP 71111.18 |
| Inspectors (proximate) | C Thompson D Jones E Duncan J Cassidy J Laughlin J Mcghee J Robbins M Holmberg M Jones N Feliz Adorno R Ng S Sheldon B Bartlett B Palagi |
| Violation of: | Technical Specification |
| CCA | H.14, Conservative Bias |
| INPO aspect | DM.2 |
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Finding - Byron - IR 05000455/2013005 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Finding List (Byron) @ 2013Q4
Self-Identified List (Byron)
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