05000277/LER-2011-002
| Peach Bottom Atomic Power Station (Pbaps) Unit 2 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 2772011002R00 - NRC Website | |
Unit Conditions Prior to the Discovery of the Event Units 2 and 3 were both in Mode 1 operating at approximately 100% rated thermal power when this condition was discovered on 6/6/11. There were no other structures, systems or components out of service that contributed to this event.
Description of the Event
On 6/6/11, the plant operations and management staff determined that certain fuel assemblies in the Unit 2 Spent Fuel Pool (SFP) (EllS: DA) racks needed to be relocated to other Unit 2 SFP rack locations to gain additional fuel assembly subcriticality margin. This determination was based on information provided by the the Nuclear Regulatory Commission (NRC) as part of the development of a Task Interface Agreement (TIA). The TIA was being performed as a result of an NRC unresolved item involving the plant staff's assessment of the degradation rate of the SFP rack (EllS: RK) neutron absorbers (i.e., Boraflex®).
Reduced margin in SFP racks containing Boraflex® panels (EllS:PL) is an industry concern and has been the subject of previous NRC generic correspondence. The condition of the Boraflex® has been closely monitored and modifications had been planned to resolve this condition. The previous station assessment for the rate of degradation of Boraflex® was found by the NRC, as part of their TIA, to not have been conservative enough.
Although no significant probability for spent fuel pool criticality existed, this event is considered to be a condition prohibited by Technical Specifications (TS) in that additional margin was determined to be appropriate to meet TS design requirements for the SFP racks. TS 4.3.1.1.b requires that at least 5% margin to criticality exists when the SFP is flooded with unborated water. This margin must also include an allowance for uncertainties as described in the plant Updated Final safety Analysis Report (UFSAR). As a result of the NRC TIA information, the actual worst case criticality margin (including an allowance for uncertainties) was less than 5%.
As a result of this occurrence, 201 Unit 2 fuel assemblies were promptly relocated to other Unit 2 SFP rack locations where more Boraflex® margin exists. Although there were no conditions SFP rack locations as a conservative measure to account for future Boraflex® degradation.
Additionally, administrative controls were revised to control usage of the rack locations of concern. The above actions were completed by 6/15/11, thereby restoring compliance with TS 4.3.1.1.b.
Analysis of the Event
This report is being submitted pursuant to:
10CFR 50.73(a)(2)(i)(B) — Condition Prohibited by TS — The PBAPS TS Section 4.3 requires certain plant design features be in place concerning new and spent fuel storage. TS 4.3.1, Criticality, requires in part, that the SFP racks are designed and shall be maintained with Keff allowance for uncertainties as described in Section 10.3 of the UFSAR. Contrary to this design feature requirement, the NRC review of the PBAPS analysis for degraded Boraflex® performance determined that the analysis was not as conservative as it should have been. Therefore, a condition prohibited by TS 4.3.1.1 was determined to have occurred on Unit 2. It was determined that the threshold for the Boraflex® degradation becoming a condition prohibited by TS was reached for the Unit 2 SFP racks in approximately the 4th quarter 2008.
There were no actual safety consequences associated with this event. No subcriticality margin concerns existed on the Unit 3 SFP. Reduced margin in SFP racks containing Boraflex® is an industry concern and has been the subject of previous NRC generic correspondence. There was no significant probability for fuel pool criticality to have occurred in either the Unit 2 or Unit 3 SFPs.
The high density spent fuel storage racks provide storage at the bottom of the fuel pool for the spent fuel received from the reactor vessel and new fuel for loading to the reactor vessel. The fuel storage racks are covered with water (normally about 23 ft above the stored fuel) for radiation shielding. The racks are full-length top entry and are designed to maintain the spent fuel in a spacial geometry with neutron absorbing material between the fuel assemblies, which precludes the possibility of criticality under any conditions. The rack modules are rectilinear in shape and are of nine different array sizes. A total of 3819 storage locations are provided per pool. The high density spent fuel racks are constructed of stainless steel materials and each rack module is composed of cell assemblies, a base plate, and a base support assembly. Each cell assembly is composed of a full length enclosure constructed of stainless steel, sections of Boraflex® (neutron absorbing material), and wrapper plates constructed of stainless steel.
Boraflex® consists of boron carbide particles as neutron absorbers held in place by a nonmetallic binder. Boraflex® contains an initial minimum B10 areal density of 0.021 gm/cm2. It is a continuous sheet centered on the length of the active fuel.
Analysis of the Event, continued NRC Generic Letter 96-04, "Boraflex Degradation in Spent Fuel Pool storage Racks" was issued on 6/26/96. The Generic Letter required licensees to assess the capability of the Boraflex® to maintain 5% subcriticality margin (i.e., Keff describing its actions if this subcriticality margin cannot be maintained because of current or projected future Boraflex® degradation. PBAPS uses an analytical code (i.e., RACKLIFE) every 6 months to calculate degradation levels and performs periodic in-situ testing (i.e., BADGER) to benchmark the RACKLIFE predictions.
The Boraflex panels installed in the PBAPS Units 2 and 3 SFP racks were manufactured by Brand Industrial Services Corporation (BISCO), and have been in service since 1986.
Cause of the Event
The primary cause of the event was due to a degradation of vendor-supplied neutron absorbing material (i.e., Boraflex®). Gamma radiation from the spent nuclear fuel induces cross-linking of the silica-based polymer, which results in Boraflex® material shrinkage. Since the gamma radiation field from the fuel assemblies is not uniform, the shrinkage of the Boraflex® was not uniform. This is believed to be the cause of gaps / cracks that form in the Boraflex® panels.
Boraflex® will also dissolve in the SFP environment after exceeding an exposure of approximately 5E8 rads of gamma dose.
The previous station assessment for the rate of degradation of Boraflex® was found by the NRC as part of their TIA to not have been conservative enough. Primarily, the NRC staff found that the PBAPS staff had under-predicted the peak Boraflex® degradation rate.
Corrective Actions
As a result of the revised Boraflex® degradation rate, the PBAPS staff has performed analyses to evaluate fuel assembly loading in specific SFP rack cell locations. These analyses have resulted in 201 Unit 2 fuel assemblies being promptly relocated to other Unit 2 SFP rack locations where more Boraflex® subcriticality margin existed. Although there were no conditions prohibited by TS for the Unit 3 SFP, 84 fuel assemblies were relocated to other Unit 3 SFP rack locations as a conservative measure to account for future Boraflex® panel degradation.
Corrective Actions, continued Additionally, administrative controls were put in place to control usage of the rack locations of concern. These controls also will ensure movement of fuel assemblies from specific Units 2 and 3 SFP cell locations prior to exceeding the subcriticaility margin requirements of the plant TS for the loaded rack cell locations.
A modification is being designed to resolve the subcriticality margin concerns caused by the Boraflex® degradation. A license amendment associated with these design changes is expected to be submitted to the NRC later this year. The modification will be completed subsequent to the NRC approval of the license amendment request.
Previous Similar Occurrences There were no previous LERs identified involving SFP subcriticality margin issues at PBAPS.
individual has been transferred from the position for which a license was required and therefore, no longer requires his license:
Brian T. Woodard (SOP-11342, Docket No. 55-63137) It is requested that the above individual be removed from the list of license holders. If you have any questions, feel free to contact Mr. Bruce Hennigan at 717-456-3385.
Thomas J. I ougherty Site Vice President Peach Bottom Atomic Power Station cc: F. L. Bower, Senior Resident Inspector, USNRC, PBAPS R. R. Janati, Commonwealth of Pennsylvania Document Control Desk, USNRC, Washington DC CCN: 11-65