05000277/LER-2011-002, For Peach Bottom, Units 2 and 3 - Condition Prohibited by Technical Specifications Due to Degraded Spent Fuel Pool Racks Boraflex Panels
| ML11213A262 | |
| Person / Time | |
|---|---|
| Site: | Peach Bottom  |
| Issue date: | 07/29/2011 |
| From: | Stathes G Exelon Nuclear |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| LER 11-002-00 | |
| Download: ML11213A262 (6) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i) 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability 10 CFR 50.73(a)(2)(ii)(A), Seriously Degraded 10 CFR 50.73(a)(2)(viii)(A) 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition 10 CFR 50.73(a)(2)(viii)(B) 10 CFR 50.73(a)(2)(iii) 10 CFR 50.73(a)(2)(ix)(A) 10 CFR 50.73(a)(2)(iv)(A), System Actuation 10 CFR 50.73(a)(2)(x) 10 CFR 50.73(a)(2)(v)(A), Loss of Safety Function - Shutdown the Reactor 10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat 10 CFR 50.73(a)(2)(i)(A), Completion of TS Shutdown 10 CFR 50.73(a)(2)(v), Loss of Safety Function 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 2772011002R00 - NRC Website | |
text
Exelkn.
Exelon Nuclear www.exeloncorp.coM Nuclear Peach Bottom Atomic Power Station 1848 Lay Rd.
Delta, PA 17314 10CFR 50.73 July 29, 2011 U. S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555-0001 Peach Bottom Atomic Power Station (PBAPS) Units 2 and 3 Renewed Facility Operating License Nos. DPR-44 and DPR-56 NRC Docket Nos. 50-277 and 50-278
Subject:
Licensee Event Report (LER) 2-11-02 Enclosed is a Licensee Event Report concerning a condition prohibited by Technical Specifications involving the need for additional subcriticality margin for the Spent Fuel Pool Racks neutron absorber panels.
In accordance with NEI 99-04, the regulatory commitment contained in this correspondence is to restore compliance with the regulations. The specific methods that are planned to restore and maintain compliance are discussed in the LER. If you have any questions or require additional information, please do not hesitate to contact us.
Sincerely,
)440 /'dfý Garey L. Stathes Plant Manager Peach Bottom Atomic Power Station GLS/djf/IR 1225840 Attachment cc:
US NRC, Administrator, Region I US NRC, Senior Resident Inspector R. R. Janati, Commonwealth of Pennsylvania S. Grey, State of Maryland P. Steinhauer, PSE&G, Financial Controls and Co-owner Affairs INPO Records Center CCN: 11-62
NRC FORM 366 U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB: NO. 3150-0104 EXPIRES: 10/31/2013 110-2010)
, the NRC may digits/characters for each block)inot conduct or sponsor, and a person is not required to respond to, the
- 3. PAGE Peach Bottom Atomic Power Station (PBAPS) Unit 2 05000277 1 OF 5
- 4. TITLE Condition Prohibited by Technical Specifications due to Degraded Spent Fuel Pool Racks Boraflex Panels
- 5. EVENT DATE
- 6. LER NUMBER
- 7. REPORT DATE
- 8. OTHER FACILITIES INVOLVED MNHDYI YAI EUNILRVFACILITY NAME DOCKET NUMBER MONTH DAY YEAR YEAR SEQUENTIAL REV MONTH DAY YEAR PBAPS Unit 3 05000278 NUMBER NO.
FACILITY NAME DOCKET NUMBER 06 06 2011 11
- - 002 00 07 29 2011 05000
- 9. OPERATING MODE
- 11. THIS REPORT IS SUBMITTED PURSUANT TO THE REQUIREMENTS OF 10 CFR §: (Check all that apply) 0l 20.2201(b)
El 20.2203(a)(3)(i)
El 50.73(a)(2)(i)(C)
El 50.73(a)(2)(vii) 1 [E
20.2201(d)
El 20.2203(a)(3)(ii)
El 50.73(a)(2)(ii)(A)
[I 50.73(a)(2)(viii)(A)
El 20.2203(a)(1)
El 20.2203(a)(4)
[I 50.73(a)(2)(ii)(B)
[I 50.73(a)(2)(viii)(B)
El 20.2203(a)(2)(i)
[E 50.36(c)(1)(i)(A)
[I 50.73(a)(2)(iii)
El 50.73(a)(2)(ix)(A)
- 10. POWER LEVEL [I 20.2203(a)(2)(ii)
[E 50.36(c)(1)(ii)(A)
[I 50.73(a)(2)(iv)(A)
[I 50.73(a)(2)(x)
[E 20.2203(a)(2)(iii)
El 50.36(c)(2)
[E 50.73(a)(2)(v)(A)
El 73.71(a)(4) 100%
[1 20.2203(a)(2)(iv)
El 50.46(a)(3)(ii)
[I 50.73(a)(2)(v)(B)
[1 73.71 (a)(5)
El 20.2203(a)(2)(v)
[I 50.73(a)(2)(i)(A)
El 50.73(a)(2)(v)(C)
El OTHER El 20.2203(a)(2)(vi) 0 50.73(a)(2)(i)(B)
El 50.73(a)(2)(v)(D)
Specify in Abstract below or in
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 Ke, < 0.95 if fully flooded with unborated water, which includes an 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 4 th 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., KIf <_ 0.95) and submit plans to the NRC 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.