05000528/LER-2013-001, Unanalyzed Condition Due to Spent Fuel Pool Criticality Analysis of Record Not Updated for Power Uprate
| ML13133A002 | |
| Person / Time | |
|---|---|
| Site: | Palo Verde  |
| Issue date: | 05/07/2013 |
| From: | Mims D Arizona Public Service Co |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| 102-06699-DCM/DFH LER 13-001-00 | |
| Download: ML13133A002 (7) | |
| 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 |
| 5282013001R00 - NRC Website | |
text
10 CFR 50.73 DWIGHT C. MIMS Senior Vice President, Nuclear a pRegulatory
& Oversight Palo Verde Nuclear Generating Station P.O. Box 52034 Phoenix, AZ 85072 Mail Station 7605 Tel 623 393 5403 102-06699-DCM/DFH May 7, 2013 ATTN: Document Control Desk U.S. Nuclear Regulatory Commission Washington, DC 20555-0001
Dear Sirs:
Subject:
Palo Verde Nuclear Generating Station (PVNGS)
Units 1, 2, and 3 Docket No. STN 50-528, STN 50-529, and STN 50-530 License No. NPF 41, NPF 51, and NPF 74 Licensee Event Report 2013-001-00 Enclosed please find Licensee Event Report (LER) 50-528/2013-001-00 that has been prepared and submitted pursuant to 10 CFR 50.73. This LER reports an unanalyzed condition due to the spent fuel pool criticality analysis of record not being updated following a power uprate.
In accordance with 10 CFR 50.4, copies of this LER are being forwarded to the Nuclear Regulatory Commission (NRC) Regional Office, NRC Region IV, and the Senior Resident Inspector. If you have questions regarding this submittal, please contact Mark McGhee, Operations Support Manager, Regulatory Affairs, at (623) 393-4972.
Arizona Public Service Company makes no commitments in this letter.
Sincerely, DCM/DFH/hsc Enclosure cc:
A.T. Howell III NRC Region IV Regional Administrator J. K. Rankin NRC NRR Project Manager for PVNGS M. A. Brown NRC Senior Resident Inspector for PVNGS A member of the STARS (Strategic Teaming and Resource Sharing) Alliance Callaway.Comanche Peak.Diablo Canyon.Palo Verde.San Onofre.South Texas.Wolf Creek
NRC FORM 366 U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB: NO. 3150-0104 EXPIRES: 10/31/2013 (10-2010)
, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.
- 3. PAGE Palo Verde Nuclear Generating Station (PVNGS) Unit 1 05000528
- 4. TITLE Unanalyzed Condition Due to Spent Fuel Pool Criticality Analysis of Record Not Updated for Power Uprate
- 5. EVENT DATE
- 6. LER NUMBER
- 7. REPORT DATE
- 8. OTHER FACILITIES INVOLVED ON SQUEN FACILITY NAME DOCKET NUMBER MR YEAR SUMBER R
MONTH DAY YEAR PVNGS Unit 2 05000529 FACILITY NAME DOCKET NUMBER 3
8 2013 2013
- - 001 00 05 07 2013 PVNGS Unit3 05000530 D. OPERATING MODE
- 11. THIS REPORT IS SUBMITTED PURSUANT TO THE REQUIREMENTS OF 10 CFR§: (Check all that apply)
[1 20.2201(b)
El 20.2203(a)(3)(i)
El 50.73(a)(2)(i)(C)
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El 20.2203(a)(4) 0 50.73(a)(2)(ii)(B)
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[I 20.2203(a)(2)(i)
El 50.36(c)(1)(i)(A)
El 50.73(a)(2)(iii)
El 50.73(a)(2)(ix)(A)
- 10. POWER LEVEL L] 20.2203(a)(2)(ii)
El 50.36(c)(1)(ii)(A)
[1 50.73(a)(2)(iv)(A)
[3 50.73(a)(2)(x)
El 20.2203(a)(2)(iii)
[E 50.36(c)(2)
[3 50.73(a)(2)(v)(A)
El 73.71(a)(4)
El 20.2203(a)(2)(iv)
El 50.46(a)(3)(ii)
[I 50.73(a)(2)(v)(B)
El 73.71(a)(5) 100 / 100 / 100 El 20.2203(a)(2)(v)
El 50.73(a)(2)(i)(A)
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Specify in Abstract below or in a safe shutdown earthquake (SSE). This arrangement is designed to maintain adequate spacing of stored fuel assemblies for control of reactivity.
The SFP criticality analysis supports TS LCO 3.7.15, Fuel Storage Pool Boron Concentration, and LCO 3.7.17, Spent Fuel Assembly Storage, as well as TS Design Features 4.3.1, Fuel Storage Criticality. The SFP is designed to prevent criticality by use of adequate spacing of fuel assemblies, maintenance of minimum soluble boron concentration, and placement restrictions for stored fuel based on fuel assembly initial enrichment, burnup and decay time. The PVNGS SFP design does not require use of neutron absorbing materials for reactivity control. The design requirements related to criticality assure an effective neutron multiplication factor (Keff) of less than or equal to 0.95 for all conditions given a minimum SFP soluble boron concentration of 900 parts per million (ppm). The design also requires that Keff remains less than 1.0 with a SFP soluble boron concentration of 0 ppm. These Keff limits include allowances for biases and uncertainties, including methodology and temperature biases, and enrichment, fuel pellet stack density, steel thickness, storage cell pitch, fuel assembly position and calculational uncertainties.
The soluble boron in the SFP water ensures that there is a sufficient amount of negative reactivity in the SFP at all times. TS LCO 3.7.15 requires SFP soluble boron concentration to be greater than or equal to 2150 ppm when any fuel assembly is stored in the SFP.
Normal soluble boron concentrations in the SFPs are maintained between 4000 ppm and 4400 ppm during all modes of operation. Maintenance of SFP boron concentration in excess of the value required by LCO 3.7.15 provides additional reactivity margin and allows the SFP to be used as a source of borated water to restore RCS shutdown margin in an emergency.
PVNGS procedures establish controls for the movement of both new and spent fuel assemblies to ensure the requirements for fuel storage and SFP criticality are met. The specific location where a fuel assembly may be stored in the SFP is governed by LCO 3.7.17, Spent Fuel Assembly Storage. LCO 3.7.17 ensures the combination of initial enrichment, burnup, and decay time of each fuel assembly stored in each of the four regions of the SFP is within the acceptable burnup domain for each region and remains subcritical.
- 3.
INITIAL PLANT CONDITIONS
On March 8, 2013, PVNGS Units 1, 2, and 3 were in Mode 1 (Power Operation), at 100 percent power and normal operating temperature and pressure. There were no structures, systems, or components inoperable that contributed to the event.
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- 4.
EVENT DESCRIPTION
On March 8, 2013, PVNGS engineering personnel determined that certain impacts on the SFP criticality AOR had not been considered as part of the project to perform a power uprate from 3876 MWth to 3990 MWth in 2003. As a result, the input parameters used in the SFP criticality AOR had not been appropriately revised for the power uprate conditions.
The non-conservative impact of the 2003 power uprate change on the SFP criticality calculation results principally from enhanced plutonium production in each fuel assembly which occurs due to higher moderator and fuel temperatures under power uprate conditions.
The higher moderator and fuel temperatures bias the neutron energy spectrum more toward fast neutrons which then results in greater uranium 238 resonance absorption and subsequent higher plutonium production. The higher plutonium concentrations in fuel assemblies that have operated under power uprate conditions are not accounted for in the SFP criticality calculations resulting in fuel assemblies being discharged into the SFP that are more reactive than previously predicted.
When informed of the condition, operations personnel performed an immediate operability determination (IOD) which considered both the borated and unborated TS requirements.
The IOD concluded that the fuel assembly storage configurations in the SFPs were operable but non-conforming. The IOD conclusion for the borated case was supported by station requirements to maintain SFP boron concentration greater than 4000 ppm. The operability determination for the unborated case was based on engineering judgment that conservatisms within the AOR adequately bound the condition. Operations requested that a prompt operability determination (POD) be developed to establish the reactivity margin for fuel that had been operated under power uprate conditions and now stored in the SFPs.
For the unborated case, the POD, completed on March 22, 2013, concluded that sufficient reactivity margin exists for fuel manufactured at the Columbia Fuel Fabrication Facility (CFFF) to comply with TS requirements. The reactivity margin for non-CFFF manufactured fuel was not sufficient for storage of this fuel in the most limiting SFP locations for the unborated case. In response, administrative controls were established to ensure TS requirements are met for the most limiting storage locations.
The POD established administrative controls to ensure that burnup penalties are applied to placement of non-CFFF manufactured fuel assemblies in order to maintain compliance with TSs.
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- 5.
ASSESSMENT OF SAFETY CONSEQUENCES
To assess the safety consequences, an evaluation was performed using the power uprate value of 3990 MWth and the SFP criticality AOR methodology. The evaluation concluded that TS 4.3.1.1.c. would require an increase in minimum SFP boron concentration from 900 ppm to 935 ppm to ensure that Keff of the storage racks remains below 0.95. The SFP boron concentration at PVNGS is required to be maintained greater than 2150 ppm by LCO 3.7.15, Fuel Storage Pool Boron Concentration, and the actual operating concentration is maintained between 4000 ppm and 4400 ppm; therefore, adequate margin was maintained at all times.
Additional evaluations were performed to ensure that fuel stored in the spent fuel storage racks would remain subcritical even if the SFP is assumed to be fully flooded with unborated water as required by TS 4.3.1.1.b. The evaluation determined that, when manufacturing tolerances were considered, fuel storage requirements were met for fuel manufactured at the CFFF due to conservatisms in the AOR. For fuel that was not manufactured at the CFFF, the evaluation determined that the AOR conservatisms were not sufficient to ensure Keff was less than 1.0 in the unborated case for the most limiting SFP storage locations. As a result, burnup penalties were applied to non-CFFF manufactured fuel to establish the needed margin when determining the allowed SFP storage locations in accordance with the requirements of TS LCO 3.7.17.
Reviews of the actual fuel storage configurations for the past three years determined that, with the burnup penalty applied, the TS requirements for the condition with the SFP flooded with unborated water were met. No conditions were identified where the non-CFFF manufactured fuel assemblies were stored in non-allowable locations.
These evaluations also show no credible deboration event that could have diluted the SFP from the TS value of 2150 ppm boron to 900 ppm boron without detection. The slight increase in boron concentration from 900 ppm to 935 ppm required to maintain Keff less than or equal to 0.95 (TS 4.3.1.1.c) is small when compared to the procedurally required SFP boron concentration of 4000 ppm to 4400 ppm.
The condition would not have prevented the fulfillment of a safety function; and, the condition did not result in a safety system functional failure as defined by 10 CFR 50.73 (a)(2)(v). This event did not result in a challenge to the fission product barriers or result in the release of radioactive materials; and the event did not adversely affect the safe operation of the plant or health and safety of the public.
6. CAUSE OF THE EVENT
The cause was procedures and processes lacked adequate rigor to identify impacts to the SFP criticality AOR. Additionally, the impacts of power uprate relative to the SFP criticality AOR were not well known or understood by personnel involved.
- 7.
CORRECTIVE ACTIONS
As an immediate corrective action, an administrative control was implemented to prevent movement of fuel assemblies in the three PVNGS SFPs. This fuel movement restriction was lifted following completion of analysis that showed that acceptable margin existed with the applied burnup penalties and implementation of administrative controls to establish burnup penalties.
The following interim corrective actions were implemented:
" As a compensatory measure, procedures were revised to establish additional reactivity margin (burnup penalties) for fuel assemblies identified as having lower margins in limiting SFP storage locations.
" The Unit 1, 2 and 3 SFP design documents, which determine the allowable SFP storage locations for each fuel assembly, were revised to implement the burnup penalties required by the revised procedures.
The following corrective actions are planned:
" To sustain compensatory measures to maintain reactivity margin for fuel assemblies discharged into the SFP in the future, procedures will be revised to establish manufacturing tolerances for new fuel assemblies.
Revise design change procedures and documentation to consider reactivity impacts on the SFP.
" Revise the SFP criticality AOR using updated methodology and input parameters and submit a license amendment request.
If information is subsequently developed that would significantly affect a reader's understanding or perception of this event, a supplement to this LER will be submitted.
- 8.
PREVIOUS SIMILAR EVENTS
No similar events have been reported to the NRC in the prior three years.PRINTED ON RECYCLED PAPER