RIS 2001-12, Nonconservatism in Pressurized Water Reactor Spent Fuel Storage Pool Reactivity Equivalencing Calculations
| ML010990300 | |
| Person / Time | |
|---|---|
| Issue date: | 05/18/2001 |
| From: | Matthews D Division of Regulatory Improvement Programs |
| To: | |
| References | |
| RIS-01-012 | |
| Download: ML010990300 (4) | |
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555-0001 May 18, 2001 NRC REGULATORY ISSUE SUMMARY 2001-12 NONCONSERVATISM IN PRESSURIZED WATER REACTOR SPENT
FUEL STORAGE POOL REACTIVITY EQUIVALENCING
CALCULATIONS
ADDRESSEES
All holders of operating licenses for pressurized water reactors.
INTENT
The U.S. Nuclear Regulatory Commission (NRC) is issuing this regulatory issue summary (RIS)
to notify pressurized water reactor (PWR) licensees of a possible nonconservatism in the practice of equating the reactivity of spent fuel to the reactivity of fresh fuel in spent fuel storage pool calculations using burnup credit.
BACKGROUND INFORMATION
Since the early 1980s the NRC has accepted the practice of reactivity equivalencing in analyses that determine the appropriate placement of fuel assemblies within PWR spent fuel storage pools. Reactivity equivalencing equates the reactivity of a fuel assembly that has a particular initial enrichment and burnup combination to the reactivity of a fuel assembly that has a different initial enrichment and zero burnup. This is a fictitious fuel assembly that is used in subsequent analyses.
SUMMARY OF ISSUE
Recent analyses done for NRC by the Oak Ridge National Laboratory (ORNL) have indicated that this practice is acceptable provided the geometric configuration and the conditions under which the equivalency was determined remain unchanged (NUREG/CR-6683 (ORNL/TM-
2000/230), A Critical Review of the Practice of Equating the Reactivity of Spent Fuel to Fresh Fuel in Burnup Credit Criticality Safety Analyses for PWR Spent Fuel Pool Storage, September
2000 [ADAMS Accession Number ML003751298]). However, the equivalent fresh fuel enrichment is often determined for a reference configuration (e.g., an infinite array of storage rack cells in unborated water) and then used for various similar, but not identical, configurations. For example, analyses of checkerboard-type storage configurations in which the spent fuel is placed in alternating locations with higher reactivity fuel assemblies (e.g., fresh or lower burned assemblies) indicate that equivalencing yields nonconservative results (on the order of a few tenths of one percent).
Likewise, analyses for storage conditions with soluble boron present reveal somewhat more significant nonconservative results. An under-estimation of reactivity of more than 3 percent was observed for an infinite array of equivalent fresh fuel assemblies stored in a soluble boron concentration of 500 parts per million (ppm).
Typical boron concentrations in PWR pools are greater than 2000 ppm and result in a subcriticality margin of about 20 to 25 percent. Therefore, the potential nonconservatisms noted above are not a safety concern. However, licensee calculations for spent fuel pool accident conditions such as the misplacement of a fresh fuel assembly in a storage rack designed for spent fuel, or for pools which take partial credit for soluble boron, may be less conservative than originally thought. Licensees are being made aware of this information so that accuracy in spent fuel pool criticality calculations can be maintained.
BACKFIT DISCUSSION
This RIS requests no action or written response and is, therefore, not a backfit under
10 CFR 50.109. Consequently, the staff did not do a backfit analysis.
FEDERAL REGISTER NOTIFICATION
A notice of opportunity for public comment was not published in the Federal Register because this RIS is informational and requires no action or written response by addressees.
PAPERWORK REDUCTION ACT STATEMENT
This RIS does not request any information collection.
If there are any questions about this matter, please contact the person listed below, or the appropriate Office of Nuclear Reactor Regulation project manager for a specific nuclear power plant.
/RA/
David B. Matthews, Director Division of Regulatory Improvement Programs Office of Nuclear Reactor Regulation
Technical Contact:
Anthony Ulses, NRR
301-415-1194 E-mail: apu@nrc.gov Attachment: List of Recently Issued NRC Regulatory Issue Summaries Likewise, analyses for storage conditions with soluble boron present reveal somewhat more significant nonconservative results. An under-estimation of reactivity of more than 3 percent was observed for an infinite array of equivalent fresh fuel assemblies stored in a soluble boron concentration of 500 parts per million (ppm).
Typical boron concentrations in PWR pools are greater than 2000 ppm and result in a subcriticality margin of about 20 to 25 percent. Therefore, the potential nonconservatisms noted above are not a safety concern. However, licensee calculations for spent fuel pool accident conditions such as the misplacement of a fresh fuel assembly in a storage rack designed for spent fuel, or for pools which take partial credit for soluble boron, may be less conservative than originally thought. Licensees are being made aware of this information so that accuracy in spent fuel pool criticality calculations can be maintained.
BACKFIT DISCUSSION
This RIS requests no action or written response and is, therefore, not a backfit under
10 CFR 50.109. Consequently, the staff did not do a backfit analysis.
FEDERAL REGISTER NOTIFICATION
A notice of opportunity for public comment was not published in the Federal Register because this RIS is informational and requires no action or written response by addressees.
PAPERWORK REDUCTION ACT STATEMENT
This RIS does not request any information collection.
If there are any questions about this matter, please contact the person listed below, or the appropriate Office of Nuclear Reactor Regulation project manager for a specific nuclear power plant.
/RA/
David B. Matthews, Director Division of Regulatory Improvement Programs Office of Nuclear Reactor Regulation
Technical Contact:
Anthony Ulses, NRR
301-415-1194 E-mail: apu@nrc.gov Attachment: List of Recently Issued NRC Regulatory Issue Summaries Distribution: RIS File PUBLIC
Accession No.ML010990300 Template No. NRR - 052 OFFICE SRXB:DSSA SRXB:DSSA SRXB:DSSA D:DSSA OGC
NAME LKopp* AUlses* JWERMIEL* GHOLAHAN* APH*
DATE 12/27/00 03/19/01 12/27/01 01/11/01 04/06/01 OFFICE TECH ED REXB:DRIP REXB REXB:DRIP D:DRIP
NAME PKLEENE* JShapaker*enf for JTappert* LMARSH* DMATTHEWS
DATE 12/1/00 03/06/01 05/04/01 05/10/01 05/19/01 OFFICIAL FILE COPY
Attachment LIST OF RECENTLY ISSUED
NRC REGULATORY ISSUE SUMMARIES
_____________________________________________________________________________________
Regulatory Issue Date of Summary No. Subject Issuance Issued to
_____ _______________________________________________________________________________
2001-11 Voluntary Submission of 05/18/2001 All holders of operating licenses Performance Indicator Data for nuclear power reactors, except those who have permanently ceased operations and have certified that fuel has been permanently removed from the reactor vessel
2001-10 Revisions to Staff Guidance on 04/02/2001 All holders of operating licenses Notices of Enforcement Discretion for nuclear power reactors, except those who have permanently ceased operations and have certified that fuel has been permanently removed from the reactor vessel
2001-09 Control of Hazard Barriers 04/02/2001 All holders of operating licenses for nuclear power reactors, except those who have permanently ceased operations and have certified that fuel has been permanently removed from the reactor vessel
2001-08 Operating Reactor Licensing 04/02/01 All power reactor licensees Action Estimates
2000-11, NRC Emergency 03/22/01 All holders of operating licenses Supp. 1 Telecommunications System for nuclear power reactors
2001-07 10 CFR 50.75(f)(1) Reports on the 02/23/01 All holders of operating licenses Status of Decommissioning Funds for nuclear power reactors (Due March 31, 2001)
2001-06 Criteria for Triggering a Review 02/15/01 All holders of operating licenses Under 10 CFR 50.80 for Non- for nuclear power reactors Owner Operator Service Companies
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OL = Operating License CP = Construction Permit