Information Notice 2011-03, Nonconsecutive Criticality Safety Analyses for Fuel Storage
| ML103090055 | |
| Person / Time | |
|---|---|
| Issue date: | 02/16/2011 |
| From: | Blount T, John Tappert Office of New Reactors, Division of Policy and Rulemaking |
| To: | |
| Purnell, B A, NRR/DPR, 415-1380 | |
| References | |
| IN-11-003 | |
| Download: ML103090055 (5) | |
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
OFFICE OF NEW REACTORS
WASHINGTON, DC 20555-0001 February 16, 2011 NRC INFORMATION NOTICE 2011-03: NONCONSERVATIVE CRITICALITY SAFETY
ANALYSES FOR FUEL STORAGE
ADDRESSEES
All holders of operating licenses or construction permits for a nuclear power reactor issued
under the provisions of Title 10 of the Code of Federal Regulations (10 CFR) Part 50, Domestic
Licensing of Production and Utilization Facilities.
All holders of or applicants for a standard design certification, standard design approval, manufacturing license, or combined license issued under 10 CFR Part 52, Licenses, Certifications, and Approvals for Nuclear Power Plants.
PURPOSE
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice (IN) to inform
addressees of possible nonconservative technical specifications related to the criticality safety
of fuel storage. The NRC expects that recipients will review the information for applicability to
their facilities and consider actions, as appropriate, to avoid similar problems. However, suggestions contained in this IN are not NRC requirements; therefore, no specific action or
written response is required.
BACKGROUND
Paragraph 50.68(b)(4) of 10 CFR 50.68, Criticality Accident Requirements, requires the
following:
If no credit for soluble boron is taken, the k-effective of the spent fuel storage
racks loaded with fuel of the maximum fuel assembly reactivity must not exceed
0.95, at a 95 percent probability, 95 percent confidence level, if flooded with
unborated water. If credit is taken for soluble boron, the k-effective of the spent
fuel storage racks loaded with fuel of the maximum fuel assembly reactivity must
not exceed 0.95, at a 95 percent probability, 95 percent confidence level, if
flooded with borated water, and the k-effective must remain below 1.0
(subcritical), at a 95 percent probability, 95 percent confidence level, if flooded
with unborated water.
NUREG/CR-6698, Guide for Validation of Nuclear Criticality Safety Calculational Methodology, January 2001 (Agencywide Document and Management System (ADAMS) Accession
No. ML050250061), provides guidance on determining the bias uncertainty for Monte Carlo
codes.
The primary NRC staff guidance regarding the depletion uncertainty is an internal NRC
memorandum from L. Kopp to T. Collins, Guidance on the Regulatory Requirements for
Criticality Analysis of Fuel Storage at Light-Water Reactor Power Plants, dated
August 19, 1998 (ADAMS Accession No. ML003728001) (Kopp Letter). The Kopp Letter is
referenced by virtually all spent fuel pool criticality license amendment requests submitted since
its issuance.
Regarding the depletion uncertainty, the Kopp Letter states the following:
A reactivity uncertainty due to uncertainty in the fuel depletion calculations should
be developed and combined with other calculational uncertainties. In the
absence of any other determination of the depletion uncertainty, an uncertainty
equal to 5 percent of the reactivity decrement to the burnup of interest is an
acceptable assumption.
DESCRIPTION OF CIRCUMSTANCES
Beaver Valley Power Station
By letter dated April 9, 2009, FirstEnergy Nuclear Operating Company submitted a license
amendment request for Beaver Valley Power Station, Unit 2 (BVPS-2), to modify the technical
specifications to support the installation of high-density storage racks in the BVPS-2 fuel pool.
A vendor provided a criticality safety analysis to support the BVPS-2 license amendment
request.
During the review of the BVPS-2 application, the NRC staff found that the statistical treatment of
the Monte Carlo code bias uncertainty used in the criticality safety analysis was inappropriately
based on the standard deviation of the mean k-effective value calculated for the critical
experiments. 10 CFR 50.68 requires that the estimated k-effective be less than 1.0 at a
95-percent probability, 95-percent confidence level (95/95 confidence). Therefore, the correct
statistical approach is to determine the Monte Carlo code bias uncertainty using the standard
deviation of the population about the mean, not the standard deviation of the mean. Using the
correct statistical approach, the bias uncertainty was found to be larger by 0.0079 over the
previous analysis. The large increase in the value for the bias uncertainty also resulted in a
significant increase in the estimated k-effective.
St. Lucie Nuclear Power Plant
By letter dated April 16, 2010, Florida Power and Light Company submitted information about
the current licensing basis for fuel storage criticality safety at St. Lucie Nuclear Power Plant, Unit 1, as part of a license amendment request for an extended power uprate. During the
acceptance review of the request, the NRC found that the current licensing basis criticality
safety analysis for fuel storage did not include the depletion uncertainty for the unborated cases.
10 CFR 50.68 requires licensees to demonstrate that k-effective is less than 1.0 with a
95/95 confidence for the unborated cases. When these demonstrations rely upon fuel depletion, the depletion uncertainty must be included in both the borated and unborated
analyses. The estimated k-effective will likely increase when the depletion uncertainty is
factored into the analysis.
DISCUSSION
The two issues discussed in this IN relate to the Monte Carlo code bias uncertainty and
the depletion uncertainty. Both of these uncertainties, if not properly treated, may lead
to nonconservative estimation of the maximum k-effective, and regulatory compliance
may not be assured. Analyses with small margins to the regulatory limit are especially
vulnerable to noncompliance with 10 CFR 50.68 and nonconservative technical
specifications if these issues are present.
In 10 CFR 50.68, the NRC requires a 95-percent probability with a 95-percent confidence level
that a calculation demonstrating subcriticality of a spent fuel storage rack actually is subcritical.
The primary NRC guidance for criticality code validation, NUREG/CR-6698, includes guidance
on the determination of the bias uncertainty for Monte Carlo codes based on the population of
critical experiments. Guidance for determining the bias uncertainty in NUREG/CR-6698 can be
used to ensure compliance with 10 CFR 50.68.
Most nuclear criticality safety analyses performed to support spent fuel pool licensing actions
include reactor depletion calculations, whether it is a boiling-water reactor establishing the most
reactive point in the life of a fuel assembly lattice or a pressurized-water reactor taking burnup
credit. Licensing actions that include reactor depletion calculations should consider the
uncertainty of those calculations to ensure the 95/95 confidence requirement for k-effective is
met. The Kopp Letter provides the NRCs current guidance for determining uncertainties for
reactor depletion calculations. As with other NRC guidance, addressees may choose an
alternative method for determining the depletion uncertainty, although additional NRC staff
review should be expected.
The NRC is working to improve its guidance in these areas. The NRC Office of Nuclear Reactor
Regulation (NRR) has established an Action Plan, On Site Spent Fuel Criticality Analyses
(ADAMS Accession No. ML101520463), to monitor and track these activities. The NRC has
received public comment on Draft Interim Staff Guidance DSS-ISG-2010-01, Staff Guidance
Regarding the Nuclear Criticality Safety Analysis of Spent Fuel Pools (ADAMS Accession
No. ML101520463), and expects to issue the final version in the first quarter of 2011. The NRC
expects to issue more durable guidance by the first quarter of 2013.
CONTACT
S
This IN requires no specific action or written response. Please direct any questions about this
matter to the technical contacts listed below or the appropriate NRR project manager.
/RA/ /RA/
John Tappert, Acting Director Thomas B. Blount, Acting Director
Division of Construction Inspection Division of Policy and Rulemaking
and Operational Programs Office of Nuclear Reactor Regulation
Office of New Reactors
Technical Contacts: Kent A. L. Wood, NRR Tony T. Nakanishi, NRR
301-415-4120 301-415-3211 E-mail: kent.wood@nrc.gov E-mail: tony.nakanishi@nrc.gov
Note: NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov, under Electronic Reading Room/Document Collections.
CONTACT
S
This IN requires no specific action or written response. Please direct any questions about this
matter to the technical contacts listed below or the appropriate NRR project manager.
/RA/ /RA/
John Tappert, Acting Director Thomas B. Blount, Acting Director
Division of Construction Inspection Division of Policy and Rulemaking
and Operational Programs Office of Nuclear Reactor Regulation
Office of New Reactors
Technical Contacts: Kent A. L. Wood, NRR Tony T. Nakanishi, NRR
301-415-4120 301-415-3211 E-mail: kent.wood@nrc.gov E-mail: tony.nakanishi@nrc.gov
Note: NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov, under Electronic Reading Room/Document Collections.
ADAMS Accession Number: ML103090055 TAC ME4423 OFFICE LPL2-2:DORL:NRR SRXB:DSS:NRR Tech Editor BC:SRXB:NRR D:DSS:NRR
NAME JPaige KWood KAzariah-Kribbs AUlses WRuland
DATE 11/05/2010 11/08/2010 11/10/ 2010 e-mail 01/24/2011 01/25/2011 OFFICE LA:PGCB:NRR PM:PGCB:NRR PM:PGCB:NRR BC:PGCB:NRR BC(A):DCIP:NRO
NAME CHawes BPurnell DBeaulieu SRosenberg JTappert
DATE 02/03/2011 02/09/2011 02/09/2011 02/10 /2011 02/11 /2011 OFFICE BC:SRSB:DSRA D:DSRA:NRO D(A):DPR:NRR
NAME JEDonoghue CEAder TBlount
DATE 02/01/2011 02/03/2011 02/16/2011 OFFICIAL RECORD COPY