Information Notice 2009-23, Nuclear Fuel Thermal Conductivity Degradation
| ML121730336 | |
| Person / Time | |
|---|---|
| Issue date: | 10/26/2012 |
| From: | Camper L, Laura Dudes, Mark Lombard, Mcginty T Division of Policy and Rulemaking, NRC/FSME/DWMEP, NRC/NMSS/SFST, Division of Construction Inspection and Operational Programs |
| To: | |
| References | |
| IN-09-023 S01 | |
| Download: ML121730336 (5) | |
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
OFFICE OF NUCLEAR MATERIAL SAFETY AND SAFEGUARDS
OFFICE OF FEDERAL AND STATE MATERIALS AND
ENVIRONMENTAL MANAGEMENT PROGRAMS
OFFICE OF NEW REACTORS
WASHINGTON, DC 20555-0001 October 26, 2012 NRC INFORMATION NOTICE 2009-23, SUPPLEMENT 1: NUCLEAR FUEL THERMAL
CONDUCTIVITY DEGRADATION
ADDRESSEES
All holders of operating licenses and construction permits for nuclear power reactors under the
provisions of Title 10 of the Code of Federal Regulations (10 CFR) Part 50, Domestic Licensing
of Production and Utilization Facilities, including those who have permanently ceased
operations and have certified that fuel has been permanently removed from the reactor vessel.
All holders of or applicants for an early site permit, standard design certification, standard
design approval, manufacturing license, or combined license under 10 CFR Part 52, Licenses, Certifications, and Approvals for Nuclear Power Plants.
All holders of, and applicants for, a certificate of compliance for a spent nuclear fuel
transportation package under 10 CFR Part 71, Packaging and Transportation of Radioactive
Material.
All holders of a certificate of compliance for a spent fuel storage cask and all holders of a
license for an independent spent fuel storage installation under 10 CFR Part 72, Licensing
Requirements for the Independent Storage of Spent Nuclear Fuel, High-Level Radioactive
Waste, and Reactor-Related Greater Than Class C Waste.
PURPOSE
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice (IN) to notify
addressees of information related to the impact of irradiation on fuel thermal conductivity. This
supplement to IN 2009-23, Nuclear Fuel Thermal Conductivity Degradation (Agencywide
Documents Access and Management System (ADAMS) Accession No. ML091550527), dated
October 8, 2009, complements the information previously presented regarding fuel thermal
performance analysis codes that do not account for fuel thermal conductivity degradation (TCD).
In particular, this IN supplement reflects that the NRC has issued letters to the major fuel
vendors requesting they evaluate the magnitude of the effects of thermal conductivity
degradation on relevant safety analysis parameters. The major fuel vendors have been and are
currently working towards improved fuel thermal performance codes that incorporate TCD
models. The NRC expects the recipients to review the information within this IN for applicability
IN 2009-23, Supp 1 to their facilities and consider actions, as appropriate. However, suggestions contained in this
IN are not NRC requirements; therefore, no specific action or written response is required.
BACKGROUND
Irradiation damage and the progressive buildup of fission products in fuel pellets result in
reduced thermal conductivity of the pellets. However, there are fuel thermal performance codes
approved by the NRC that do not include models for this behavior. NUREG/CR-6534, Volume 1, FRAPCON-3: Modifications to Fuel Rod Material Properties and Performance
Models for High-Burnup Application (ADAMS Accession No. ML092950544), issued in October
1997, describes that TCD had been considered negligible when end-of-life burnup levels were
less than 4 atom percent but may no longer be negligible as commercial fuel has operated to
higher burnup levels of 7 atom percent and greater. NUREG/CR-6534 describes that
measurements collected from an instrumented assembly at the Halden ultra-high-burnup
experiment during the 1990s showed TCD of approximately 5 to 7 percent for every 10
gigawatt-days per metric tonne of exposure. Based on these experimental data, the NRC
updated its confirmatory fuel thermal-mechanical performance software, FRAPCON, to account
for TCD as a function of exposure.
Since that time, several reactor fuel vendors have submitted improved fuel thermal performance
codes to the NRC for review and approval. These new codes incorporate updates to the fuel
thermal conductivity models that account for degradation caused by irradiation. The improved
vendor models generally considered experimental qualification data that were substantially
similar to the data considered in NUREG/CR-6534. However, the NRC staff is aware that many
computer codes that do not account for TCD are still used to perform safety analyses.
DESCRIPTION OF CIRCUMSTANCES
The NRC staff has concerns that fuel thermal performance codes that do not model TCD as a
function of burnup, when used at multiple points within the body of the safety analyses, may
result in the downstream effect of calculated safety limit margins that are less conservative than
previously understood. Following the issuance of IN 2009-23, the NRC staff completed a
preliminary review of the impact of fuel thermal conductivity models on the reactor safety
analysis codes by the major fuel vendors. The NRC staff determined through this review that
several currently approved analysis methods provide results that are less conservative than
previously understood. As a result, the NRC issued letters containing the NRC staffs
assessment to the major fuel vendors (ADAMS Accession Nos. ML11166A052, ML120580690,
and ML120680571) requesting the vendors evaluate the magnitude of the effect of fuel thermal
conductivity degradation on relevant safety analysis parameters (e.g., fuel centerline
temperature, peak cladding temperature, and rod internal pressure) and determine whether
specified acceptable fuel design limits (SAFDLs) for any licensing basis analysis using relevant
models and codes are exceeded if TCD as a function of burnup is included in the analysis.
The NRC staff was presented with information from Westinghouse Electric Company that
showed the inclusion of an updated thermal conductivity degradation model in the safety
analysis could cause compliance issues with 10 CFR 50.46, Acceptance Criteria for
Emergency Core Cooling Systems for Light-Water Nuclear Power Reactors. In response, the
NRC staff issued IN 2011-21, Realistic Core Cooling System Evaluation Model Effects
Resulting from Nuclear Fuel Thermal Conductivity Degradation (ADAMS Accession No.
ML113430785), dated December 13, 2011, which addresses the potential for TCD to cause
IN 2009-23, Supp 1 significant errors in realistic emergency core cooling system (ECCS) evaluation models. The
NRC staff also issued letters pursuant to 10 CFR 50.54(f) to several licensees that use
Westinghouse-furnished realistic ECCS evaluation models to request additional information
regarding the effects of the error associated with TCD.
The responses from the major fuel vendors and licensees included interim solution methods that
are being evaluated by the NRC staff. The NRC staff understands that the major fuel vendors
are currently working towards an appropriate resolution of this issue.
DISCUSSION
General Design Criterion (GDC) 10, Reactor Design, in Appendix A, General Design Criteria
for Nuclear Power Plants, of 10 CFR Part 50, establishes that licensees should not exceed
SAFDLs during any condition of normal operation, including the effects of anticipated
operational occurrences, to ensure that the fuel is not damaged. The general requirements to
maintain control rod insertability and core coolability appear in GDC 27, Combined Reactivity
Control Systems Capability, and GDC 35, Emergency Core Cooling. In particular,
10 CFR 50.46, Acceptance Criteria for Emergency Core Cooling Systems for Light-Water
Nuclear Power Reactors, provides the specific coolability requirements for a loss-of-coolant
accident. In addition, 10 CFR 50.46(a)(3) specifies requirements for evaluating and reporting
each change to, or error discovered in, an acceptable evaluation model.
Technical specifications require licensees to submit a report on core operating limits that
incorporates the revised cycle-specific parameters resulting from the new core configuration
implemented during the refueling outage. Technical specifications also require that the
analytical methods used to determine the core operating limits be those previously reviewed
and approved by the NRC. Licensees rely on computer codes for fuel performance calculations
and to perform safety analyses. Within the scope of reload licensing evaluations, they use
these computer codes to establish cycle operating limits to ensure that all applicable
requirements (e.g., fuel thermal-mechanical limits, core thermal-hydraulic limits, ECCS limits, and nuclear design limits) are met.
The simulation of the fuel element is an integral part of the safety analyses. Within the
analyses, the fuel pellet thermal conductivity model determines the rate at which heat is
transferred from the fuel pellet, first to the gas gap, then to the fuel cladding, and finally to the
coolant. A lower fuel pellet conductivity results in higher fuel temperatures at a given linear
heat-generation rate. Therefore, the analytical prediction of the fuel thermal conductivity will
affect the results of several types of safety analyses. If fuel thermal performance codes contain
models that misrepresent fuel thermal conductivity, then calculated margins to SAFDLs and
other limits may be less conservative than previously understood.
GENERIC IMPLICATIONS
Safety analyses performed for reactors using methods that do not model TCD as a function of
burnup may be less conservative than previously understood.
Lower fuel pellet conductivity does not appear to significantly influence spent nuclear fuel
cladding temperatures that are typically estimated for aged spent nuclear fuel during dry cask
storage and transportation operations.
IN 2009-23, Supp 1
CONTACT
This IN requires no specific action or written response. Please direct any questions about this
matter to the technical contacts listed below or to the appropriate NRC project manager.
/RA/ /RA/
Mark D. Lombard, Director Timothy J. McGinty, Director
Division of Spent Fuel Storage Division of Policy and Rulemaking
and Transportation Office of Nuclear Reactor Regulation
Office of Nuclear Material Safety
and Safeguards
/RA/ /RA by JLuehman for/
Larry W. Camper, Director Laura A. Dudes, Director
Division of Waste Management Division of Construction Inspection
and Environmental Protection and Operational Programs
Office of Federal and State Materials and Office of New Reactors
Environmental Management Programs
Technical Contacts: Andrew Proffitt, NRR A. Kevin Heller, NRR
301-415-1418 301-415-8379 E-mail: Andrew.Proffitt@nrc.gov E-mail: Kevin.Heller@nrc.gov
301-492-3272 E-mail: Jimmy.Chang@nrc.gov
Note: NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov, under NRC Library.
ML121730336 TAC ME8862 OFFICE SNPB/DSS/NRR Tech Editor BC:SNPB/DSS/NRR BC: DSFST/NMSS D:DSS/NRR
NAME AProffitt JDougherty AMendiola MSampson WRuland
DATE 08/03/12 e-mail 08/01/12 e-mail 09/06/12 e-mail 09/18/12 e-mail 09/26/12 OFFICE BC:RDB/FSME LA:PGCB:NRR PM:PGCB:NRR BC:PGCB:NRR
NAME BWatson CHawes DBeaulieu DPelton
DATE 08/02/12 e-mail 09/28/12 9/27/12 10/01/12 OFFICE D:DWMEP/FSME D:DCIP/NRO D:DSFST/NMSS D:DPR:NRR
NAME LCamper LDudes(JLuehman ) MLombard TMcGinty
OFFICE 10/18/12 10/15/12 10/10/12 10/26/12