Information Notice 2012-09, Irradiation Effects on Fuel Assembly Spacer Grid Crush Strength
| ML113470490 | |
| Person / Time | |
|---|---|
| Issue date: | 06/28/2012 |
| From: | Laura Dudes, Mcginty T Division of Construction Inspection and Operational Programs, Division of Policy and Rulemaking |
| To: | |
| Beaulieu, D P, NRR/DPR, 415-3243 | |
| References | |
| IN-12-009 | |
| Download: ML113470490 (4) | |
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
OFFICE OF NEW REACTORS
WASHINGTON, DC 20555-0001
June 28, 2012
NRC INFORMATION NOTICE 2012-09:
IRRADIATION EFFECTS ON FUEL ASSEMBLY
SPACER GRID CRUSH STRENGTH
ADDRESSEES
All holders of an operating license or construction permit for a nuclear power reactor under
Title 10 of the Code of Federal Regulations (10 CFR) Part 50, Domestic Licensing of
Production and Utilization Facilities, except 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.
PURPOSE
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice (IN) to inform
addressees of operating experience involving evaluations of fuel assembly structural response
to external loads and associated issues the NRC staff identified during recent reviews of fuel
designs for design certification applications. The NRC expects that recipients will review the
information for applicability to their facilities and consider actions, as appropriate, to avoid
similar problems. Suggestions contained in this IN are not NRC requirements; therefore, no
specific action or written response is required.
DESCRIPTION OF CIRCUMSTANCES
Operating experience regarding the effects of in-reactor service on fuel assembly component
response to externally applied forces challenge existing NRC staff guidance. Operating
experience from Framatome ANP, Inc. and a fuel design certification application submitted
under 10 CFR Part 52 by Mitsubishi Heavy Industries, Ltd., show that the crush strength of fuel
assembly spacer grids may decrease during the life of a fuel assembly; whereas, the review
guidance contained in the NRC standard review plan is based on an assumption regarding the
combined effects of operating conditions on grid strength. Additional information is documented
in Framatome ANP, Inc., Closure of Interim Report 02-002, Spacer Grid Crush Strength -
Effects of Irradiation, dated August 8, 2003, which can be found on the NRCs public Web site
in the Agencywide Documents Access and Management System (ADAMS) at Accession No.
ML032240425, and in Mitsubishi Heavy Industries, Ltd., Transmittal of the Revised Technical
Report MUAP-08007-P/NP(R2), dated December 20, 2010 (ADAMS Accession No.
BACKGROUND
Earthquakes and postulated pipe breaks in the reactor coolant system can result in external
forces on fuel assemblies. The fuel assembly structural response to these externally applied
forces is evaluated to ensure the fuel system satisfies requirements to maintain control rod
insertability and core coolability in 10 CFR Part 50, Appendix A, General Design Criteria [GDC]
for Nuclear Power Plants, (e.g., GDC 27, Combined Reactivity Control Systems Capability
and GDC 35, Emergency Core Cooling) and 10 CFR 50.46, Acceptance criteria for
emergency core cooling systems for light-water nuclear power reactors.
The NRC review guidance that is used to evaluate these external forces is NUREG-0800,
Standard Review Plan for the Review of Safety Analysis Reports for Nuclear Power Plants:
LWR Edition, (SRP) Section 4.2, Fuel System Design, Revision 3, March 2007, Appendix A,
Evaluation Of Fuel Assembly Structural Response To Externally Applied Forces (ADAMS
Accession No. ML070740002). SRP 4.2, Appendix A,Section III, states, While P(crit) [the
crushing load] will increase with irradiation, ductility will be reduced. The extra margin in P(crit)
for irradiated spacer grids is thus assumed to offset the unknown deformation behavior of
irradiated spacer grids beyond P(crit). The assumption in the SRP concerning irradiated grids
may suggest that only the beginning-of-life condition for spacer grid strength needs to be
evaluated for fuel assembly integrity under externally applied forces. However, recent operating
experience challenges this assumption. Effects that can influence structural strength include
neutron fluence (e.g., grid spring relaxation, irradiation hardening, growth, cladding creep
down), corrosion (e.g., thinning, hydrogen uptake), and operating conditions (e.g., temperature)
up to the approved limits on fuel assembly burnup and service life, as applicable.
Core dynamics models simulate the behavior of a row of fuel assemblies within a shaking core.
The results are used to determine peak stresses in fuel assembly components, and with respect
to spacer grid performance, determine if plastic deformation occurs. Spacer grid spring
relaxation could have a significant affect on two aspects of the core dynamics model: fuel
bundle stiffness and spacer grid strength. Reduced spring force would lower the effective
bundle stiffness and may lower the strength of the spacer grid. Both of these phenomena
directly affect the fuel assembly structural evaluation.
In addition, SRP 4.2, Appendix A,Section III, states, The consequences of grid deformation are
small. Gross deformation of grids in many PWR [pressurized-water reactor] assemblies would
be needed to interfere with control rod insertion during an SSE [safe-shutdown earthquake] (i.e.,
buckling of a few isolated grids could not displace guide tubes significantly from their proper
location), and grid deformation (without channel deflection) would not affect control blade
insertion in a BWR [boiling-water reactor]. In a LOCA [loss-of-coolant accident], gross
deformation of the hot channel in either a PWR or a BWR would result in only small increases in
peak cladding temperature. This SRP statement conveys that the overall fuel assembly design
is robust with respect to being able to function with grid formation. However, this SRP
statement does not negate other SRP statements such as SRP,Section II, Acceptance
Criteria, which states. Fuel damage criteria should assure that fuel system dimensions remain
within operational tolerances and that functional capabilities are not reduced below those
assumed in the safety analysis. Complete damage criteria should address.stress, strain, or
loading limits for spacer grids, guide tubes, thimbles, fuel rods, control rods, channel boxes, and
other fuel system structural members should be provided. For instance, to demonstrate
compliance with GDC 27, spacer grid strength, loading, and deformation must be evaluated to determine that the guide tubes (or fuel skeleton structure) remains sufficiently straight so as not
to impede control rod insertion following a seismic event and/or loss-of-coolant accident.
DISCUSSION
The SRP delineates the scope and depth of NRC staff review of licensee submittals associated
with various licensing activities and provides an NRC staff interpretation of measures that, if
taken, will satisfy the requirements of the more generally stated, legally binding body of
regulations primarily found in 10 CFR. SRP 4.2 provides NRC review guidance regarding the
evaluation of fuel assembly integrity under externally applied forces. While SRP 4.2 indicates
that it is acceptable to assume that fuel spacer grid strength at the beginning-of-life is most
limiting, operating experience discussed in this IN challenges this assumption. The NRC staff is
considering appropriate steps to clarify SRP 4.2 and will continue to evaluate this issue when
reviewing submittals involving fuel assembly structural response.
CONTACT
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 Office of Nuclear Reactor
Regulation (NRR) or Office of New Reactors (NRO) project manager.
/RA by JLuehman for/
/RA/
Laura A. Dudes, Director
Timothy J. McGinty, Director
Division of Construction Inspection
Division of Policy and Rulemaking
and Operational Programs
Office of Nuclear Reactor Regulation
Office of New Reactors
Technical Contacts: Fred M. Forsaty, NRO
Paul M. Clifford, NRR
301-415-8523
301-415-4043 E-mail: Fred.Forsaty@nrc.gov
E-mail: Paul.Clifford@nrc.gov
Note: NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov, under NRC Library.
TAC ME7565 OFFICE
SRSB/NRO
Tech Editor
DSS/NRR
BC:SRSB/NRO
NAME
FForsaty
JDougherty
PClifford
JDonoghue
DATE
5/18/2012 via e-mail
05/08/12 via email
5/18/2012
5/17/2012 via e-mail
OFFICE
BC:SNPB/NRR
D:DSS/NRR
D:DSRA/NRO
LA:PGCB/NRR
NAME
AMendiola
WRuland
CAder
CHawes
DATE
5/18/2012 via email
6/7/2012
6/12/2012
6/13/2012 OFFICE
PM:PGCB/NRR
BC:PGCB/NRR
LA:PGCB/NRR
D:DCIP/NRO
NAME
DBeaulieu
KMorganButler
CHawes
LDudes (JLuehman for)
DATE
6/13/2012
6/14/2012
6/14/2012
6/18/2012 OFFICE
D:DPR/NRR
NAME
TMcGinty
DATE
6/28/2012