ML20141J824
| ML20141J824 | |
| Person / Time | |
|---|---|
| Issue date: | 07/15/1997 |
| From: | Callan L NRC OFFICE OF THE EXECUTIVE DIRECTOR FOR OPERATIONS (EDO) |
| To: | Diaz K, Dicus G, Shirley Ann Jackson, Mcgaffigan E, The Chairman NRC COMMISSION (OCM) |
| References | |
| M970325, WITS-9700097, NUDOCS 9708210324 | |
| Download: ML20141J824 (6) | |
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UNITED STATES g
j NUCLEAR REGULATORY COMMISSION t
WASHINGTON, D.C. 20555 4001 July 15, 1997 MEMORANDUM T0:
Chairman Jackson Commissioner Dicus Commissioner Diaz Commissioner McGaffi an FROM:
L. Joseph Callan i
Executive Director Operations 1
L
SUBJECT:
REGULATORY GUIDELINES AND' LICENSING CRITERIA FOR HIGH BURNUP FUEL (M970325) - (WITS Control 9700097)
This memorandum responds to the staff requirements memorandum (SRM) dated l
April 16, 1997, regarding the March 25, 1997 Commission briefing concerning high burnup fuel issues. The SRM instructed the staff to assess the effects of-high burnup fuel on the adequacy of reaulatory guidelines and licensing
- criteria, The SRM stated that the assessment snould cover not only reactivity 1
accidents but also design basis accidents such as loss of coolant accidents.
In addition, the staff was requested to identify issues, which would need to 4
be addressed, that are associated with transportation, storage, and long term geologic burial of high burnup fuels.
The staff was also requested to ensure that 10 CFR 50.59, Millstone, and Maine Yankee lessons learned recommendations, which are related to licensing bases, capture the necessary m
issues related to fuel design.
BACKGROUND In a memorandum to the Commission dated November 25, 1996, the Executive--
. Director of Operations (ED0) provided an uodate on staff and industry safety assessments of new research data on low enthalpy fuel failures during reactivity insertion transients.
In-the memorandum, the staff re)orted that detailed generic analyses of reactivity transients performed by t1e nuclear
. industry, showed that the fuel failures (and radiolo remain bounded by existing licensing basis analyses. gical conseamnces)
The analyses were based on-conservative treatment of the experimental fuel failure data applied to existing and planned core operations within a> proved burnup limits (62 GWD/MTU lead fuel rod exposure) for U.S. reactors. Tiere is broad agreement among the
-staff, the industry, and'the international comunity that any burnup-dependent degradation in the margin to _ low enthalpy fuel failures is likely to be regained by the application of m re detailed 3-dimensional analysis methods to the evaluation of the fuel response to reactivity transients.
Since all plants are in compliance with existing licensing acceptance criteria for these transients, the staff believes that all plants could demonstrate compliance with new criteria-by applying detailed analytical models to the licensing safety. analyses. The staff concludes that, although the regulatory criteria for fuel failure enthalpy limits are not conservative, the generic anal performed by the industry that assume low enthalpy fuel failures (e.g..yses 100 calories per gram) provide reasonable assurance that the radiological I k
. consequences of reactivity insertion accidents will not exceed design basis limits for cores operating within current NRC approved burnup limits.
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Both the staff and the nuclear industry safety assessments concluded that reactivity accidents are of low safety significance and are not a limiting constraint in fuel behavior for core coerations within approved burnup limits.
The industry has indicated that further extension of burnup limits is needed to improve fuel economy, reduce spent tuel storage problems, and to keep nuclear plants competitive with other alternative fuel options.
The staff will not approve further extensions of burnup limits until additional experimental information on fuel behavior is available to demonstrate that the fuel cladding will satisfy the regulatory acce)tance criteria used in the licensing basis analyses.
Although the staff las concluded that the recent test results have no significant risk implications for reactivity accidents, the reactivity accidents in question must be evaluated in the licensing basis analyses. The regulatory acceatance criteria, therefore, should have a technical basis consistent wit 1 the current state of knowledge before extending the burnup limits.
The SRM asked the staff to as: as the effects of high burnup fuel on the adequacy of the regulatory guidelines ano licensing criteria in a number of areas, which are discussed in more detail below.
STAFF ASSESSMENT OF REGULATORY CRITERIA i
Fuel-related criteria are used throughout the licensing basis analysis to evaluate the adequacy of plant designs and operations.
The general requirements that must be complied with are as follow:
The fuel system is not damaged as a result of normal operation and e
anticipated operational occurrences (10 CFR 50 Appendix A, Criterion 10).
Fuel system damage is never so severe as to prevent control rod insertion e
when it is required (10 CFR 50 Appendix A. Criteria 26 and 27).
The number of fuel rod failures is not underestimated for postulated e
accidents (10 CFR 50 Part 100.11),
Coolability is always maintained (10 CFR 50 Part 46).
a One of the design basis accidents considered in a safety analysis involves a reactivity transient.
For these accidents, the staff has completed an interim technical assessment of the criteria and has concluded that 100 cal /g is a reasonable enthalpy limit to ensure the integrity of Zircaloy fuel cladding within currently approved burnup limits.
Fuel vendors are developing new cladding materials that can be expected to behave differently and must be sup wrted by individual databases.
Since the failure threshold will vary wit 1 cladding material properties and burnup level. design acceptance enthalpy l
limits equal to or exceeding Zircaloy performance at 62 GWD/MTU (e.g., 100 l
cal /g) should be maintained at the extended burnup level requested.
Limits on l
oxidation and other important behavioral indicators should also be imposed as needed to ensure consistency of perfo mance.
Experimental data that ensure continued fuel integrity up to the specified licensing limit must be provided, and fuel failure must be assumed for transients exceeding the licensing limit.
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The subsidiary issues of fuel particle dispersal, and the attendant issues of core coolabi.11ty.-plant contamination. and enhanced plant releases need to be addressed in conjunction with any substantial extension of burnup limits.
Additional studies are planned to asse;s thc fuel dispersal behavior of high burnup fuel during large power oscillations that may occur during operator response to a boiling water reactor ATWS (anticipated transients without scram) event.
The effects of high burnup levels are not a factor in the evaluation of other design basis accidents, except that it may be important for LOCAs (loss of i
coolant accident). The staff has initiated an ex)erimental program to assess criteria for a LOCA. The LOCA research has only aegun to determine the effects of high burnup levels. Because the expected outcome of a LOCA is so strongly dependent on flow blockage and cladding embrittlement, the large L
amounts of cor cosion, enhanced ox1datwn rates, and reduced cladC?.9 ductility experienced at high burn up levels are likely to have a significant effect.
The RES pr agram has two main objectives:
(1) determine the behavior of high burnup fuel under (simulated) LOCA conditions, and (2) establish a database of mechanical properties of high burnup cladding needed to analyze transients that are important in licensing safety analyses.
Decisiens relating to implementation of revised criteria have not been made except that implementation must preceda aay extension of approved burnup limits. An agency )lan for dealing with high burnup fuel issues is being developtd and will ae provided to the Commission in the fall of 1997.
-HIGH ENRICHMENT Many licensees are going to 2-year fuel cycles thereby increasing the burnup levels of the fuel.
A nough initial fuel enrichment has been increc.;;o to support these longer fuel cycles, the industry has expressed its desire to use still higher enrichment fuel.
Current NRC approvals for the conaercial reactor fuel cycle allow uranium enrichments up to 5%.
Any increase above this level raises concerns about the criticality safety of fuel production facilities and fresh fuel transportation activities..
For enrichments up to 5%. uranium fuel must have a moderator present, such as light water, in order to achieve criticality. Above 5% enrichment there is a transition region in which the dominant physics phenomena begin to change and under-moderated criticality situations become more important.
Criticality code performance and accuracy are assessed by comparing code calculations to criticality benchmark experiments.
To date, the development and validation of criticality codes for commercial reactor fuels have concentrated on enrichments less than 5t.
The applicability of current
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- neutr:n cross section sets in the enrh.hment range just above 5% is not well characterized.
No significant benchmark data points applicable to commercial power reactor fuel exist in that range.
In addition, a basis for extrapolating code performance and adjusting for calculational accuracy in this enrichment range has not yet been established.
If a basis for extrapolation cannot be shown, experimental benchmark data will be needed to validate the code behavior in the higher enrichment range.
These issues on code performance and criticality safety apply to the full range of commercial fuel production, transportation, and storage activities.
-These-activities include operations at fuel fabrication facilities, as well as packages used to transport uranium hexafluoride, uranium oxide powder, pellets, and final fuel assemblies.
A move to enrichments above 5% may require rede.,lgn of some processing equipment and transportatinn packages.
SPENT FUEL STORAGE AND TRANSPORTATION Cladding temperature limits have been established for the dry storage of spent fuel.
These limits are based on data frua fuel with moderate burnup levels.
During the vacuum drying of a cask after loading, the fuel temperatures g
gradually rise until-an inert backfill gas is introduced.
In addition the vacuum drying process reduces the external pressure on the fuel clad.
Both of
-these effects increase the chance of rupture if significant cladding degradation has occurred with higher burnup levels, The ability of-the fuel clad to maintain its integrity over long interim storage times of 20 years or more when cladding degradation and higher internal rod pressures may exist will need to be evaluated.
Not only is cladding integrity im ataat on a fuel rod by fuel rod basis, but caskanalyses.suchasthoseoftIlecriticality, shielding,andthermal 1
-performance, use basic assumptions about the ability of the fuel assemblies to
-maintain their orioinal configuration.
If clad integrity is in question, the validity of assumptions-about fuel configurations and the applicability of cask analyses must be reassessed. Theae concerns apply to spent fuel transportation, storage, and the attendant fuel handling, such as shifting the fuel to a permanent dis)osal package. A particular fuel handling concern is the preparation for casc unloading when a cask is cooled down before reflooding it with water.
The fuel clad will need to be evaluated to ensure that it can withstand the thermal stresses produced during this evolution.
NMSS has several actions in progress to respond to the concerns described above. The Spent Fuel Project Office is developing a user need request for help from RES to review and update technical bases and approval criteria as they relate to cladding integrity of high burnup fuel in dry storage conditions. The request would also seek assistance on assessing the validity of radionuclide source term codes for the higher burnup levels and the longer-cooling times of interest to spent fuel storaga, transportation. and disposal.
Meanwhile, on the basis of current fuel management practices and operating experience, the staff believes that licensees currently meet all regulatory requirements and t'nat these issues neither involve a significant increase in risk nor pose an immediate public health or safety problem.
Heightening of 1
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industry and NRC sensitivity to th % issues has been accomplished.
The staff further believes that current plans to address the issues are commensurate with their safety significance.
MILLSTONE AND MAINE YANKEE LESSONS LEARNED The chance in fuel 3roperties during the fuel cycle is included in the A)pendix K LOCA metlodologies through sensitivity studies that demonstrate tlat LOCA limiting conditions occur at or near beginning of cycle (BOCL If burnup was extended beyond 62 GWd/t. further justification would be required to use current Appendix K LOCA methodologies, and it is conceivable that BOC may not be limiting for burnup beyond 62 GWd/t.
The change in fuel properties during the fuel cycle is incorporated into existing best-estimate LOCA methodology: however, additional material data would be required to validate the use of the methodology beyond 62 GWd/t.
RES is conducting a program to collect additional material data for extended burnup. Also, the industry is collecting additional oxidation data on various cladding materials.
More studies are needed to evaluate appropriate oxidation limits, including high burnup fuel performance under LOCA condu. ions l
The 10 CFR 50.59 and Millstone lessons-learned recommendations do not specifically address fuel-related issues.
They primarily address process issues related to the licensing basis and the design basis, The Maine Yankee lessons-learned recommendations have contributed to an increased focus on LOCA methodologies for application to burnup levels not considered in the original design.
SUMMARY
RESPONSE TO SRM The staff has completed an interim technical 3ssessment of the regulatory guidance and fuel performance criteria for i, censing evaluation of reactor cores o)erating within approved burnup liniits. The staff concludes that, althoug1 the fuel failure enthalpy limits used for licensing analysis of reactivity insertion transients are non-conservative detailed generic analyses provide reasonable assurance that the radiological consecuences of these accidents will not exceed design basis licensing limits, mJ1tional studies are planned to address the subsidiary issues of high burnup fuel particle dispersal behavior during large power oscillations that may occur during operator response to a boiling water reactor ATWS.
The effects of high-burnup levels are not a factor in the Evaluation of other design basis accidents, except that it may be important i'or LOCAs.
The staff has initiated an experimental program to assess regulatory criteria for a LOCA.
There are no special issues associated with enrichment increases to achieve higher burnup up to current approveo limits of 5%.
Any increase above this level would raise _ concerns about the criticality safety of fuel production facilities and fresh fuel transportation and storage activities.
In addition.
validation of physics codes used for criticality calculations would become an issue for a richments above 5%.
Concerns exist about fuel cladding inte of high burnup fuel over long interim storage times of 20 years or more.grity
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These concerns apply to spent fuel transportation, storage, end the attendant 1
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The' Commissioners 6-fuel handling. such as shifting the fuel to a permanent disposal package.
However, the staff believes that licensees currently meet-all regulatory i
requirements for these activities.
The 10 CFR 50.59 and Millstone lessons-learned recommendations do not specifically address fuel-related issues.
The Maine Yankee lessons-learned recommendations have contributed to an increased focus on LOCA methodologies for_ application to burnup levels not considered in the criginal design.
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