ML20249A824

Discusses 453rd Meeting of ACRS Held on 980603-05 Re Proposed Plans for NRC Research Program on Reactor Fuels

ML20249A824
Person / Time
Issue date:	06/15/1998
From:	Seale R Advisory Committee on Reactor Safeguards
To:	Shirley Ann Jackson, The Chairman NRC COMMISSION (OCM)
References
ACRS-R-1764, NUDOCS 9806180286
Download: ML20249A824 (4)

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NUCLEAR REGULATORY COMMISSION 8

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June 15,1998 4

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The Honorable Shirley Ann Jackson Chairman U.S. Nuclear Regulatory Commission Washington, DC 20555-0001

SUBJECT:

NRC REACTOR FUELS RESEARCH PROGRAM During the 453rd meeting of the Advisory Committee on Reactor Safeguards, June 3-5,1998, we discussed with the NRC staff the proposed plans for the NRC research program on reactor fuels.

Our Subcommittee on Reactor Fuels also discussed this program during a meeting on April 23-24, 1998. We also had the benefit of the document referenced.

BACKGROUND There are large economic incentives for licensees to extend the bumup of reactor fuels. Extended reactor fuel bumup also has significant societal benefits, it is not surprising, then, that over the last two decades fuel bumups at discharge from reactor cores have more than doubled. Today, some nuclear units are approved to extend bumup to 62 GWd/t (peak rod average). The nuclear industry has indicated an interest in further extending fuel bumups to as high as 75 GWd/t. The nuclear industry is currently reluctant to go to yet higher bumups, since fuel enrichments in excess of.5%

would be required.

in the past, the NRC maintained an active experimental research program to study the performance of reactor fuels under accident and off-normal conditions. The NRC developed codes (FRAPCON and FRAPTRAN) for predicting changes in fuel and fuel cladding with bumup and these codes are l

used in reviewing and approving licensee proposals for core reloads. This research program enabled the NRC to establish fuel performance criteria. Licensees were expected to perform analyses and tests needed to demonstrate that their fuel met these criteria. Funding reductions ad the press of other needs forced the NRC to curtail its fuel performance research at a time when the expenmental database extended to only about 33 GWd/t. Similarly, models of fuel and cladding properties were restricted to this limited database. Although the models were developed based on a database for low bumup fuels, they would still be acceptable at higher bumups if there were no l

significant changes in the fuel or the cladding.

Unfortunately, changes do occur in fuel and cladding starting at bumups in the vicinity of 50 to 60 GWd/t. The fuel develops a high porosity " rim" of low thermal conductivity. The cladding can undergo rapid or" breakaway" oxidation. Zirconium hydrides precipitate and embrittle the cladding.

The effects of these changes in high bumup fuels have been demonstrated in French and Japanese teste of high bumup fuel behavior during reactivity insertions such as might be caused by design-basis control md ejection accidents or control rod drop accide* Cladding rupture and fuel dispersal were observed in the tests at energy inputs of 1/3 to 1/10 the levels expected based on current j g regulatorv guides. Analyses of the test results show that cladding oxidation and embrittlement are 9906190286 990615C

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2 important contributors to the poor fuel performance. These findings raise questions about the capacity of high bumup fuel to survive other design-basis accidents such as loss-of-coolant accidents (LOCAs) and anticipated transients without scram (ATWS). The test findings and operational events, such as control rod sticking and axial distortion of the neutron flux in reactor cores, show that high bumup fuel does not behave in ways anticipated by simple extrapolation of data'forlower bumups. The problems are made more complicated by some evidence that the route to high bumup as well as the bumup level may affect fuel performance.

Regulatory response to these findings has been to limit bumups to 62 GWd/t. The Office of Nuclear Reactor Regulation (NRR) has concluded that the degradation,of fuel and cladding at these bumups does not pose a significant threat to the public health and safety. The Office of Nuclear Regulatory Research (RES) has been asked to conduct research to confirm this regulatory decision. RES has not been asked to resume the broad program of exploratory research that it had in the past.

Licensees will be expected to provide all the data and analyses needed to support approval to extend fuel bumups beyond the cunrent limit. In addition, it is expected that licensees will implement more aggressive lead test assembly programs and will establish fuel performance monitoring programs.

THE CONFIRMATORY RESEARCH PROGRAM RES has formulated a confirmatory research program that consists of the following elements:

continued collaboration with international, experimental studies of fuel behavior during reactivity insertions, experimental studies of high bumup fuel behavior under LOCA conditions, experimental studies of high bumup fuel behavior under ATWS conditions, determination of the oxidation behavior and mechanical properties of cladding as a function of bumup, upgrading properties correlations in the FRAPCON and FRAPTRAN codes, and I

analyses of uncertainties in neutronic codes used by NRC.

The experimental and analytical studies in this program are limited to the confirmation of the regulatory decision to permit bumups to 62 GWd/t. RES has used risk insights to focus experiments and analyses on the issues of most importance. Resources have been leveraged by collaboration with continued intemational programs and with an industry-sponsored program.

in addition to the RES confirmatory research program, the Office of Nuclear Material Safety and Safeguards (NMSS) proposes to benchmstk its criticality models for fuel with enrichments in excess of 5%.

COMMENTS AND RECOMMENDATIONS I

We make the following comments and recommendations:

i The strategy to require licensees to provide all the data and analyses to support extension i

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3 of fuel bumups.beyond current' limits seems appropriate. It' places the burdens and.

- responsibilities un those who will gain the rewards that come from extending fuel bumup.

The strategy does, however, place limits on the technical independence that the NRC will have in establishing fuel performance acceptance criteria. NRR should make it clear that this

- strategy applies also to bumups within current limits for fuels with new cladding types not previously tested.

The NRC should ensure that it has the knowledge and the tools to respond quickly to adequately formulated proposals from licensees to extend fuel bumups beyond 62 GWd/t.

t The staff noods to make clear what data and what analyses will be required to gain approval for extended fuel bumup.

. The ovear. RES confirmatory research program is well conceived and deserves the support, of the Commission.

The experimental studies of fuel behavior under LOCA conditions need to be augmented to -

include tests with more realistic time temperature histories that may impose harsher thermal and mechanical stresses on the cladding and cladding oxides to ensure that Appendix K (10 CFR Part 50) requirements are adequate for high bumup fuels. Greater realism in the LOCA tests may be especially important if, as is now expected, licensees take advantage of the option of using realistic analyses to comply with the requirements of Appendix K. Time-temperature histories need to be selected such that realistic or at least conservative thermal and mechanical stresses are placed on cladding oxides and any potential for breakaway

_ oxidation is revealed. The suggestion from NRR that the route to bumup as well as the bumup level affects fuel performance needs to be addressed by the research program. Tests with more cladding types than are now approved for high bumup operation may be needed.

The test mainces would benefit from application of well-known experiment design methods.

Plans to address issues of fuel performance during ATWS events have not yet been developed. ATWS may be an especially critical event for high bumup fuels.

The conclusion that high bumup does not affect radionuclides source terms used in regulatory safety analyses is not supported by the technicalliterature. The research program should include consideration of how bumup may affect core degradation behavior under severe accidents. - it may be necessary to confirm the validity of existing core degradation models-used to estimate risk for high bamup fuelt.

The confirmatory research program needs to be augmented with an anticipatory component to give the NRC line organizations the tools to respond to inevitable proposals from licensees for extended fuel bumup. RES has stated that fuel performance models are being refurbished to better predict the thermal and mechanical loads on cladding, as well as the embrittlement of the cladding. We have heard no details on this portion of the program.

Furthermore, RES should begin now to develop criteria for use by the licensees in proposing fuel performance monitoring programs. ~

We can find no immedete justificahon for work proposed by NMSS for fuel with enrichments in excess of 5%. The nuclear industry indicates a reluctance to use fuels that have higher enrichment Should this change, there will be ample time to carry out activities proposed by NMSS.

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Dr. William Mt sck did not participate in the Committee's deliberation regarding this matter.

Sincerely, R. L Seale Chairman

Reference:

Memorandum dated May 22,1998, from Thomas L. King, RES, to John T. Larkinsl Executive Director, ACRS,

Subject:

Transmittal of Advance Copy of Agency Program Plan for High-Bumup Fuel (Predecisional) e e

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