ML20211F815
| ML20211F815 | |
| Person / Time | |
|---|---|
| Issue date: | 07/30/1999 |
| From: | Brach E NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS) |
| To: | NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS) |
| Shared Package | |
| ML20211F806 | List: |
| References | |
| NUDOCS 9908310080 | |
| Download: ML20211F815 (4) | |
Text
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UNITED STATES g
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NUCLEAR REGULATORY COMMISSION 2
WASHINGTON, D.C. 2006dWlo01
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July 30, 1999 MEMORANDUM TO:
r O Staff Members FROM:
E. William Brach, Director
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l Spent Fuel Project Office
' Office of Nuclear Material Safety and Safeguards
SUBJECT:
ISSUANCE OF REVISION 1 OF SFPO DIRECTOR'S INTERIM STAFF GUIDANCE DOCUMENT 8 Attached for your use and information is Revision 1 of the Spent Fuel Project Office (SFPO)
Director's Interim Staff Guidance Document 8 (ISG-8). This revised interim staff guidance concerns the issue of "Burnup Credit in the Criticality Safety Analyses of PWR Spent Fuelin Transport and Storage Casks."
This document is being provided to you to help ensure consistent reviews by the SFPO staff. If you have any comments or questions about the attached guidance document, please contact your immediate supervisor.
Attachment:
As stated
-e.
CONTACT:
Donald E. Carlson, SFPO/NMSS 301 415-8507 Francis 1. Young, SFPO/NMSS 301-415-3207 Lawrence E. Kokajko, SFPO/NMSS 301-415 1309 e
9908310080 990820 PDR ORG NOMA PDR Enclosure
4 Spent Fuel Project Office Interim Staff Guidance-8 Revision 1 issue: Burnup Credit in the Criticality Safety Analyses of PWR Spent Fuel in Transport snd Storage Casks introduction:
Unirradiated reactor fuel has a well-specified nuclide composition that provides a straightforward and bounding approach to the criticality safety analysis of transport and storage casks. As the fuel is irradiated in the reactor, the nuclide composition changes and, ignoring the presence of bumable poisons, this composition change will cause the reactivity of the fuel to decrease. Allowance in the criticality safety analysis for the decrease in fuel reactivity resu! ting from irradiation is typically termed burnup credit. Extensive investigations have been performed both within the United States and by other countries in an effort to understand and document the technicalissues related to burnup credit. Much of this work has been considered in the development of the U.S. Department of Energy's Topical Report (TR) on Actinide Only Burnup Credit for Pressurized Water Reactor (PWR) Spent Nuclear Fuel Packages (DOE /RW-0472).
The technical information provided in the literature and in the various TR hevisions, together with the initial confirmatory analyses by the NRC research program, have provided a sufficient oasis for the staff to proceed with acceptance of a burnup credit approach in the criticality safety analysis of PWR spent fuel casks as discussed in the Recommendations below.
Although insights gained from reviewing the TR submittals form a part of the basis for the staff's position, this interim staff guidance does not approve the Topical Report or its supporting documentation. The following recommendations provide a cask specific basis for granting burnup credit, based on actinide composition. The U.S. Nuclear Regulatory Commission's (NRC) staff will issue additional guidance and/or recommendations as information is obtained from its research program on burnup credit and as experience is gained through future licensing activities. Except as specified in the following recommendations, the application of bumup credit does not alter the current guidance and recommendations provided by the NRC staff for criticality safety analysis of transport and storage casks.
Recommendations:
1.
Limits for the Licensing Basis. The licensing-basis analysis performed to demonstrate criticality safety should limit the amount of burnup credit to that available from actinide compositions associated with PWR irradiation of UO, fuel to an assembly-average bumup value of 40 GWd/MTU or less. This licensing-basis analysis should assume an out-of-reactor cooling time of five years and should be restricted to intact assemblies that have not used bumable absorbers. The initial enrichment of the fuel assumed for the licens!.,g-basis analysis should be no more than 4.0 wt% rasU unless a loading offset is applied. The
' loading offset is defined as the minimum amount by which the assigned bumup loading value (see Recommendation 5) must exceed the burnup value used in the licensing safety basis analysis. The loading offset should be at least 1 GWd/MTU for every 0.1 wt%
increase in initial enrichment above 4.0 wt%. In any case, the initial enrichment shall not exceed 5.0 wt%. For example,if the applicant performs a safety analysis that demonstrates an appropriate suberitical margin for 4.5 wt% fuel burned to the limit of 40 GWd/MTU, then the loading curve (see Recommendation 4) should be developed to
J ISG-8 Rev.1 2
ensure that the assigned burnur loading value is at least 45 GWd/MTU (i.e., a 5 GWd/MTU loading offset resulting from the 0.5 wt% excess enrichment over 4.0 wt%).
I Applicants requesting use of actinide compositions associated with fuel assemblies, burnup values, or cooling times outside these specifications, or applicants requesting a relaxation of the loading offset for initial enrichments between 4.0 and 5.0 wt%, should provide the measurement data and/or justify extrapolation techniques necessary to adequately extend the isotopic validation and quantify or bound the bias and uncertainty.
2.
Code Validation. The applicant should ensure that the analysis methodologies used for predicting the actinide compositions and determining the neutron multiplication factor (k-effective) are properly validated. Bias and uncertainties associated with predicting the actinide compositions should be determined from benchmarks of applicable fuel assay measurements. Bias and uncertainties associated with the calculation of k-effective should be derived from benchmark experiments that represent important features of the cask design and spent fuel contents. The particular set of nuclides used to determine the k-effective value should be limited to that established in the validation process. The bias and uncertainties should be applied in a way that ensures conservatism in the licensing safety analysis. Particular consideration should be given to bias uncertainties arising from the lack of critical experiments that are highly prototypical of spent fuelin a cask.
3.
Licensing-Basis Model Assumptions. The applicant a,hould ensure that the actinide comporitions used in analyzing the licensing safety basis (as described in Recommendation 1) are calculated using fuel design and in reactor operating parameters selected to provide conservative estimates of the k-effective value under cask conditions.
The calculation of the k-effective value should be performed using cask models, appropriate analysis assumptions, and code inputs that allow adequate representation of the physics. Of particular concern should be the need to account for the axial and horizontal variation of the bumup within a spent fuel assembly (e.g., the assumed axial burnup profiles), the need to consider the more reactive actinide compositions of fuels bumed with fixed absorbers or with control rods fully or partly inserted, and the need for a l
k-effective model that accurately accounts for local reactivity effects at the less-bumed axial ends of the fuel region.
4.
Loading Curve. The applicant should prepare one or more loading curves that plot, as a function of initial enrichment, the assigned bumup loading value above which fuel assemblies may be loaded in the cask. Loading curves should be established based on a 5-year cooling time and only fuel cooled at least five years should be loaded in a cask approved for bumup credit.
l 5.
Assigned Burnup Loading Value. The applicant should describe administrative procedures that should be used by licensees to ensure thrit the cask will be loaded with fuel that is within the specifications of the approved contents. The administrative procedures should include an assembly measurement that confirms the reactor record assembly burnup. The measurement technique may be calibrated to the reactor records
'for a representative set of assemblies. For an assembly reactor burnup record to be confirmed, the measurement should provide agreement within a 95 percent confidence interval based on the measurement uncertejnty. The assembly burnup value to be used for loading acceptance (termed the assigned burnup loading value) should be the l
confirmed reactor record value as adjusted by reducing the record value by the combined uncertainties in the records and the measurement.
I
I l
l ISG-8 Rev.1 3
6.
Estimate of Additional Reactivity Margin. The applicant should provide design specific analyses that estimate the additional react 9ity margins available from fission product and actinide nuclides not included in the licensing safety basis (as described in Recommendation 1). The analysis methods used for determining these estimated reactivity margins should be verified using available experimental data (e.g., isotopic assay data) and computational benchmarks that demonstrate the performance of the applicant's methods in comparison with independent methods and analyses. The Organization for Economic Cooperation and Development Nuclear Energy Agency's Working Group on Burnup Credit provides a source of computational benchmarks that may be considered. The design-specific margins should be evaluated over the full range of initial enrichments and burnups on the burnup credit loading cunie(s). The resulting estimated margins should then be assessed against estimates of: (a) any uncertainties not directly evaluated in the modeling or validation processes for actinide-only burnup credit (e.g., k-effective validation uncertainties caused by a lack of critical experiment benchmarks with either actinide compositions that match those in spent fuel or material geometries that represent the most reactive ends of spent fuelin casks); and (b) any potential nonconservatisms in the models for calculating the licensing-basis actinide" inventories (e.g., any outlier assemblies with higher-than-modeled reactivity caused by the use of control rod insertion during burnup).
l Approved E. William Brach Date i
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