ML18353A958
| ML18353A958 | |
| Person / Time | |
|---|---|
| Site: | 07003103 |
| Issue date: | 01/08/2019 |
| From: | Jacob Zimmerman NRC/NMSS/DFCSE/FLB |
| To: | Padgett W Louisiana Energy Services |
| KJSturzebecher NMSS/FCSE/FLB 415.8534 | |
| Shared Package | |
| ML18353A957 | List: |
| References | |
| EPID L-2018-LLA-0270 | |
| Download: ML18353A958 (3) | |
Text
REQUEST FOR ADDITIONAL INFORMATION LICENSE AMENDMENT REQUEST 18-04 ADDITION OF MCNP6 FOR NUCLEAR CRITICALITY SAFETY ANALYSIS (ENTERPRISE PROJECT IDENTIFIER: L-2018-LLA-0270)
- 1. Characterization Provide the information identified below to allow the U.S. Nuclear Regulatory Commission (NRC) staff to evaluate the proposed changes contained in License Amendment Request 18-04, UUSA ADDITION OF MCNP6 FOR NUCLEAR CRITICALITY SAFETY ANALYSIS.
- a. Safety Analysis Report, Section 5.2.1.2, Limits on Control and Controlled Parameters, states that the minimum subcritical margin, SM, is taken as 0.03 per the justification provided in the Urenco USA (UUSA) MCNP6 validation report.
NCS-REP-002-00, UUSA MCNP6 Validation, (i.e., the UUSA MCNP6 validation report), Section 6.1.2, Establishing of USL - LEU, states that the minimum subcritical margin is justified, in part, by conservatisms beyond the worst-case normal or credible abnormal conditions, as appropriate, including allowance for any tolerances. It further states that the added conservatism arises from the inclusion of various normal and credible abnormal conditions with a set of inherent or bounding assumptions in modeling practices.
- i. Section 6.1.2 states that under normal conditions an enrichment of 6 percent is assumed for added conservatism. Provide information as to how this supports the justification of the minimum subcritical margin for operations under enrichment control.
ii Section 6.1.2 states that under credible abnormal conditions internal moderation of optimum or physical maximum is assumed; however, it later states that safe-by-design (SBD) components are assumed to be filled with wet uranyl fluoride at optimum or maximum moderation. State whether this assumption applies to non-SBD components, such as the small component decontamination train, various aspects of the multi-function decontamination train and the liquid effluent collection and transfer system bulk tanks. If this assumption does apply to non-SBD components, provide information as to how this is conservative for cases potentially involving heterogeneous solutions. If this assumption does not apply to non-SBD components, provide information as to how this supports the justification of the minimum subcritical margin for operations involving such components.
iii Section 6.2.2, Establishing of USL - LEU and IEU, states that higher enrichment applications are anticipated to operate with the same conservatisms as outlined in Section 6.1.2. Section 6.1.2 states that under normal conditions an enrichment of 6 percent is assumed for added conservatism. Provide information as to how this is appropriate and conservative for intermediate enriched uranium (IEU) applications.
Enclosure
- b. NCS-REP-002-00, Section 4, UUSA Design Application Classification, states that UUSA has only one area of applicability (AOA) for the entire plant, covering uranyl fluoride/water mixtures as well as uranium hexafluoride mixtures. Section 5.1.1, LEU
- Benchmark Selection, states that for the enrichment range of 2-10 wt% U-235, 13 sets of benchmarks consisting of 110 critical configurations were selected for validation. Section 5.2.1 states that for the enrichment range of 2-30 wt% U-235, three additional sets of benchmarks consisting of 23 critical configurations were selected.
i The 13 sets of benchmarks selected for the 2-10 wt% range appropriately involve enrichments between 2-10 wt%. However, the three additional sets of benchmarks selected for the 2-30 wt% range involve enrichments of 12.5 percent, 30.45 percent, and 14.7 percent (wt% U-235). Provide information as to how the selected benchmarks sufficiently validate the 10-30 wt% range as the data is heavily weighted towards enrichments below 10 wt% and include only one benchmark set near the 30 wt% range. Provide information that justifies grouping the entire enrichment range of 2-30 wt% into a single AOA as opposed to two or more separate enrichment ranges.
- c. Section 5.2.1.1, Methods Validation, states that any revision to validation will be performed using ANSI/ANS-8.24, Validation of Neutron Transport Methods for Nuclear Criticality Safety Calculations, as a guideline with exceptions pertaining to the use of positive bias and the rejection of statistical outliers as identified in Regulatory Guide 3.71, Nuclear Criticality Safety Standards for Fuels and Materials Facilities (Revision 2.2010).
i In accordance with Regulatory Guide 3.71, Revision 3 (issue date October 2018),
the NRC no longer takes exception to the use of positive bias and the rejection of statistical outliers. Rather, it provides a clarification that the NRC may choose to evaluate the use of positive bias on a case-by-case basis, and a clarification that the verification of computer code systems must be completed both prior to validation as well as on a periodic basis. State explicitly whether UUSA commits to following the requirements of ANSI/ANS-8.24, or otherwise state exceptions. State whether UUSA acknowledges the clarifications to ANSI/ANS-8.24 detailed in Regulatory Guide 3.71, Revision 3.
- d. Section 5.2.1.2, provides two separate upper subcritical limits (USLs) of 0.958 and 0.96053 for the low enriched uranium (LEU) and LEU-IEU ranges, respectively, and further states that for conservatism the lower of the two USLs (0.958) will be used for all enrichment applications. However, NCS-REP-002-00, Section 7, Conclusions, maintains their separation and does not discuss the use of the lower of the two USLs.
Provide information to explain this inconsistency.
- e. Section 5.2.1.2, Limits on Control and Controlled Parameters, provides a calculated lower tolerance limit, KL, of 0.98837; however, NCS-REP-002-00 states that the lower tolerance limit is 0.98894. Provide information to explain this discrepancy.
- f. NCS-REP-003-00, Urenco USA (UUSA) MCNP6 Verification, states that for keff verification and mathematical testing the differences between the analytical and calculated keff values are mostly within two standard deviations, with the exception of Problems 37, 43, and 71.
2
i LA-UR-16-24254, New Version of the MCNP6 Analytic Criticality Benchmark Suite, issued June 2016, states that Problems 34, 37, 42, 43, and 71 were removed from the MCNP6 benchmark suite due to the fact that anisotropic P1 scattering with the absolute value of the scattering angle, µ, greater than 1/3 is nonphysical. Provide information that justifies the use of an earlier than current version of the MCNP6 benchmark suite for MCNP6 verification.
Regulatory Requirement:
Title 10 of the Code of Federal Regulations (10 CFR) Paragraph 70.61(d).
Regulatory Guidance:
NUREG-1520, Rev. 2; Standard Review Plan for Fuel Cycle Facilities License Applications, June 2015 Basis: 10 CFR 70.61(d) requires that the risk of nuclear criticality accidents be limited by assuring that under normal and credible abnormal conditions, all nuclear processes are subcritical, including use of an approved margin of subcriticality for safety. NUREG-1520, Section 5.4.3.4.4, states that there must be margin that includes, among other uncertainties, adequate allowance for uncertainty in the methodology, data, and bias to assure subcriticality.
Additionally, NUREG-1520 states that the primary purpose of review is to determine, with reasonable assurance, whether the applicant (or licensee) has designed a facility that will provide adequate protection against criticality hazards related to the storage, handling, and processing of licensed materials, as required by 10 CFR Part 70.
3