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UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION & OFFICE OF NUCLEAR MATERIAL SAFETY AND SAFEGUARDS WASHINGTON, D.C. 20555 October 18, 1991 NRC INFORMATION NOTICE 91-66: (1) ERRONEOUS DATA IN "NUCLEAR SAFETY GUIDE, TID-7016, REVISION 2," (NUREG/CR-0095, ORNL/NUREG/CSD-6 (1978)) AND (2) THERMAL SCATTERING DATA LIMITATION IN THE CROSS-SECTION SETS PROVIDED WITH THE KENO AND SCALE CODES Addressees All fuel cycle licensees, critical mass licensees, interim spent fuel storage licensees, and all holders of operating licenses or construction permits for test, research, and nuclear power reactors. Purpose The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to alert addressees to errors in a common reference for nuclear criticality safety and to potential problems resulting from limitations in a common cross-section library used in support of criticality safety. It is expected that recipients will review the information for applicability to their facilities and consider actions, as appropriate, to avoid similar problems. However, suggestions contained in this information notice are not NRC requirements; therefore, no specific action or written response is required. Description of Circumstances (1) Erroneous data exists in the "Nuclear Safety Guide, TID 7016, Revision 2," further identified as NUREG/CR-0095, ORNL/NUREG/CSD-6 (1978). Table 2.8 of this guide presents calculated subcritical limits for mixtures of U(93.5) metal, water, and graphite. The values in the table had intended to indicate limits corresponding to a multiplication factor of 0.95. However, two independent groups working with such mixtures discovered errors in the table. Calculations performed at Oak Ridge National Laboratory (ORNL) suggested that limits presented in the table correspond to multiplication factors as high as 1.09. ORNL has generated a corrected table. Validation was based on an extrapolation of experimental data for U(100) mixtures. Copies of the corrected table may be obtained from G. Elliot Whitesides, Site Manager, Computing and Telecommunications Division, Oak Ridge National Laboratory, Post Office Box 2008, Oak Ridge, Tennessee 37831. (2) Holtec International notified ORNL of a discrepancy between CASMO-3 and NITAWL-KENO-V.a calculations involving fuel storage at elevated temperatures. 9110100267 . IN 91-66 October 18, 1991 Page 2 of 3 Subsequently, the Nuclear Engineering Application Division (NEAD) of ORNL confirmed the discrepancy and released the following article in the July 1991, SCALE Newsletter: THERMAL SCATTERING DATA LIMITATION Stan Turner of Holtec International brought to the attention of NEAD a discrepancy between KENO-V.a and CASMO-3 calculations for light-water reactor fuel assemblies in a storage configuration as a function of temperature. The two codes were in relative agreement at low temperatures (20�C) and small water gaps between assemblies. However, as the temperature of the system was increased to 120�C, the CASMO calculations gave increasingly larger values of keff relative to KENO. Larger water gaps (between assemblies) enhanced the temperature effects on calculated keff. The KENO results were as much as 3% lower than CASMO at 120�C. The cause of this discrepancy has been identified as a limitation in processing the thermal scattering data when NITAWL makes a working library. The SCALE 27-group ENDF/B-IV hydrogen has scattering matrices at 293 K and 550 K (20�C and 277�C). When NITAWL processes hydrogen, the scattering matrix with a temperature closest to that specified is used. Room temperature hydrogen (293 K) was used by NITAWL in all of the calculations performed by Turner. WORKER, a new SCALE module under development, was used to interpolate the thermal scatter matrix for one of the 120�C cases and the effective multiplication factor agreed to within 0.5% of the CASMO-3 value. Thus, it appears that the temperature dependence of the scattering can increase the multiplication factor by approximately 2.5%. Another KENO case was run with the 277�C scattering matrix and resulted in an additional increase of 1.7% in keff which was conservative in comparison to CASMO. Users should be aware of this problem and ensure that the hydrogen scattering matrices used in their analyses are either appropriate or conservative with respect to keff. With regard to the other criticality libraries in SCALE, hydrogen has kernels at 295 K and 345 K in the 123-group library and only at room temperature in the 16-group library. Information on the thermal kernels for other nuclides may be found in Section M4 of the SCALE manual. No changes are planned for any of the existing SCALE-4 cross section libraries, but there are plans to eventually release new ENDF/B-V cross section libraries which will have hydrogen kernels at 296, 350, 400, 450, 500, 600, 800, and 1,000 K. Discussion of Safety Significance There may exist criticality safety evaluations that are based on the incorrect data provided in the Nuclear Safety Guide, TID-7016, or on calculations with . IN 91-66 October 18, 1991 Page 3 of 3 inappropriate hydrogen scattering matrices. It is possible that some evaluations are significantly nonconservative. User review of those evaluations for continuing operations would ensure that adequate safety margins are retained. For safety calculations, it is important for users to adequately validate the code, physics data, and modeling methods, for the intended application, with appropriate experiments. It is important to recognize the limitations of computer programs and cross-section libraries. This information notice requires no specific action or written response. If you have any questions about the information in this notice, please contact the technical contact listed below or the appropriate NRC project manager. Charles E. Rossi, Director Richard E. Cunningham, Director Division of Operational Events Assessment Division of Industrial and Office of Nuclear Reactor Regulation Medical Nuclear Safety Office of Nuclear Material Safety and Safeguards Technical contact: Charles H. Robinson, NMSS (301) 492-0576 Attachments: 1. List of Recently Issued NRC Information Notices 2. List of Recently Issued NMSS Information Notices .
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