United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 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

                                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
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                                                            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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