Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
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 (20øC) and small water gaps between assemblies. However,
as the temperature of the system was increased to 120øC, 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 120øC.
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 (20øC and 277øC). 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 120øC 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 277øC 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
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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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