Information Notice No. 91-26: Potential Nonconservative Errors in the Working Format Hansen-Roach Cross-Section Set 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

                                April 2, 1991

Information Notice No. 91-26:  POTENTIAL NONCONSERVATIVE ERRORS IN THE 
                                   WORKING FORMAT HANSEN-ROACH CROSS-SECTION
                                   SET PROVIDED WITH THE KENO AND SCALE 
                                   CODES 


Addressees: 

All fuel cycle licensees and other licensees, including all holders of 
operating licenses for nuclear power reactors, who use physics codes to 
support criticality safety in the use of fissile material. 

Purpose: 

This information notice is intended to alert addressees to potential 
problems resulting from errors in a common calculational method used in the 
support of criticality safety.  These errors could result in nonconservative 
calculations forming the bases for criticality safety limits and practices.  
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 do not 
constitute Nuclear Regulatory Commission (NRC) requirements; therefore, no 
specific action or written response is required.  

Description of Circumstances: 

Various users of the KENO family of codes have noticed anomalous results 
when using some parts of the neutron cross-section data distributed with the 
KENO code.  The KENO code, a three-dimensional Monte Carlo physics tool, has 
become the dominant method for nuclear criticality safety calculations in 
the U.S. fuel cycle industry.  In the past, the various versions of KENO 
were commonly used in a "stand-alone" mode and used a version of the 
16-group Hansen-Roach cross-section set.  Users referred to this as the 
"stand-alone" Hansen-Roach set; code developers referred to the set as the 
"working-format" library.  Dozens of criticality safety organizations 
developed a library of benchmark calculations based on this "working-format" 
library data set.  The developers of the KENO codes also released a system 
of codes (SCALE) to process physics data into problem-dependent 
cross-section sets for KENO to use.  The SCALE package also included the 
"working-format" library, to allow running the code-manual's sample cases 
and to check past calculations with the current version of the KENO code.  
At this time, KENO IV and KENO V.a are the most commonly used versions of 
the criticality code.  For the code system, SCALE-4 has been recently 
released; SCALE-3 remains the most widely used. 

Since the release of the SCALE system some years ago, the "working-format" 
library was not intended to be used for safety calculations.  It has, 
however, 

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remained in wide use to support criticality safety.  Recently, an NRC 
licensee used the full SCALE-3 system to check a safety evaluation that had 
used the "working-format" set.  The latter was shown to be 3 percent 
nonconservative in keff.  The licensee did not pursue finding examples of 
larger nonconservatism. In conversation with the code developers, the 
licensee learned that there were known errors, for various nuclides, in the 
cross-section "working-format" data set, that had not been corrected because 
the data set was not intended for safety calculations.  

In response to this conversation and others, a warning from the code 
developers to SCALE users and users of the PC version of the code 
(KENO5a-PC) was published in the December 1990, RSIC Newsletter.  As a SCALE 
PC version is not available, PC users need to know about errors in the 
"working-format" library. 

The January 1991, SCALE Newsletter contained the announcement reprinted 
below: 

     HANSEN-ROACH WORKING FORMAT LIBRARY 

     "In addition to the AMPX master format libraries utilized by SCALE, all 
     releases have included a Hansen-Roach 16 group AMPX working format 
     cross section library provided for use with the KENO V.a and XSDRNPM 
     sample problems.  All releases prior to SCALE-4 contain errors in this 
     working format cross section library for the following nuclides:  Mg 
     (12100), SI (14100), S (16100), Mn (25100), Pu-242 (94200), U-238 
     SIGP=400 (92834), and U-238-6R SIGP=400 (92861).  The impact of these 
     errors depends on the amount of the nuclide present and the geometric 
     conditions of the problem.  One problem utilizing Mg, Si, and S in a 
     mixture was observed to experience a 3 percent increase in k-effective 
     when it was run using the corrected cross sections." 
     
     "It was recently recognized that some of the premixed mixtures on the 
     working format library (all releases, including SCALE-4) were created 
     using the incorrect nuclide cross sections mentioned above.  These 
     mixtures include Stainless Steel 304 (200), Oak Ridge Concrete (300), 
     Ordinary Concrete (301), Magnuson Concrete (302), and Magnuson 
     Vermiculite (701).  The corrected working format library is now being 
     distributed by RSIC.  To date, no statistically significant differences 
     due to the changes in these mixtures have been observed.  However, it 
     is recommended that those who have utilized one or more of these 
     mixtures reverify their results using the new working format library." 
     
     "Note that only the working format library contains the above errors.  
     This library was never intended for any use other than running KENO V.a 
     and XSDRNPM sample problems.  The working library will be removed from 
     the next release of SCALE-4."

Discussion of Safety Significance: 

There may exist a large number of criticality safety evaluations that are 
based on the "working-format" (also called stand-alone) Hansen-Roach data 
set distributed with the KENO or SCALE codes.  It is possible that some of 
these evaluations are 
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significantly nonconservative.  User review of those evaluations for 
continuing operations would ensure that adequate safety margins are 
retained.  Note that a corrected version of the data is available.  Versions 
of the Hansen-Roach data from other sources may not have these errors. 

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.  Had appropriate experiments been performed, 
the error described above would likely have been discovered earlier. 

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:  Robert E. Wilson, NMSS
                    (301) 492-0126


Attachments:
1.   List of Recently Issued NRC Information Notices
2.   List of Recently Issued NMSS Information Notices
.