Handling of erroneous ACE data

Report any suspected bugs and unexpected behavior here

Handling of erroneous ACE data

Postby Jaakko Leppänen » Wed Aug 24, 2011 9:55 pm

This is not really a problem in the code, but something the user should be aware of when comparing results to other codes, especially MCNP using the same ACE format data libraries...

The NJOY processing code sometimes produces errors in the data files, which may not be recognized by the transport code. So far I have encountered at least two problems in the Serpent data libraries, which result in an entirely different outcome in Serpent and MCNP.

1) Negative reaction cross sections are reset to zero when Serpent reads the data. MCNP apparantly uses the data as it is. Even though the probability of sampling such reaction is zero in both cases, the negative values in the MCNP calculation reduce the total cross section, which in turn increases the neutron mean free path. Both codes produce systematically incorrect results (garbage in - garbage out), but the results are not compatible with each other either. This problem is most prominent in Ti-46 data in ENDF/B-VI.8, JEF-2.2, JEFF-3.1 and JEFF-3.1.1 libraries, which contains 21 negative values in elastic scattering cross section at high energy.

2) Reaction modes are identified not only by the ENDF MT number, but also by another value that defines neutron multiplicity and the frame-of-reference for scattering laws. This number should be set to 19 for fission reactions, but for some nuclides the value is set to zero, and MCNP consequently interprets the reaction modes as absorption. Serpent, on the other hand, identifies fission reactions from the MT number, and the incorrect type number is ignored. The result is that MCNP cannot sample fission for these nuclides, but Serpent reads the data without problems. This issue conserns at least Am-242 data in ENDF/V-II and Cm-243 and -245 in ENDF/B-VI.8.

Both the above problems concern the Serpent data libraries distributed with the NEA/RSICC installation package with code version 1.1.7. I will add warning messages to identify the problems in the next update (1.1.16).
- Jaakko
User avatar
Jaakko Leppänen
Site Admin
 
Posts: 1936
Joined: Thu Mar 18, 2010 10:43 pm
Location: Espoo, Finland

Return to Bug reports

Who is online

Users browsing this forum: No registered users and 1 guest