How Accurately Can We Calculate Neutrons Slowing Down In Water ?

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We have compared the results produced by a variety of currently available Monte Carlo neutron transport codes for the relatively simple problem of a fast source of neutrons slowing down and thermalizing in water. Initial comparisons showed rather large differences in the calculated flux; up to 80% differences. By working together we iterated to improve the results by: (1) insuring that all codes were using the same data, (2) improving the models used by the codes, and (3) correcting errors in the codes; no code is perfect. Even after a number of iterations we still found differences, demonstrating that our ... continued below

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Cullen, D E; Blomquist, R; Greene, M; Lent, E; MacFarlane, R; McKinley, S et al. March 30, 2006.

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We have compared the results produced by a variety of currently available Monte Carlo neutron transport codes for the relatively simple problem of a fast source of neutrons slowing down and thermalizing in water. Initial comparisons showed rather large differences in the calculated flux; up to 80% differences. By working together we iterated to improve the results by: (1) insuring that all codes were using the same data, (2) improving the models used by the codes, and (3) correcting errors in the codes; no code is perfect. Even after a number of iterations we still found differences, demonstrating that our Monte Carlo and supporting codes are far from perfect; in particularly we found that the often overlooked nuclear data processing codes can be the weakest link in our systems of codes. The results presented here represent the today's state-of-the-art, in the sense that all of the Monte Carlo codes are modern, widely available and used codes. They all use the most up-to-date nuclear data, and the results are very recent, weeks or at most a few months old; these are the results that current users of these codes should expect to obtain from them. As such, the accuracy and limitations of the codes presented here should serve as guidelines to code users in interpreting their results for similar problems. We avoid crystal ball gazing, in the sense that we limit the scope of this report to what is available to code users today, and we avoid predicting future improvements that may or may not actual come to pass. An exception that we make is in presenting results for an improved thermal scattering model currently being testing using advanced versions of NJOY and MCNP that are not currently available to users, but are planned for release in the not too distant future. The other exception is to show comparisons between experimentally measured water cross sections and preliminary ENDF/B-VII thermal scattering law, S({alpha},{beta}) data; although these data are strictly preliminary they are currently available and undergoing testing and these results were judged to be within the scope of this report.

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  • Report No.: UCRL-TR-220605
  • Grant Number: W-7405-ENG-48
  • DOI: 10.2172/889442 | External Link
  • Office of Scientific & Technical Information Report Number: 889442
  • Archival Resource Key: ark:/67531/metadc873489

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  • March 30, 2006

Added to The UNT Digital Library

  • Sept. 21, 2016, 2:29 a.m.

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  • Dec. 5, 2016, 7:45 p.m.

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Cullen, D E; Blomquist, R; Greene, M; Lent, E; MacFarlane, R; McKinley, S et al. How Accurately Can We Calculate Neutrons Slowing Down In Water ?, report, March 30, 2006; Livermore, California. (digital.library.unt.edu/ark:/67531/metadc873489/: accessed October 17, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.