How Accurately Can We Calculate Neutrons Slowing Down In Water ?

Abstract

DISCLAIMERThis document was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor the University of California nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or the University of California. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or the University of California, and shall not be used for advertising or product endorsement purposes.Work performed under the auspices of the U. S. Department of Energy by the University of California Lawrence Livermore National Laboratory under Contract W-7405-Eng-48.This report has been reproduced directly from the best available copy. IntroductionWe 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 2) correcting errors in the codes; no code is perfect.-2 -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 compariso...

Neutron-induced transmutation effects in W and W-alloys in a fusion environment

“…Note that a proper S(α, β) bound thermalization treatment was included for H 2 O (with a free gas model applied to all other nuclides). Without any thermal scattering (stationary), neutrons continue to slow down in energy and never form a thermal spectrum, while in contrast, using free atom scattering (proper bound or free gas model), the spectrum diverges from this continuous slowing down and eventually forms a Maxwellian-like thermal spectrum [27]. In the event, the S(α, β) treatment was not found to have any noticeable effect on reaction rates because, in the absorption reactions considered here, the giant resonances generally occurred at slightly higher neutron energies than those influenced by it, which are below 4 eV [26].…”

Verification of High Temperature Free Atom Thermal Scattering in MERCURY Compared to TART

“…In earlier studies comparing results from a variety of neutron transport codes each using whatever neutron data it felt to be appropriate, we found a spread in even the simple integral parameters, such as K-eff, in excess of 1 % [4]. In earlier comparisons of MERCURY and TART we found that by having both codes use the same neutron data, namely TART's data, the differences in Keff could be reduced to less than 0.1% [7], indicating that most of the differences between codes is due to differences in nuclear data. In this study both codes are again using the same TART data and same physics models, so we expect results to be at the same 0.1 % or better level.…”

“…In particular we want to verify the free atom scattering treatment in MERCURY as used over an extended temperature range; by comparison to many other codes for TART this has already been verified over many years [4,7].…”

Impact of the thermal scattering law of H in H2O on the isothermal temperature reactivity coefficients for UOX and MOX fuel lattices in cold operating conditions

“…They modify the energy and angular distributions of secondary neutrons. A description of the model for water is given in references [24,25], and studies that investigate how we can accurately calculate neutrons slowing down in water are reported in reference [26]. The double differential incoherent inelastic scattering cross-section of a single bound atom in molecule (H bound in H 2 O) can be written as a function of the symmetric scattering law S(a,b):…”