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Title: Progress with On-The-Fly Neutron Doppler Broadening in MCNP

Abstract

The University of Michigan, ANL, and LANL have been collaborating on a US-DOE-NE University Programs project 'Implementation of On-the-Fly Doppler Broadening in MCNP5 for Multiphysics Simulation of Nuclear Reactors.' This talk describes the project and provides results from the initial implementation of On-The-Fly Doppler broadening (OTF) in MCNP and testing. The OTF methodology involves high precision fitting of Doppler broadened cross-sections over a wide temperature range (the target for reactor calculations is 250-3200K). The temperature dependent fits are then used within MCNP during the neutron transport, for OTF broadening based on cell temperatures. It is straightforward to extend this capability to cover any temperature range of interest, allowing the Monte Carlo simulation to account for a continuous distribution of temperature ranges throughout the problem geometry.

Authors:
 [1];  [1];  [2];  [3]
  1. Los Alamos National Laboratory
  2. Argonne National Laboratory
  3. University of Michigan
Publication Date:
Research Org.:
Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
Sponsoring Org.:
DOE/LANL
OSTI Identifier:
1044100
Report Number(s):
LA-UR-12-22277
TRN: US1203330
DOE Contract Number:
AC52-06NA25396
Resource Type:
Conference
Resource Relation:
Conference: 2012 ANS Summer Meeting ; 2012-06-24 - 2012-06-28 ; Chicago, Illinois, United States
Country of Publication:
United States
Language:
English
Subject:
22 GENERAL STUDIES OF NUCLEAR REACTORS; 72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS; 97 MATHEMATICAL METHODS AND COMPUTING; ACCURACY; ANL; CROSS SECTIONS; DISTRIBUTION; DOPPLER BROADENING; GEOMETRY; IMPLEMENTATION; LANL; NEUTRON TRANSPORT; NEUTRONS; SIMULATION; TARGETS; TESTING

Citation Formats

Brown, Forrest B., Martin, William R., Yesilyurt, Gokhan, and Wilderman, Scott. Progress with On-The-Fly Neutron Doppler Broadening in MCNP. United States: N. p., 2012. Web.
Brown, Forrest B., Martin, William R., Yesilyurt, Gokhan, & Wilderman, Scott. Progress with On-The-Fly Neutron Doppler Broadening in MCNP. United States.
Brown, Forrest B., Martin, William R., Yesilyurt, Gokhan, and Wilderman, Scott. Mon . "Progress with On-The-Fly Neutron Doppler Broadening in MCNP". United States. doi:. https://www.osti.gov/servlets/purl/1044100.
@article{osti_1044100,
title = {Progress with On-The-Fly Neutron Doppler Broadening in MCNP},
author = {Brown, Forrest B. and Martin, William R. and Yesilyurt, Gokhan and Wilderman, Scott},
abstractNote = {The University of Michigan, ANL, and LANL have been collaborating on a US-DOE-NE University Programs project 'Implementation of On-the-Fly Doppler Broadening in MCNP5 for Multiphysics Simulation of Nuclear Reactors.' This talk describes the project and provides results from the initial implementation of On-The-Fly Doppler broadening (OTF) in MCNP and testing. The OTF methodology involves high precision fitting of Doppler broadened cross-sections over a wide temperature range (the target for reactor calculations is 250-3200K). The temperature dependent fits are then used within MCNP during the neutron transport, for OTF broadening based on cell temperatures. It is straightforward to extend this capability to cover any temperature range of interest, allowing the Monte Carlo simulation to account for a continuous distribution of temperature ranges throughout the problem geometry.},
doi = {},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Mon Jun 18 00:00:00 EDT 2012},
month = {Mon Jun 18 00:00:00 EDT 2012}
}

Conference:
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  • A new method to obtain Doppler broadened cross sections has been implemented into MCNP, removing the need to generate cross sections for isotopes at problem temperatures. When a neutron of energy E enters a material region that is at some temperature T, the cross sections for that material at temperature T are immediately obtained 'on-the-fly' (OTF) by interpolation using a high order functional expansion for the temperature dependence of the Doppler-broadened cross section for that isotope at the neutron energy E. The OTF cross sections agree with the NJOY-based cross sections for all neutron energies and all temperatures in themore » range specified by the user, e.g., 250 K - 3200 K. The OTF methodology has been successfully implemented into the MCNP Monte Carlo code and has been tested on several test problems by comparing MCNP with conventional ACE cross sections versus MCNP with OTF cross sections. The test problems include the Doppler defect reactivity benchmark suite and two full-core VHTR configurations, including one with multiphysics coupling using RELAP5-3D/ATHENA for the thermal-hydraulic analysis. The comparison has been excellent, verifying that the OTF libraries can be used in place of the conventional ACE libraries generated at problem temperatures. In addition, it has been found that the OTF methodology greatly reduces the complexity of the input for MCNP, resulting in an order of magnitude decrease in the number of input lines for full-core configurations. Finally, for full-core problems with multiphysics feedback, the memory required to store the cross section data is considerably reduced with OTF cross sections and the additional computational effort with OTF is modest, on the order of 10-15%. (authors)« less
  • The capture cross section in the vicinity of the 0.18 eV neutron resonance is calculated for a metallic cadmium sample. The description of Doppler broadening starts from the lattice vibration spectrum of Cd. The result shows only a tiny difference if compared with the free gas approximation for the Doppler broadening. This justifies the application of the free gas approximation in the case of this low energy resonance and the weakly bound Cd lattice. Moreover, for cases of strong lattice binding it can be concluded that the Doppler broadened shape of neutron resonances can be determined with high precision ifmore » a realistic model is used which includes lattice dynamics.« less
  • The neutron capture yield of the 16.3 eV resonance in /sup 107/Ag was measured, using targets of Ag metal, AgCl and Ag/sub 2/O at room, liquid N/sub 2/ and He temperatures. The effective temperature T' defined by Lamb for weak binding, and subsequently the Debye temperature theta/sub D/, were extracted for each case by a least squares shape fit. For Ag metal the measured theta/sub D/ values agree with both the simple Debye theory and a prediction of the Ag lattice dynamics. The theta/sub D/ values for AgCl and Ag/sub 2/O show some variations with temperature. The results for AgClmore » at 77.2 and 4.066/sup 0/, however, agree with the inelastic scattering and calorimetric data. 12 references.« less
  • It is well known that Doppler-broadened cross sections satisfy the usual heat equation of the form delta/sup 2/F/deltax/sup 2/ = deltaF/deltat, where x is the variable in either energy or momentum space and t is proportional to the physical temperature in question. Given the initial condition F(x,O) for -infinity < x < infinite and the boundary condition F(infinity,t) = F (infinity,O); F(-infinity,t) = F(-infinity,O), the function can be calculated by use of finite difference methods. One inherent problem in using such an approach is that excessive numbers of mesh points in both x and t are generally required near themore » peaks of the resonances where the maxima of the function and its higher order derivatives occur. The problem is inevitable where the explicit or implicit scheme is used. An explicit finite difference scheme that is believed to be more suitable than the implicit method in treating the Doppler broadening of the resonance cross sections is described. Pertinent issues and inherent limitations of the method are also addressed.« less