On the use of S(α,β) tables for nuclides with well pronounced resonances

Abstract

The use of S(α,β) tables for evaluating the secondary energy distribution is restricted in the MCNP code [Briesmeister, J.F. (Ed.), 1997. MCNP – A General Monte Carlo N-Particle Transport Code, LA-12625-M] to light isotopes only. The reason is the free gas model in NJOY, [Macfarlane, R.E., Muir, D.W., 1994. The NJOY Nuclear Data Processing System Version 91, LA-12740-M] which does not account for heavy isotopes with strongly energy dependent cross-sections. The joint scattering kernel developed by [Rothenstein, W., Dagan, R., 1998. Ann. Nucl. Energ. 25, 209–222] improved the secondary energy distribution treatment in a manner consistent with the BROADR module in NJOY. [Rothenstein, W., 2004. Ann. Nucl. Energ. 31, 9–23] enabled the generation of S(α,β) by implementing a new formalism for the modified kernel into the NJOY module THERMR. The new generated S(α,β) tables for heavy isotopes with pronounced resolved resonances were added to the MCNP library data files and the MCNP code itself, was modified accordingly. The quantitative effects of using scattering kernel tables in the vicinity of resonances were analyzed by introducing them into the Tellier et al. [Tellier, H., Costa, M., Raepsaet, C., Van der Gucht, C., 1993. 113, 20–30] benchmark problem. The absorption rate and the Doppler effect were calculated for the pronounced eight S-type resonances of 238U within the energy range 2.7–210 eV. The introduced S(α,β) tables for 238U increases the Doppler effect by 23.5% in comparison with the existing MCNP calculation method. Over the entire Resolved Resonance Region (RRR) this means an increase of 17.5%. The overall absorption rate over the entire RRR is increased by 1.4% at 1200 K and by 2.1% at 1800 K.