Development of the Window Multipole Library Generation Code

Abstract

This paper presents the window multipole library generation code, which is under development in Ulsan National Institute of Science and Technology (UNIST). This code has been developed to produce multipole library with the high efficiency, which can be used in on-the-fly Doppler broadening of microscopic cross section during the Monte Carlo based multi-physics coupling simulations. ENDF/B-VII.1 neutron induced library contains in total 423 isotopes information. There are more than 300 isotopes containing resonance parameters, which are used to reconstruct the resolved resonance region cross section at 0 K relying on the R-Matrix theory. The remaining isotopes without resonance parameters contain the point-wise cross section in the nuclear data files. The code converts those resonance parameters into the rigorous multipoles, which can be directly used to reproduce the cross section at the various temperatures. In order to obtain the poles, the high order polynomial with complex coefficient are solved using the Newton-Raphson method. The point-wise background cross sections from evaluated nuclear data file are added to the reconstructed resolved resonance cross section. Next, the optimization of the library is performed using the energy window concept. The resolved resonance region is evenly divided into sub-regions (windows). Only several poles contributing to the resonance peaks in the specific window are used to reproduce cross section, the remaining contribution from omitted poles is compensated using polynomial fitting. The given approach is allowed to reduce memory and required time to generate accurate cross section at the temperature of the interest. The code finds the optimal number of windows, important poles for the every window, and order of polynomial, which satisfy the user error criteria for the reconstructed cross section. Finally, the code results in optimized window multipole library with polynomial coefficients, number of windows, and crucial multipole parameters for every window.