Tuomas Viitanen

Explicit Treatment of Thermal Motion in Continuous-Energy Monte Carlo Tracking Routines

Abstract This paper introduces a new stochastic method for taking the effect of thermal motion into account on the fly in a Monte Carlo neutron transport calculation. The method is based on explicit treatment of the motion of target nuclei at collision sites and, consequently, requires simply cross sections at a temperature of 0 K regardless of the number of temperatures in the problem geometry. It utilizes rejection sampling techniques to manage the fact that total cross sections become distributed quantities. The method has a novel capability of accurately modeling continuous temperature distributions. The new stochastic method is verified using a simple test program, which compares its results to an analytical reference solution based on NJOY-broadened cross sections. Future implementation to Monte Carlo reactor physics code Serpent is also discussed shortly.

Target Motion Sampling Temperature Treatment Technique with Elevated Basis Cross-Section Temperatures

Abstract The target motion sampling (TMS) temperature treatment technique, previously known as “explicit treatment of target motion,” is a stochastic method for taking the effect of thermal motion on reaction rates into account on-the-fly during Monte Carlo neutron tracking. The method is based on sampling target velocities at each collision site and dealing with the collisions in the target-at-rest frame using cross sections below the actual temperature of the nuclide or, originally, 0 K. Previous results have shown that transport with the original implementation of the TMS method requires about two to four times more CPU time than conventional transport methods, depending on the case. In the present paper, it is observed that the overhead factor may increase even above 10 in cases involving burned fuel. To make the method more practical for everyday use, some optimization is required. This paper discusses a TMS optimization technique in which the temperatures of the basis cross sections are elevated above 0 K. Comparisons show that the TMS method is able to reproduce the NJOY-based reference results within statistical accuracy, both with and without the newly implemented optimization technique. In the specific test cases, the optimization saved 35% to 83% of the calculation time, depending on the case.

Temperature Majorant Cross Sections in Monte Carlo Neutron Tracking

Abstract This paper discusses the generation of temperature majorant cross sections, the type of cross sections required by two separate techniques related to Monte Carlo neutron tracking, namely, the Doppler-broadening rejection correction (DBRC) and target motion sampling (TMS) temperature treatment methods. In the generation of these cross sections, the theoretically infinite range of thermal motion must be artificially limited by applying some sort of a cutoff condition, which affects both the accuracy and the performance of the calculations. In this paper, a revised approach for limiting thermal motion is first introduced, and then, optimal cutoff conditions are determined for both the traditional majorant, commonly used in DBRC implementations and old implementations of the TMS method, and the revised majorant. Using the revised type of temperature majorant cross sections increases the performance of the TMS method slightly, but no practical difference is observed with the DBRC method. It is also discovered that in ordinary reactor physical calculations, the cutoff conditions originally adopted from the SIGMA1 Doppler-broadening code can be significantly relieved without compromising the accuracy of the results. By updating the cutoff conditions for majorant generation, the CPU time requirement of Serpent 2.1.17 is reduced by 8% to 23% in TMS calculations and by 1% to 6% in problems involving DBRC.

Internal Neutronics-Temperature Coupling in Serpent 2

Abstract This paper describes the built-in calculation routines in the reactor physics code Serpent 2 that provide a novel method for solving the coupled problem of the power distribution, temperature distribution, and material property distributions in nuclear fuel elements. All of the coupled distributions are solved during a single simulation with no coupling to external codes. The temperature feedback system consists of three separate built-in parts: an explicit treatment of the thermal motion of target nuclides during the transport calculation, an internal analytic radial temperature profile solver, and internal material property correlations. The internal structure and couplings of the calculation routines are described in detail, after which the results of an assembly-level problem are presented to demonstrate the capabilities and functionality of the system.