Discrete Ordinates Radiation Transport Code Research Articles

Application of the Denovo Discrete Ordinates Radiation Transport Code to Large-Scale Fusion Neutronics

Fusion energy systems pose unique challenges to the modeling and simulation community. These challenges must be met to ensure the success of the ITER experimental fusion reactor. ITER’s complex systems require detailed modeling that goes beyond the scale of comparable simulations to date. In this work, the Denovo radiation transport code was used to calculate neutron fluence and kerma for the JET streaming benchmark. This work was performed on the Titan supercomputer at the Oak Ridge Leadership Computing Facility. Denovo is a novel three-dimensional discrete ordinates transport code designed to be highly scalable. Sensitivity studies have been completed to examine the impact of several deterministic parameters. Results were compared against experiment as well as the MCNP and Shift Monte Carlo codes.

A Stable 1D Multigroup High-Order Low-Order Method

ABSTRACTThe high-order low-order (HOLO) method is a recently developed moment-based acceleration scheme for solving time-dependent thermal radiative transfer problems, and has been shown to exhibit orders of magnitude speedups over traditional time-stepping schemes. However, a linear stability analysis by Haut et al. (2015) revealed that the current formulation of the multigroup HOLO method was unstable in certain parameter regions. Since then, we have replaced the intensity-weighted opacity in the first angular moment equation of the low-order (LO) system with the Rosseland opacity. This results in a modified HOLO method (HOLO-R) that is significantly more stable.This paper has two primary components. First, we present results from a linear stability analysis of the HOLO-R method for a 2-group problem. These results show that HOLO-R is stable in all regions of the parameter space we considered, and strongly suggest that HOLO-R is unconditionally stable. The predicted decay factors from the stability analysis are verified with numerically computed decay factors from a spectral HOLO-R code. Second, we describe a preliminary (unoptimized) implementation of HOLO-R in Capsaicin – a deterministic, discrete-ordinates radiation transport code at Los Alamos National Laboratory. The results from this implementation further verify the stability of the HOLO-R method. Moreover, in the simulation of a 50-group, nonlinear Marshak wave problem, away from the 2-group, linearized stability analysis, we assess HOLO-R’s accuracy, efficiency, and robustness as compared to current methods in Capsaicin. To our knowledge, this is the first implementation of HOLO that uses a mass-lumped discontinuous Galerkin spatial discretization for both the high-order (HO) and LO systems, and details of this discretization are provided in the Appendix.

Massively Parallel, Three-Dimensional Transport Solutions for the k-Eigenvalue Problem

We have implemented a new multilevel parallel decomposition in the Denovo discrete ordinates radiation transport code. In concert with Krylov subspace iterative solvers, the multilevel decomposition allows concurrency over energy in addition to space-angle, enabling scalability beyond the limits imposed by the traditional Koch-Baker-Alcouffe (KBA) space-angle partitioning. Furthermore, a new Arnoldi-based k-eigenvalue solver has been implemented. The added phase-space concurrency combined with the high-performance Krylov and Arnoldi solvers has enabled weak scaling to O(105) cores on the Titan XK7 supercomputer. The multilevel decomposition provides a mechanism for scaling to exascale computing and beyond.

Sensitivity Analysis and Data Assimilation in a Subcritical Plutonium Metal Benchmark

This work presents the application of first-order adjoint sensitivity analysis, uncertainty quantification, and data assimilation to a subcritical plutonium benchmark experiment using a modified version of the discrete ordinates radiation transport code Denovo. Previous Monte Carlo simulations of this benchmark saw a consistent overprediction of the mean and variance of the measured neutron multiplicity distribution. It was observed that a small scalar reduction in the value of the 239Pu-induced fission neutron multiplicity was capable of significantly reducing the discrepancies. This work extends those results by computing first-order sensitivities to each nuclide, reaction type, energy, and material region in the benchmark. The sensitivities are then used in a data assimilation methodology to simultaneously calibrate all responses and multigroup nuclear data. The resulting best-estimate values for the energy group differential multiplicity ν̄(Eg) are 1σ to 2σ less than the nominal values found in ENDF/B-VII for energies less than ˜1.5 MeV.

A C5 Benchmark Problem with the Discrete Ordinates Radiation Transport Code Denovo

The C5 benchmark problem proposed by the Organisation for Economic Co-operation and Development/Nuclear Energy Agency was modeled to examine the capabilities of Denovo, a three-dimensional (3-D) parallel discrete ordinates (SN) radiation transport code, for problems with no spatial homogenization. Denovo uses state-of-the-art numerical methods to obtain accurate solutions to the Boltzmann transport equation. Problems were run in parallel on Jaguar, a high-performance supercomputer located at Oak Ridge National Laboratory. Both the two-dimensional (2-D) and 3-D configurations were analyzed, and the results were compared with the reference MCNP Monte Carlo calculations. For an additional comparison, SCALE/KENO-V.a Monte Carlo solutions were also included. In addition, a sensitivity analysis was performed for the optimal angular quadrature and mesh resolution for both the 2-D and 3-D infinite lattices of UO2 fuel pin cells. Denovo was verified with the C5 problem. The effective multiplication factors, pin powers, and assembly powers were found to be in good agreement with the reference MCNP and SCALE/KENO-V.a Monte Carlo calculations.

Comparison of Three-Dimensional Flux Synthesis and Full Three-Dimensional Discrete Ordinates Methods for the Calculation of Reactor Cavity Bioshield Heat Generation Rates

In the computational fluid dynamics analysis to determine the necessary cooling airflow rates in the reactor cavity of a nuclear power plant during operation, the heat generated in the sacrificial bioshield and adjacent components is a significant source term. Traditionally, a three-dimensional (3-D) flux synthesis method is used to calculate the heat generation rate in the bioshield for reactors with a cylindrical reactor cavity because there is minimal azimuthal variation. However, the AP1000™ reactor incorporates an octagonal reactor cavity design with 12 ex-core detectors, leading to potentially significant impacts on the azimuthal heat generation rate distribution. Therefore, it was of interest to benchmark the traditional flux synthesis method with full 3-D discrete ordinates methods. Because of an uncertainty in the amount of mesh refinement necessary to have confidence in the results, a sensitivity study on the mesh refinement was performed with a parallel 3-D discrete ordinates code. This allowed a comparison with an industry-standard serial 3-D discrete ordinates code in terms of both execution speed and calculated results.The results suggest that for angular positions where the flux synthesis method incorporates an axial model, there is relatively good agreement with 3-D methods (within ±20%). In areas remote from axial models, there are differences of up to a factor of 2 in a nonconservative direction. Furthermore, a recently developed parallel 3-D discrete ordinates radiation transport code was shown to produce results generally consistent with the industry-standard 3-D code used (within 2.5%). Finally, the parallel code completed its calculations in 10% of the time required by the serial code for an identically sized problem.

Twin Ionisation Chambers for Dose Determiniations in Phantom in an Epithermal Neutron Beam

The validity of the twin ionisation chamber technique for determination of gamma and neutron doses in an epithermal neutron beam has been studied by comparisor of measured and calculated dose distributions in a water phantom. The measurements were performed in two epithermal neutron beams, designed for boron neutron capture therapy (BNCT), at the FiR I TRIGA nuclear reactor using two pairs of A 150 tissue-equiv alent and non-hydrogenous Mg(Ar) ionisation chambers. Doses were calculated by the two-dimensional discrete ordinates radiation transport code (DORT) applying the fluence-to-dose conversion factors by ICRU. At the dose maximum in the phantom 5% consistency between measured and calculated results was achieved for both neutron and gamma doses. Improv ements in chamber materials and precise knowledge of the fluence perturbation effects of the chambers are needed to decrease the measurement uncertainty to an acceptable level.

Radiation exposure inside reinforced concrete buildings at Nagasaki.

In this study, the radiation doses to occupants of two reinforced concrete buildings at Nagasaki, who survived the immediate effects of the nuclear weapon detonation, are determined using state-of-the-art radiation transport techniques. The radiation doses at all locations in the buildings are calculated using the Three-Dimensional Oak Ridge Discrete Ordinates Radiation Transport Code which was constructed especially for this task. This code represents a new and unique capability that has been previously reported. This study resulted in case-by-case lists of doses to occupants and an uncertainty analysis. These data have been used in a companion study as the basis for determining a new value of the dose producing a 50% risk of fatality.