Ahmad M. Ibrahim

ADVANTG An Automated Variance Reduction Parameter Generator

The primary objective of ADVANTG is to reduce both the user effort and the computational time required to obtain accurate and precise tally estimates across a broad range of challenging transport applications. ADVANTG has been applied to simulations of real-world radiation shielding, detection, and neutron activation problems. Examples of shielding applications include material damage and dose rate analyses of the Oak Ridge National Laboratory (ORNL) Spallation Neutron Source and High Flux Isotope Reactor (Risner and Blakeman 2013) and the ITER Tokamak (Ibrahim et al. 2011). ADVANTG has been applied to a suite of radiation detection, safeguards, and special nuclear material movement detection test problems (Shaver et al. 2011). ADVANTG has also been used in the prediction of activation rates within light water reactor facilities (Pantelias and Mosher 2013). In these projects, ADVANTG was demonstrated to significantly increase the tally figure of merit (FOM) relative to an analog MCNP simulation. The ADVANTG-generated parameters were also shown to be more effective than manually generated geometry splitting parameters.

Nuclear Assessment of Final Optics of a KrF Laser Driven Fusion Power Plant

Abstract In the HAPL program, power plant designs are assessed with targets driven by 40 KrF laser beams. The final optics system that focuses the laser onto the target may include a grazing incidence metallic mirror (GIMM) located at 24 m from the target with 85° angle of incidence. The GIMM is in direct line of sight of the target and has a 50 micron thick aluminum coating. Two options were considered for the substrate material; SiC and AlBeMet. The impact of the GIMM design options on the nuclear environment at the dielectric focusing and turning mirrors was assessed. Using AlBeMet results in about a factor of two higher neutron flux. We considered beam duct configuration modifications such as utilizing neutron traps to reduce radiation streaming. In addition, we investigated the impact of lining the beam ducts and neutron traps with different materials that help slowing down and absorbing neutrons.

The Multi-Step CADIS method for shutdown dose rate calculations and uncertainty propagation

Abstract Shutdown dose rate (SDDR) analysis requires (a) a neutron transport calculation to estimate neutron flux fields, (b) an activation calculation to compute radionuclide inventories and associated photon sources, and (c) a photon transport calculation to estimate final SDDR. In some applications, accurate full-scale Monte Carlo (MC) SDDR simulations are needed for very large systems with massive amounts of shielding materials. However, these simulations are impractical because calculation of space- and energy-dependent neutron fluxes throughout the structural materials is needed to estimate distribution of radioisotopes causing the SDDR. Biasing the neutron MC calculation using an importance function is not simple because it is difficult to explicitly express the response function, which depends on subsequent computational steps. Typical SDDR calculations do not consider how uncertainties in MC neutron calculation impact SDDR uncertainty, even though MC neutron calculation uncertainties usually dominate SDDR uncertainty. The Multi-Step Consistent Adjoint Driven Importance Sampling (MS-CADIS) hybrid MC/deterministic method was developed to speed SDDR MC neutron transport calculation using a deterministically calculated importance function representing the neutron importance to the final SDDR. Undersampling is usually inevitable in large-problem SDDR simulations because it is very difficult for the MC method to simulate particles in all space and energy elements of the neutron calculation. MS-CADIS can assess the degree of undersampling in SDDR calculations by determining the fraction of the SDDR response in the space and energy elements that did not have any scores in the MC neutron calculation. It can also provide estimates for upper and lower limits of SDDR statistical uncertainties resulting from uncertainties in MC neutron calculation. MS-CADIS was applied to the ITER SDDR benchmark problem that resembles the configuration and geometrical arrangement of an upper port plug in ITER. Without using the hybrid MC/deterministic methods to speed MC neutron calculations, SDDR calculations were significantly undersampled for all tallies, even when MC neutron calculation computational time was 32 CPU-days. However, all SDDR tally results with MC neutron calculations of only 2 CPU-days converged with the standard Forward-Weighted CADIS (FW-CADIS) method and the MS-CADIS method. Compared to the standard FW-CADIS approach, MS-CADIS decreased the undersampling in the calculated SDDR by factors between 0.9% and 0.3% for computational times between 4 and 32 CPU-days, and it increased the computational efficiency of the SDDR neutron MC calculation by factors between 43% and 69%.

Shutdown Dose Rate Analysis Using the Multi-Step CADIS Method

Abstract The Multi-Step Consistent Adjoint Driven Importance Sampling (MS-CADIS) hybrid Monte Carlo (MC)/deterministic radiation transport method was proposed to speed up the shutdown dose rate (SDDR) neutron MC calculation using an importance function that represents the neutron importance to the final SDDR. In this work, the MS-CADIS method was applied to the ITER SDDR benchmark problem. The MS-CADIS method was also used to calculate the SDDR uncertainty resulting from uncertainties in the MC neutron calculation and to determine the degree of undersampling in SDDR calculations because of the limited ability of the MC method to tally detailed spatial and energy distributions. The analysis that used the ITER benchmark problem compared the efficiency of the MS-CADIS method to the traditional approach of using global MC variance reduction techniques for speeding up SDDR neutron MC calculation. Compared to the standard Forward-Weighted-CADIS (FW-CADIS) method, the MS-CADIS method increased the efficiency of the SDDR neutron MC calculation by 69%. The MS-CADIS method also increased the fraction of nonzero scoring mesh tally elements in the space-energy regions of high importance to the final SDDR.

THREE DIMENSIONAL ANALYSIS OF RADIATION STREAMING THROUGH ARIES-CS HE-ACCESS PIPES

Abstract The effects of neutron streaming through the divertor He-access pipes of the ARIES compact stellarator fusion power plant on the shielding performance of its components were investigated in this analysis. A 3-D analysis for the most promising design of the He-access pipe with shielding plug and inserts indicated that neutron attenuation through the shielded pipe is not sufficient to eliminate the issue of neutron streaming. The results show that the damage exceeded the limits near the pipe for the manifold, vacuum vessel, and magnet. Precautions should be taken that include changing the pipe design and orientation, avoiding rewelding the manifold and vacuum vessel near the pipe, and/or relocating the magnet away from the pipe. The neutron flux behind the pipe is excessive, mandating additional local shield (∼1 m) to protect the externals.

NUCLEAR ASSESSMENT OF SHIELDING CONFIGURATION OPTIONS FOR FINAL OPTICS OF HAPL LASER FUSION POWER PLANT

Abstract The High Average Power Laser (HAPL) power plant has targets that are directly driven by forty KrF laser beams. Three-dimensional neutronics calculations were performed directly in the exact CAD model of the HAPL final optics system to assess the impact of the biological shielding configuration on the nuclear environment at the GIMM and dielectric focusing and turning mirrors. In the initial configuration, the biological shield fully encloses the GIMM sand associated dielectric mirrors. We assessed another configuration where the shield is moved farther from the target to fully enclose the dielectric mirrors leaving the GIMM in the open space between the chamber and the biological shield. A variation of this configuration utilizes 40 neutron traps attached to the inner surface of the biological shield behind the GIMMs. It is concluded that the shielding configuration with all optics including the GIMM being fully enclosed in the biological shield is the preferred option since it results in the lowest nuclear environment at the dielectric mirrors, provides better GIMM support, reduces the volume to be maintained under vacuum, and requires the least amount of concrete shield.

Validation of the MS-CADIS Method for Full-Scale Shutdown Dose Rate Analysis

Abstract Fusion energy systems present increasingly significant computational challenges as they grow in size and complexity. Once constructed, ITER will be a full-size nuclear facility with highly complicated structures and support systems, with an array of scientific equipment in close proximity to the neutron-emitting deuterium-tritium plasma. Characterization of shutdown dose rate (SDDR) distributions caused by the neutron activation of these structures is important to the final design and full-power operation of the device. This work summarizes the theoretical basis and parallel implementation of the Multi-Step Consistent Adjoint-Driven Importance Sampling (MS-CADIS) method designed specifically for highly efficient execution of multistep activation problems. Fusion SDDR benchmark problems have been solved with these new tools, and the results have been compared to experimental and other computational results to establish their validation basis.

Analysis of Radiation Transport due to Activated Coolant in the ITER Neutral Beam Injection Cell

Abstract Detailed spatial distributions of the biological dose rate due to a variety of sources are required for the design of the ITER tokamak facility to ensure that all radiological zoning limits are met. During operation, water in the Integrated loop of Blanket, Edge-localized mode and vertical stabilization coils, and Divertor (IBED) cooling system will be activated by plasma neutrons and will flow out of the bioshield through a complex system of pipes and heat exchangers. This paper discusses the methods used to characterize the biological dose rate outside the tokamak complex due to 16N gamma radiation emitted by the activated coolant in the Neutral Beam Injection (NBI) cell of the tokamak building. Activated coolant will enter the NBI cell through the IBED Primary Heat Transfer System (PHTS), and the NBI PHTS will also become activated due to radiation streaming through the NBI system. To properly characterize these gamma sources, the production of 16N, the decay of 16N, and the flow of activated water through the coolant loops were modeled. The impact of conservative approximations on the solution was also examined. Once the source due to activated coolant was calculated, the resulting biological dose rate outside the north wall of the NBI cell was determined through the use of sophisticated variance reduction techniques. The AutomateD VAriaNce reducTion Generator (ADVANTG) software implements methods developed specifically to provide highly effective variance reduction for complex radiation transport simulations such as those encountered with ITER. Using ADVANTG with the Monte Carlo N-particle (MCNP) radiation transport code, radiation responses were calculated on a fine spatial mesh with a high degree of statistical accuracy. Advanced visualization tools were also developed and used to determine pipe cell connectivity, to facilitate model checking, and to post-process the transport simulation results.

Automatic Mesh Adaptivity for Hybrid Monte Carlo/Deterministic Neutronics Modeling of Difficult Shielding Problems

Abstract The well-established Consistent Adjoint Driven Importance Sampling (CADIS) and the Forward Weighted Consistent Adjoint Driven Importance Sampling (FW-CADIS) hybrid Monte Carlo/deterministic techniques have dramatically increased the efficiency of neutronics simulations, yielding accurate solutions for increasingly complex problems through full-scale, high-fidelity simulations. However, for full-scale simulations of very large and geometrically complex nuclear energy systems, even the CADIS and FW-CADIS techniques can reach the CPU and memory limits of all but the very powerful supercomputers. In this work, three mesh adaptivity algorithms were developed to reduce the computational resource requirements of CADIS and FW-CADIS without sacrificing their efficiency improvements. First, a macromaterial approach was developed to enhance the fidelity of the deterministic models without changing the mesh. Second, a deterministic mesh refinement algorithm was developed to generate meshes that capture as much geometric detail as possible without exceeding a specified maximum number of mesh elements. Finally, a weight window (WW) coarsening (WWC) algorithm was developed to decouple the WW mesh and energy bins from the mesh and energy group structure of the deterministic calculations. By removing the memory constraint of the WW map from the resolution of the mesh and the energy group structure of the deterministic calculations, the WWC algorithm allows higher-fidelity deterministic calculations that, consequently, increase the efficiency and reliability of the CADIS and the FW-CADIS simulations. The three algorithms were used to enhance an FW-CADIS calculation of the prompt dose rate throughout the ITER experimental facility. Using these algorithms increased both the number of mesh tally elements in which nonzero results were obtained (+23.3%) and the overall efficiency of the calculation (a factor of >3.4). The three algorithms enabled this difficult calculation to be accurately solved using an FW-CADIS simulation on a 94-CPU computer cluster, eliminating the need for a world-class supercomputer.