Zoltán Perkó

Minor Actinide Transmutation in GFR600

Abstract Within the Generation IV initiative, the gas-cooled fast reactor (GFR) is one of the reactors dedicated to minor actinide (MA) transmutation. This paper summarizes the research performed with the GFR600 reference design in order to assess its MA burning capabilities. For the study, modules of the SCALE program system were used. Single-cycle parametric studies were performed with cores having different MA content and spatial distribution. It was shown that the addition of MAs to the fuel greatly reduced the reactivity loss during burnup. Moreover, the higher the MA content of the core, the higher the fraction of it that was fissioned; however, the more the delayed neutron fraction and the fuel temperature coefficient degraded. Significant reduction can be achieved in the amounts of neptunium and americium, while curium isotopes accumulate. The study of multiple consecutive cycles showed that by adding only depleted uranium (DU) to the reprocessed actinides in fuel fabrication (pure DU feed strategy), up to 70% of the initially loaded MAs can be fissioned in the first five cycles. Moreover, the reactor can be made critical during that time if the initial MA content is higher than 3%. By feeding MAs as well (constant MA content strategy), the reactivity has a steady increase from cycle to cycle, predominantly due to 238Pu breeding from 237Np. The effects of the isotopic composition of the plutonium and MAs were also examined by performing calculations with data specific to the spent fuel of traditional western pressure water reactors and Russian type VVER440 reactors. Despite the considerably different MA vectors, no significant deviation was found in their overall transmutation. However, the Pu composition had a strong effect on the reactivity and the delayed neutron fraction in the first cycles. Finally, cores having nonuniform MA content were investigated. It was found that though the MA destruction efficiency was significantly higher in the middle of the core than at the edge, moving some of the MAs from the outer regions to the center resulted in only minor improvement in their destruction. However, the spectral changes caused by the rearrangement increased the k-effective, which allowed higher burnups and increased MA destruction. Unfortunately, some of the safety parameters of the reactor degraded.

Fast and accurate sensitivity analysis of IMPT treatment plans using Polynomial Chaos Expansion

The highly conformal planned dose distribution achievable in intensity modulated proton therapy (IMPT) can severely be compromised by uncertainties in patient setup and proton range. While several robust optimization approaches have been presented to address this issue, appropriate methods to accurately estimate the robustness of treatment plans are still lacking. To fill this gap we present Polynomial Chaos Expansion (PCE) techniques which are easily applicable and create a meta-model of the dose engine by approximating the dose in every voxel with multidimensional polynomials. This Polynomial Chaos (PC) model can be built in an automated fashion relatively cheaply and subsequently it can be used to perform comprehensive robustness analysis. We adapted PC to provide among others the expected dose, the dose variance, accurate probability distribution of dose-volume histogram (DVH) metrics (e.g. minimum tumor or maximum organ dose), exact bandwidths of DVHs, and to separate the effects of random and systematic errors. We present the outcome of our verification experiments based on 6 head-and-neck (HN) patients, and exemplify the usefulness of PCE by comparing a robust and a non-robust treatment plan for a selected HN case. The results suggest that PCE is highly valuable for both research and clinical applications.

Ambiguities in the Sensitivity and Uncertainty Analysis of Reactor Physics Problems Involving Constrained Quantities

Abstract The nuclear community relies heavily on computer codes both in research and in the operation of installations. The results of such computations are useful only if they are augmented with sensitivity and uncertainty studies. This technical note presents some theoretical considerations regarding traditional first-order sensitivity analysis and uncertainty quantification involving constrained quantities. The focus is on linear constraints, which are often encountered in reactor physics problems due to energy and angle distributions, or the correlation between the isotopic abundances of elements. A consistent theory is given for the derivation and interpretation of constrained first-order sensitivity coefficients; covariance matrix normalization procedures; their interrelation; and the treatment of constrained inputs with polynomial chaos expansion, which was the main motivation of this research. It is shown that if the covariance matrix violates the “generic zero column and row sum” condition, normalizing it is equivalent to constraining the sensitivities, but since both can be done in many ways, different sensitivity coefficients and uncertainties can be derived. This makes results ambiguous, underlining the need for proper covariance data. Furthermore, it is highlighted that certain constraining procedures can result in biased or unphysical uncertainty estimates. To confirm our conclusions, we demonstrate the presented theory on three analytical and two numerical examples including fission spectrum, isotopic distribution, and power distribution-related uncertainties, as well as the correlation between mass, volume, and density.

Adjoint-Based Sensitivity Analysis of Coupled Criticality Problems

Abstract Sensitivity analysis is a technique that is widely used in reactor physics calculations to efficiently obtain first-order changes in responses of interest due to variations of input parameters. This paper presents an extension of the well-known perturbation procedures for the critical eigenvalue and flux functionals. The extended method makes it possible to determine sensitivities in coupled criticality problems with mutual feedback between neutronics and one or more augmenting systems (e.g., thermal hydraulics or fission product poisoning). The technique uses appropriate neutronic and augmenting adjoint functions, which can be obtained by solving a system of coupled adjoint equations. Three different approaches are presented for considering the effects of perturbations in coupled criticality problems with feedback: The steady-state power level is allowed to adjust to maintain criticality with the perturbed parameters (power perturbation), a change is allowed in the critical eigenvalue while the flux is constrained (eigenvalue perturbation), or simultaneous perturbations are made to ensure criticality at the unperturbed power level (control parameter perturbation). In the case of power and eigenvalue perturbations, sensitivities can be obtained with or without power- and k-reset procedures, respectively, yielding identical results to control parameter perturbation. The paper presents the theoretical background, an application to a one-dimensional slab problem with thermal and fission product feedback, and a numerical procedure to obtain the necessary adjoint functions. The proposed technique relies on using the neutronics and augmenting codes separately as a preconditioner for Krylov methods employed to the coupled adjoint problem. This makes the development of new codes unnecessary and provides a means of large-scale implementation.

The Relative Biological Effect of Spread-Out Bragg Peak Protons in Sensitive and Resistant Tumor Cells

Purpose Variations in the radiosensitivity of tumor cells within and between tumors impact tumor response to radiation, including the dose required to achieve permanent local tumor control. The increased expression of DNA-PKcs, a key component of a major DNA damage repair pathway in tumors treated by radiation, suggests that DNA-PKcs-dependent repair is likely a cause of tumor cell radioresistance. This study evaluates the relative biological effect of spread-out Bragg-peak protons in DNA-PKcs-deficient cells and the same cells transfected with a functional DNA-PKcs gene. Materials and Methods A cloned radiation-sensitive DNA-PKcs-deficient tumor line and its DNA-PKcs-transfected resistant counterpart were used in this study. The presence of functional DNA-PKcs was evaluated by DNA-PKcs autophosphorylation. Cells to be proton irradiated or x-irradiated were obtained from the same single cell suspension and dilution series to maximize precision. Cells were concurrently exposed to 6-MV x-rays or mid 137-MeV spread-out Bragg peak protons and cultured for colony formation. Results The surviving fraction data were well fit by the linear-quadratic model for each of 8 survival curves. The results suggest that the relative biological effectiveness of mid spread-out Bragg peak protons is approximately 6% higher in DNA-PKcs-mediated resistant tumor cells than in their DNA-PKcs-deficient and radiation-sensitive counterpart. Conclusion DNA-PKcs-dependent repair of radiation damage is less capable of repairing mid spread-out Bragg peak proton lesions than photon-induced lesions, suggesting protons may be more efficient at sterilizing DNA-PKcs-expressing cells that are enriched in tumors treated by conventional fractionated dose x-irradiation.

An angular multigrid preconditioner for the radiation transport equation with Fokker–Planck scattering

In a previous paper (Hennink and Lathouwers, 2017) we developed a finite element discretization for the Boltzmann transport equation with forward peaked scatter modeled by the Fokker–Planck approximation. The discretization was based on the discontinuous Galerkin method in both space and angle. It was expected and found that the regular source iteration algorithm for the Boltzmann equation is not effective in solving the discretized system and becomes excessively expensive for problems with many angular degrees of freedom. The purpose of this paper is to develop a multigrid scheme as preconditioner for the above mentioned discretization. The method exploits the nested nature of the meshes and the natural prolongation/restriction between meshes by Galerkin projection. A set of test problems ranging from pure spherical diffusion to the complete Boltzmann transport problem in 3D are presented to illustrate that the method is very effective, resulting in iteration counts nearly independent of problem size even for highly non-isotropically refined angular meshes.

SP-0010: Selection of patients for proton therapy: a physicists view

ESTRO 35 2016 _____________________________________________________________________________________________________ attention to the radiotherapy planning and delivery elements, and careful systematic and prospective documentation of tumor and normal tissue outcomes. Even if randomised trials are deemed unsuited to the setting, protocol based approaches in registered phase I/II trials are appropriate to enhance standards and should probably include audit and quality assurance processes, as well as realistic stopping rules to address unexpected or aberrant outcomes.