Monte Carlo simulation of radionuclide transport through fractured media

The presence of fractures within the host matrix may strongly affect the radionuclide mobility in the far-field of the current proposed solutions for underground radioactive waste repositories. In this paper a Monte Carlo simulation approach, based on analogies with neutron transport, has been developed for describing radionuclide transport in fractured media. The Monte Carlo approach here adopted is verified by means of comparison with analytic and numerical solutions for two different case studies. An application to a more complex and realistic case is also reported to show the potential of the proposed approach.     

Applications of an adjoint Monte Carlo method to the underground migration of radionuclides over large scales

An adjoint Monte Carlo method has been applied recently to the transport of dissolved radionuclide chains through a saturated porous medium [Smidts, O.F., Devooght, J., 1998a. Analysis of the transport of radionuclide chains in a stochastic geological medium by a biased Monte Carlo simulation. Nuclear Science and Engineering 129, 224–245. (+corrigendum, 130, 164); Smidts, O.F., 1998. Point and surface estimations by a non-analog Monte Carlo simulation for the transport of radionuclide chains in porous media. Monte Carlo Methods and Applications 4(4), 289–318; Smidts, O.F., 1999. TRACKS: an efficient particle tracking code for the migration of radionuclide chains with uncertain site characterisation. In: Proceedings ENS Topseal'99: Commitment to the Future Environment, Vol. 2.]. We give, first, a general introduction of the method. We emphasize on the problem of the accurate estimation of local concentrations or cumulative concentrations over the geosphere–biosphere interface and the migration of multiple species in a radionuclide chain.Then, to study the impact of highly heterogeneous retardation factor on transport, we present an approximate semi-analytical solution for the transport of a radionuclide. Finally, numerical tests, including migration of radionuclide chains over large distances, are defined and several comparisons are made with analytical and semi-analytical solutions to verify the Monte Carlo method and its related transport code TRACKS on 1D and 3D transport problems. The results are presented and discussed for highly heterogeneous retardation factor.     

Monte Carlo estimation of radionuclide release at a repository scale

Prediction of radionuclide release is a central issue in the performance assessment of nuclear waste repositories. This requires modeling of the radionuclide migration processes through the repository barriers, accounting for the related uncertainties. The present paper illustrates a Monte Carlo simulation-based compartment model in which detailed, local-scale modeling feeds a global-scale analysis of the repository, at reasonable computational expenses. An application to a realistic case study is presented to verify the feasibility of the approach.     

A hybrid method for the simulation of radionuclide contaminant plumes in heterogeneous unsaturated formations

The numerical simulation of multiphase flow and contaminant transport problems that take place around nuclear waste repositories is an important tool for selecting appropriate sites for these facilities. The goal of this paper is to present an accurate and efficient simulation tool to approximate flows in heterogenous unsaturated hydrogeological environments. We employ a hybrid method that combines a Eulerian finite volume approximation scheme for the underlying water–air flow problem with a locally conservative Lagrangian particle tracking method to approximate the transport of a radionuclide within the water phase. The Lagrangian scheme is computationally efficient and theoretically free of numerical diffusion. Numerical simulations showing the effectiveness of the new procedure are presented and discussed.     

Development of an effective dose calculation method for the downstairs room of a linac facility

ABSTRACT

An effective dose calculation method is important in the design of efficient shields in radiation facilities. Some analytical methods have been shown to provide a simple and quick design analysis; however, no suitable method exists that can be applied to a room located directly under an X-ray irradiation room. We propose a new analytical method that uses the multiple reflection ratio predetermined by a Monte Carlo simulation and the differential dose albedo given by the Chilton–Huddleston semi-empirical equation. Our method is verified by comparison with the Monte Carlo simulation, performed for the case of an electron linac facility with an accelerated energy of 10 MeV, where the shielding floor has a thickness of 1.6–2.0 m and the downstairs room has a height of 0.5–1.5 m. The difference between the effective X-ray doses in the downstairs room calculated via the proposed analytical method and the Monte Carlo simulation is less than 25% when the horizontal distance from the X-ray beam to the evaluation point exceeds 3 m and the evaluation point is set at half of the height of the room. The new analytical method can be efficiently and accurately applied to the calculation of the effective dose.     

A hybrid method for the simulation of radionuclide contaminant plumes in heterogeneous unsaturated formations

The numerical simulation of multiphase flow and contaminant transport problems that take place around nuclear waste repositories is an important tool for selecting appropriate sites for these facilities. The goal of this paper is to present an accurate and efficient simulation tool to approximate flows in heterogenous unsaturated hydrogeological environments. We employ a hybrid method that combines a Eulerian finite volume approximation scheme for the underlying water-air flow problem with a locally conservative Lagrangian particle tracking method to approximate the transport of a radionuclide within the water phase. The Lagrangian scheme is computationally efficient and theoretically free of numerical diffusion. Numerical simulations showing the effectiveness of the new procedure are presented and discussed.     

Groundwater contaminant transport in presence of colloids I. A stochastic nonlinear model and parameter identification

In this paper we present an extension of a stochastic model of groundwater contaminant transport, previously proposed by the authors, to account for the presence of colloids in the system. These have been shown experimentally to facilitate significantly the transport of contaminants. The stochastic model is based on the Kolmogorov–Dmitriev theory of branching stochastic processes and is capable of explicitly accounting for the individual interactions, possibly nonlinear and not at equilibrium, which occur during the transport process. We tackle the problem by means of a Monte Carlo method properly devised to account for the nonlinearities introduced by the presence of colloids. For the determination of the model parameters an approach is followed which is based on a comparison of the stochastic model equations with those of the classical advection-dispersion one. This has enabled us to establish relationships which link the transition rates to the classically measurable quantities.     

核素在非饱和带中迁移的非平衡吸附数值模式——NESOR

本文将对流弥散方程与非平衡吸附关系式相结合 ,开发了核素在非饱和孔隙介质中迁移的二维非平衡吸附数值模式与计算机程序—— NESOR,给出了 NESOR中使用的吸附、解吸速率参数的计算方法。对中国辐射防护研究院 (CIRP)野外包气带黄土试验场 F坑 85Sr两年的迁移试验资料和同步测得的黄土剖面的含水量资料 ,分别用 NESOR模式及二维饱和 -非饱和溶质迁移的平衡吸附模式 FEMWASTE进行了模拟计算。结果表明 :NESOR的模拟结果比 FEMWASTE的模拟结果更接近试验实际     

水体放射性连续监测系统设计与仿真

论文设计了内陆水体中放射性核素连续在线监测系统,并采用蒙特卡罗方法对系统参数进行了优化。论文通过计算~(131)I、~(137)Cs、~(60)Co在不同水体厚度下探测灵敏度,结合系统造价,确定了系统最优化探测体积,并依据四川省水体中天然放射性核素浓度,计算了~(131)I、~(137)Cs、~(60)Co的判断限、探测限和定量限,该结果为内陆水体中放射性核素连续在线监测系统构建提供了技术参考和依据。     

MONTE CARLO CALCULATION OF ENERGY DEPOSITION BY DELTA RAYS AROUND ION TRACKS

The radial distribution of dose around the path of a heavy ion has been studied by a Monte Carlo transport analysis of the delta rays produced along the track of a heavy ion based on classical binary collision dynamics and a single scattering model for the electron transport process.Result comparisons among this work and semi-empirical expression based delta ray theory of track structure,as well as other Monte Carlo calculations are made for 1,3MeV protons and several heavy ions.The results of the Monte Carlo simulations for energetic heavy ions are in agreement with experimental data and with results of different methods.The characteristic of this Monte Carlo calculation is a simulation of the delta rays theory of track structure.     

Normal and anomalous transport across an interface: Monte Carlo and analytical approach

We present a Monte Carlo scheme to simulate particles going across an interface separating two layers of a medium characterized by different physical properties, together with an analytical formulation of the same problem, for both normal diffusive and subdiffusive regimes. We relate the Monte Carlo simulation parameters to the coefficients and boundary conditions appearing in the companion analytical equations. Under suitable physical hypotheses on the constraints to be imposed on such parameters, we show that the Monte Carlo simulation results are in good agreement with the corresponding analytical solutions. In particular, we remark that – while in the normal diffusion case the conservation of particle local velocities across the interface leads to a smoothly varying concentration profile – in the subdiffusive case the same condition leads to a neat jump in resident concentration.     

Groundwater contaminant transport in presence of colloids II: sensitivity and uncertainty analysis on literature case studies

In a previous paper, the authors have presented a newly developed stochastic model of groundwater contaminant transport which is capable of accounting for the contribution of colloids flowing in the porous medium. The model is nonlinear and accounts explicitly for the exchange interactions among contaminants, colloids and the porous matrix, by means of properly introduced transition rates. The physical model is straightforward and intuitive, and it results in a system of differential equations whose dimensionality, in practical cases, is such to entail a Monte Carlo simulation to approach its solution. In this second part of the work, we explore in details the role played by those parameters which characterize the contaminant-colloid and the colloid-solid matrix interactions. A case study of Pu-239 transport inspired by a potential site for high level waste deposit at Mol (Belgium) is considered and the effects of colloid-facilitated transport is investigated under various conditions. Furthermore, an uncertainty analysis is performed to account for the uncertainties which in realistic situations affect the system and the transport phenomena. In this regard, a case study of Pu-239 transport inspired by the test site in Nevada has been considered.     

Highly efficient γ-ray generation by 10 PWclass lasers irradiating heavy-ion plasmas

In this article, the bunched transport of photoexcited carriers in a GaAs photoconductive semiconductor switch (PCSS) with interdigitated electrodes is investigated under femtosecond laser excitation. Continuous outputs featuring high gain are obtained for single shots and at 1 kHz by varying the optical excitation energy. An ensemble three-valley Monte Carlo simulation is utilized to investigate the transient characteristics and the dynamic process of photoexcited carriers. It demonstrates that the presence of a plasma channel can be attributed to the bunching of high-density electron–hole pairs, which are transported in the form of a high-density filamentary current. The results provide a picture of the evolution of photoexcited carriers during transient switching. A photoinduced heat effect is analyzed, which reveals the related failure mechanism of GaAs PCSS at various repetition rates.     

基于RMC程序的用康普顿轮廓进行束缚态电子多普勒展宽修正研究

堆用蒙特卡罗程序RMC具备中子、光子、电子耦合输运能力，能完成精确的屏蔽计算，其中光子输运过程采用光子数据库进行了康普顿散射模拟。本文对康普顿散射物理原理及多普勒展宽方法进行分析，使用康普顿轮廓数据对束缚态电子进行多普勒展宽修正，实现了RMC程序对自由电子和束缚态电子的选择性处理。通过核素算例测试，观察到了多普勒能谱展宽的效应，证明了该方法的正确性。通过对典型压水堆组件的计算和对比，验证了用康普顿轮廓进行束缚态电子多普勒展宽修正的必要性和正确性。     

A new high speed solution for the evaluation of Monte Carlo radiation transport computations

Advancements in parallel and cluster computing have made many complex Monte Carlo simulations possible in the past several years. Unfortunately, cluster computers are large, expensive, and still not fast enough to make the Monte Carlo technique useful for calculations requiring a near real-time evaluation period. For Monte Carlo simulations, a small computational unit called a Field Programmable Gate Array (FPGA) is capable of bringing the power of a large cluster computer into any personal computer (PC). Because an FPGA is capable of executing Monte Carlo simulations with a high degree of parallelism, a simulation run on a large FPGA can be executed at a much higher rate than an equivalent simulation on a modern single-processor desktop PC. In this paper, a simple radiation transport problem involving moderate energy photons incident on a three-dimensional target is discussed. By comparing the evaluation speed of this transport problem on a large FPGA to the evaluation speed of the same transport problem using standard computing techniques, it is shown that it is possible to accelerate Monte Carlo computations significantly using FPGAs. In fact, we have found that our simple photon transport test case can be evaluated in excess of 650 times faster on a large FPGA than on a 3.2 GHz Pentium-4 desktop PC running MCNP5.     

核应急下水源放射性快速检测系统的研究

在核应急条件下,需要对水源放射性进行快速检测。本文利用降低本底和能谱分析的方法,通过优化物理设计,给出系统快速检测水中放射性核素的能力。系统采用Ф50 mm×200 mm NaI(TI)晶体作为伽马探测器,结合蒙特卡罗模拟计算,优化屏蔽装置和探测系统。能谱分析采用两次拟合算法,对18 L含有~(137)Cs的放射性标准溶液进行检测,实验结果达到设计要求。     

LIGHT ION REFLECTION STUDIED BY MONTE CARLO SIMULATION AND TRANSPORT THEORY

The reflection of light ions, such as H+,3He+ and 4He+, with energies of 0.1- 10 keV, from Cu and Ni surface has been studied by Monte Carlo simulation and transport theory. The Monte Carlo simulation gives the detail energy spectra for the reflected particles and their angular distribution for different incident angles. It shows that the reflected particle energy spectra can be approximately described by an analytical formula for the whole energy range, all the incident angles and different ion- target combination studied here. The reflected particle energy vs its average reflection angle to the surface normal can almost be expressed by a universal curve for all cases studied here. The reflection energy spectra are used for the calculation of the reflection coefficient by transport theory including the realistic surface correction. The present work is compared with both experimental measurement and other simulation codes.     

Simulating nuclear particle transport in stochastic media using Perlin noise functions

Many nuclear particle transport problems require the simulation of stochastic media – that is, media whose composition varies randomly as a function of position. Examples include biological tissues, mineral slurries and crushed mineral ores. Such materials are usually represented in particle transport codes as being homogeneous with an appropriate average composition. This approximation can introduce significant errors.A new method is described for simulating the transport of neutral particles in such media using a three-dimensional Perlin noise function [Comput. Graph. 19 (3) (1985) 287]. The technique allows many properties of the stochastic medium to be specified, including the mean scale of the inhomogeneities, their shape and surface roughness and the proportions of the different media making up the stochastic medium. A key property of the new method is that the tracking speed is independent of the number density of the inhomogeneities.The tracking algorithm has been implemented into the EGSnrc [The EGSnrc code system: Monte Carlo simulation of electron and photon transport, NRCC Report PIRS-701, 2000] Monte Carlo code. An example application is described, modelling X-ray fluorescence from a particulate mineral sample. In contrast to the homogeneous approximation, the new algorithm accurately reproduces the observed particle size dependence seen experimentally. The increase in computational time is fairly modest, even for very small particle sizes.     

Fast three-dimensional radiation field characterization in large-scale nuclear installations

The requirement of the fast three-dimensional radiation field calculation is raised during the decommissioning of large-scale nuclear installations. The most often used methods, such as the Monte Carlo and the discrete ordinates methods, have shortcomings in their simulations of such problems. The coupled Monte Carlo–point kernel computational scheme is developed to meet the requirement. The facility is separated into the source region and the bulk shielding region, with a common interface. The transport within the source region is simulated using the Monte Carlo method, which is by nature good at treating complex geometries. The radiation field in the bulk shielding region is treated by the point kernel approach to avoid the extremely expensive computation for deep penetration problems. The flow rate through the interface,which is given by the Monte Carlo simulation, is considered as the equivalent surface source for the point kernel calculation. Test calculations from simplified typical waste storage facilities have been performed to validate the coupled scheme by comparing them with the results conducted by the Monte Carlo method directly. The good agreement of the results, as well as the significant saving in computing time, indicates that the coupled method is suitable for the fast three-dimensional radiation field calculation.     

锰浴法测量中子源发射率中各修正因子的蒙特卡罗计算

在采用锰浴法对放射性同位素中子源的发射率进行测量时,MnSO₄溶液中⁵⁵Mn仅俘获部分源中子,故需考虑对未被⁵⁵Mn俘获部分的修正。用蒙特卡罗粒子输运程序MCNP对中子源强标准装置的锰浴系统进行模拟计算,对实验测量结果进行修正,并通过由国际计量局（BIPM）组织的中子源强度国际比对,验证了计算所建立模型的可靠性。利用MCNP程序的微扰计算功能,通过考虑MnSO₄溶液的密度、源及其承托物材料的密度、源的位置、锰浴半径以及反应截面的改变对计算结果的影响,对模拟计算结果的不确定度进行了详细评定,提供了一种蒙特卡罗模拟计算结果不确定度的评定方法。