Rigorous 2-step Research Articles

A Generic Implementation of the D1S Methodology for Dose Rate Assessment by Monte Carlo Codes in Fusion Applications

Dose rate assessment is of utmost importance in fusion reactors because of the maintenance operations that these facilities will require. The standard way to perform this assessment in ITER is use of the Direct 1-Step (D1S) methodology, which consists of performing neutron transport simulation, material activation, and decay photon transport simulation in one step and thus in one simulation only. Usually, implementation of the D1S methodology requires changing the source files in a reference Monte Carlo code. The purpose of the present work is to develop an easy-to-implement method for Monte Carlo codes to help calculate the dose rate in fusion reactors. To do so, the proposal is to act on nuclear data only and not on source files of the simulation codes. This is done by replacing prompt photon production in evaluation files by suitable decay photon production, taking into account radioactive decays of radionuclides and irradiation history. This study was to be applied first on the TRIPOLI-4® Monte Carlo code for the sake of simplicity. TRIPOLI-4 is the reference code for particle transport at CEA. The verification and validation process relies first on a comparison to the reference Rigorous 2-Step (R2S) methodology and then on an experiment, the so-called Frascati Neutron Generator (FNG) dose experiment. The nuclear database to be changed is of the ENDF-6 format, a recurrent format in neutronics studies. The analysis studied both neutron and photon responses to check if the simulation was performing normally in a physical way and to compare the results with references provided by simulations based on the R2S methodology or by the FNG dose experiment. The simulations have proven to be in good agreement with the experimental results.

Development and validation of fully open-source R2S shutdown dose rate capabilities in OpenMC *

We present the first fully open-source capabilities for shutdown dose rate (SDR) calculations of fusion energy facilities based on the Rigorous 2-Step (R2S) methodology. These capabilities have been implemented in the OpenMC Monte Carlo particle transport code, building on its existing capabilities while also leveraging new features that have been added to the code to support SDR calculations, such as decay photon source generation. Each of the individual physics components in the R2S workflow—neutron transport, activation, decay photon source generation, and photon transport—have been verified through code-to-code comparisons with MCNP6.2 and FISPACT-II 4.0. These comparisons generally demonstrate excellent agreement between codes for each of the physics components. The full cell-based R2S workflow was validated by performing a simulation of the first experimental campaign from the Frascati Neutron Generator (FNG) ITER dose rate benchmark problem from the Shielding INtegral Benchmark Archive and Database (SINBAD). For short cooling times, the dose calculated by OpenMC agrees with the experimental measurements within the stated experimental uncertainties. For longer cooling times, an overprediction of the shutdown dose was observed relative to experiment, which is consistent with previous studies in the literature. Altogether, these features constitute a combination of capabilities in a single, open-source codebase to provide the fusion community with a readily-accessible option for SDR calculations and a platform for rapidly analyzing the performance of fusion technology.

Uncertainty Quantification of the Decay Gamma Source in Mesh-Based Shutdown Dose Rate Calculations

In fusion energy systems, part of the design effort is dedicated to the assessment of the shutdown dose rate (SDR) due to the decay photons that will be emitted from activated components. Monte Carlo transport codes are often used to obtain the neutron flux distribution in the problem domain. The neutron flux distribution is used in the rigorous 2-step (R2S) workflow to obtain the photon emission density distribution of decaying radionuclides. The photon emission density is then used as an input for a dedicated photon transport step to calculate the SDR. In this paper, the uncertainty of the decay gamma source due to the uncertainty of the neutron flux distribution in the R2S workflow is investigated. A scheme is developed to estimate the uncertainty of the decay gamma source, building on the concept of groupwise transmutation and using standard error propagation techniques. The applicability of the newly developed scheme is then demonstrated on one of the conceptual designs of the fusion nuclear science facility.

A shutdown dose rates analysis of the Korean fusion demonstration reactor using MCNP5 mesh-based R2S approach

In the fusion reactor, lots of high energy neutrons are produced by the nuclear fusion reaction, which produces unstable nuclides emitting gamma rays in various components of fusion energy systems. So, estimation of shut down dose rate in such facilities is very important to check if the dose rates are below the radiational safety limits. Generally, two computational analysis schemes of the cell-based Rigorous-2-Step (R2S) method and the Direct-1-Step (D1S) method have been used widely in evaluating the shutdown dose rates. In this work, the mesh-based R2S method is automated with in-house auxiliary programs for our practical applications. This mesh-based R2S procedure was applied to the Korean fusion demonstration reactor (K-DEMO) facility. From the analysis for K-DEMO, it was shown that the automated mesh-based R2S procedure gives much higher computational efficiencies in terms of FOM than the cell-based R2S method and gives higher shutdown dose rates than the cell-based R2S and D1S method.

Development of advanced rigorous two-step code system for evaluation of radioactive waste with high-resolution activation calculation

Nowadays, evaluation of amounts and distributions of radioactive waste is an important preparatory step in the process of nuclear reactor decommissioning. For tentative estimation of radioactive waste, a cell-based rigorous 2 step (R2S) method usually is used; however, a poor resolution caused by the averaged flux and spectrum in a cell is still a great challenge because of leading to underestimated or overestimated results. To overcome the poor resolution, several systems were introduced. Neither system, however, provides any function for evaluation of radioactive waste amount and distribution. Thus, it is additionally required to classify radioactive waste based on the results of activation calculation. In this study, the advanced R2S (AR2S) system was developed. To verify the performance of the system, its results for a verification problem were compared with those of the cell-based R2S method. The results showed good agreement, which is to say, within 2.0% relative error. Also, several characteristics of fine/coarse mesh were analyzed. To demonstrate the performance of the AR2S system, the radioactive waste from the Japan Power Demonstration Reactor (JPDR) was estimated, and the result indicated a high-resolution distribution. Therefore, it is expected that the AR2S system will prove useful for precise evaluation of radioactive waste.

Shutdown dose rate studies for the DTE2 campaign at JET

• DTE2 is a unique opportunity to validate the numerical tools for ITER shutdown dose rate analysis. • Simulations are performed with the R2Smesh (Rigorous 2-step) approach at the end of DTE2 to predict dose rate levels. • The impact of TT and DTE2 campaigns on the dose rate is presented. The EUROfusion Work Package JET3 programme, established to enable the technological exploitation of the future Deuterium-Tritium (DT) operations at JET over the next years, includes, within the NEXP subproject, a novel Shutdown Dose Rate (SDR) benchmark experiment. The measurement of the SDR due to neutron activation in a fusion machine operating with Deuterium and Tritium is of primary importance for planning its operation in respect of dose limits for the external radiation exposure. The next high-performance DT campaign at JET (DTE2) which will follow a Tritium-Tritium (TT) operation phase, is a unique opportunity to validate the numerical tools for ITER shutdown dose rate analysis, through the comparison between numerical predictions and measured quantities in terms of C/E (Calculation/Experiment) ratios. Within this framework, this work is a pre-analysis of the impact of the future campaigns on the shutdown dose rate. Three-dimensional simulations are performed with the R2Smesh (Rigorous 2-step) approach at the end of DTE2 to predict dose rate levels at different cooling times. The results are presented and discussed in the paper with the major objective to contribute to the optimization of the planned SDR benchmark experiments.

Activation of ITER Divertor Neutron Flux Monitor

The paper presents results of numerical modelling for the ITER Divertor Neutron Flux Monitor (DNFM) detector modules life time activation. In particular, the rigorous-2-step (R2S) computational methodology was implemented. In addition to MCNP and FISPACT codes the special integration framework MCKIT was applied. The MCKIT provides models preparation and data processing tools. To provide appropriate performance we replaced direct FISPACT inventory calculations with the superposition method described below. The activation characteristics (DPA, contact dose, etc.) were obtained and compared with our results for the previous DNFM design.

Calculation of Shutdown Dose Rate in Fusion Nuclear Science Facility During a Proposed Maintenance Scheme

In this work, a preliminary assessment of the shutdown dose rate (SDR) in the latest Fusion Energy Systems Studies–Fusion Nuclear Science Facility conceptual design was calculated for one sector at different maintenance stages. The third operational phase, deuterium-tritium for 2.75 years, was considered to define the neutron source and the Rigorous 2-Step workflow was used. SDR levels were obtained at times that correspond to major maintenance operations and were found to be above 105 µSv/h, which necessitates robotic handling of all maintenance operations.

Global shutdown dose rate maps for a DEMO conceptual design

For the calculations of highly reliable shutdown dose rate (SDR) maps in fusion devices like a DEMO plant, the Rigorous-2-step (R2S) method is nowadays routinely applied using high-resolution decay gamma sources from initial high-resolution neutron flux meshes activating all materials in the system. This approach has been utilized in the present paper with the objective to provide SDR results relevant for RH systems of a conceptual DEMO design developed in the EU.The primary objective was to assess specific locations of interest for RH equipment inside the vessel and along the extension of maintenance ports. To this end, a provisional DEMO MCNP model has been used, featuring HCLL-type blankets, tungsten/copper divertor, manifolds, vacuum vessel with ports and toroidal field coils. The operational scenario assumed 2.1GW fusion power and a life-time of 20 years with plant availability of 30%, where removable parts will be extracted after 5.2 years. Results of absorbed dose rate distributions for several relevant materials are presented and discussed in terms of the different contributions from the various activated components.

Novel hybrid Monte Carlo/deterministic technique for shutdown dose rate analyses of fusion energy systems

The rigorous 2-step (R2S) computational system uses three-dimensional Monte Carlo transport simulations to calculate the shutdown dose rate (SDDR) in fusion reactors. Accurate full-scale R2S calculations are impractical in fusion reactors because they require calculating space- and energy-dependent neutron fluxes everywhere inside the reactor. The use of global Monte Carlo variance reduction techniques was suggested for accelerating the R2S neutron transport calculation. However, the prohibitive computational costs of these approaches, which increase with the problem size and amount of shielding materials, inhibit their ability to accurately predict the SDDR in fusion energy systems using full-scale modeling of an entire fusion plant. This paper describes a novel hybrid Monte Carlo/deterministic methodology that uses the Consistent Adjoint Driven Importance Sampling (CADIS) method but focuses on multi-step shielding calculations. The Multi-Step CADIS (MS-CADIS) methodology speeds up the R2S neutron Monte Carlo calculation using an importance function that represents the neutron importance to the final SDDR. Using a simplified example, preliminary results showed that the use of MS-CADIS enhanced the efficiency of the neutron Monte Carlo simulation of an SDDR calculation by a factor of 550 compared to standard global variance reduction techniques, and that the efficiency enhancement compared to analog Monte Carlo is higher than a factor of 10,000.

Shutdown dose rate assessment with the Advanced D1S method: Development, applications and validation

The present paper addresses the recent developments and applications of Advanced-D1S to the calculations of shutdown dose rate in tokamak devices. Results of benchmarking with measurements and Rigorous 2-Step (R2S) calculations are summarized and discussed as well as limitations and further developments. The outcomes confirm the essential role of the Advanced-D1S methodology and the evidence for its complementary use with the R2Smesh approach for the reliable assessment of shutdown dose rates and related statistical uncertainties in present and future fusion devices.

Shutdown dose calculations for the IFMIF test facility and the high flux test module

This work presents neutronic analyses to support the design of the test facility of the International Fusion Materials Irradiation Facility (IFMIF) in the framework of the Broader Approach activities. In order to evaluate the necessary biological shielding thickness and the reliability of remote handling tools for activated components inside the Test Cell, a detailed activation analysis has been carried out to obtain a detailed residual dose map resulting from the entire High Flux Test Module and Test Cell by utilizing the McDeLicious-11 Monte Carlo code and the mesh-tally based rigorous 2-Step approach (R2Smesh) developed at KIT. The absorbed dose rate expected at the working place of remote handling tools inside the Test Cell is approximately 100 Gy/h. A wall thickness of 150 cm for heavy concrete can be used as the design basis for hot cells in the test facility.

Novel approach for efficient mesh based Monte Carlo shutdown dose rate calculations

A novel approach for the Monte Carlo based calculation of high resolution shutdown dose rate distributions was elaborated at KIT developing the previous R2Smesh technique. The method utilizes the MCNP code for particle transport calculations and the FISPACT code for the activity calculations providing the decay photon source distribution. The spatial distributions both of neutrons and decay photons are obtained by making use of the mesh tally technique available with MCNP5. This procedure has no limitation for the complexity of the geometry used for particle transport simulations. As a unique novelty, the present approach does not need a particle track output (PTRAC) file for the determination of the material composition in a mesh element. Instead a newly developed subroutine, linked to the MCNP code, takes care about the geometry cells, materials and their volume fractions in the mesh cell passing this information directly to the MCNP's mesh tally output file. The new approach is verified by means of benchmark calculations for JET and compared to the cell based rigorous 2 step (R2S) method. The results obtained fit very well experimental data. The successful application of the new interface to JET has demonstrated that this approach can be applied with confidence for shutdown dose rate assessments in ITER.

Neutronics analysis of the IVVS/GDC plug in ITER

A neutronics analysis has been performed to provide the input required for the design strategy for the In-Vessel Viewing System (IVVS) and the Glow Discharge Cleaning (GDC) plug units in the ITER tokamak. The focus of the analysis has been on operational loads to the GDC electrode head in the shielding position and on the activation and the decay photon radiation absorbed in the structural components of the entire system. To estimate the conditions for maintenance scenarios, the occupational dose rate around the isolated IVVS/GDC head has been calculated assuming the ITER SA2 irradiation scenario. The Rigorous 2 Step (R2S) method, developed previously at KIT, has been employed for the calculation of the shutdown dose rates. The GDC head, which is subjected to the highest neutron loads, gets heavily activated and dominates the decay gamma activity of the entire plug. Accordingly, the shutdown dose rate around the IVVS/GDC plug is dominated by the GDC electrode head. It is therefore recommended to separate the GDC head from the system prior to further operations inside the Hot Cell. All components, except the Be protective layer of the GDC probe, were shown to be classifiable as low level radwaste according to the French regulations.

Verification and validation of the R2Smesh approach for the calculation of high resolution shutdown dose rate distributions

The well validated Rigorous-2-Step (R2S) approach for the calculation of decay gamma shutdown dose rates has been extended utilizing mesh tally capabilities of the MCNP5 code. Contrary to other similar approaches, the new R2Smesh system pays special attention to achieving highly accurate and reliable results for neutron flux intensities and spectral shape in the initial MCNP5 neutron transport step. To this end, non-uniform fine and coarse meshes are adopted. Also several enhancements to the performance of activation calculations by FISPACT2007 and to sampling from repeated structures geometry have been implemented. The R2Smesh system has been verified by application to an ITER (International Thermonuclear Experimental Reactor) computational benchmark. An extensive validation exercise has been conducted on the FNG (Frascati Neutron Generator) shutdown dose rate benchmark. In this case several variants of the fine/coarse mesh spacing have been tested showing evidence for convergence toward R2S values obtained with the original cell-based approach. It thus has been shown that R2Smesh is validated for ITER applications and offers additional capabilities for high resolution shutdown dose rate calculations.

Neutronic Analyses for the Upper Ports in the Neutral Beam Cell of ITER

Neutronic analyses have been performed assessing the performance of the new radiation shielding design of the upper ports in the Neutral Beam (NB) cell of ITER. The scope of the work includes neutron and gamma spectra and nuclear heating calculations inside the port, as well as assessments of fluences, nuclear heating, and insulator radiation doses in the superconductive magnets in the vicinity of the upper port. For radiation transport calculations, the MCNP5 code has been applied with the Alite 4.1 standard 3D model of ITER, in which the inner structure of the upper port was converted from the underlying CAD (CATIA) data. The conversion has been accomplished by means of the McCad interface code. To address human safety issues, maps of shutdown dose rates have been produced using the Rigorous 2 Step (R2S) method enhanced with the mesh-tally capability. The mesh-based R2S approach couples the MCNP5 mesh-tallies with the radioactive inventory results calculated with the FISPACT-2007 activation code allowing automated calculations of shutdown doses including transport of decay gammas. All results satisfy the ITER radiation design requirements.

Radiation shielding analyses for the ECRH launcher in the ITER upper port

Radiation shielding analyses have been performed for the ECRH system in the ITER upper port to complete the neutron streaming analysis performed previously. The analyses aimed at assessing and optimising the performance of the radiation shield to prove that the shielding requirements can be met by the proposed design variants. The radiation transport calculations have been performed by means of the Monte Carlo programme MCNP in 3D geometry using the standard ITER neutronics model with ECRH launcher and plug integrated into the upper port. The interface programme MCAM was used to convert the 3D ECRH launcher models available from the CAD-system for use with MCNP-calculations. It was shown that the launcher design with the proposed radiation shield can satisfy the design limits for the radiation loads to both the launcher and the neighbouring components such as the Vacuum Vessel and the TF coils. Radiation dose levels were assessed for reactor shutdown at the rear side of ECRH launcher at locations where personnel access for maintenance may be required. The shutdown dose rate calculations were also performed in 3D geometry by applying the rigorous 2-step (R2S) method and comparing the results to those obtained with the direct 1-step (D1S) method. The R2S method includes activation calculations for the launcher materials by means of the inventory code FISPACT. It was proven that the shutdown dose rates inside the port with straight waveguides will be below the ITER radiation limit of 100 µSv/hr after 10 days decay time.

Rigorous mcnp based shutdown dose rate calculations: computational scheme, verification calculations and application to ITER

A rigorous 2-step (R2S) computational system has been developed for the calculation of shutdown dose rate distributions in full three-dimensional geometry. The system is based on the use of the mcnp transport and the fispact inventory code linked through a suitable coupling scheme for the automated routing of decay gamma sources and neutron flux spectra. The decay gamma source distribution is sampled in a source routine linked to mcnp according to the distribution provided by preceding mcnp neutron transport and fispact inventory calculations. A detailed description of the flow scheme and the interfaces of the R2S code system is given. Validation of the system has been achieved through the computational analysis of an International Thermonuclear Experimental Reactor (ITER) shutdown dose rate experiment. The application to the ITER device has demonstrated the suitability of the R2S system for handling shutdown dose rate analyses of large and complex fusion reactor systems.