Radionuclide Transport Research Articles

ESTIMATION OF RADIOLOGICAL CONSEQUENCES FOR A HYPOTHETICAL RADIOACTIVE RELEASE AT CERNAVODA NPP

An analysis regarding the release and atmospheric transport ofradionuclides in case of a hypothetical nuclear incident at Cernavoda Nuclear Power Plant was performed in the present paper. For that purpose, the computer code JRodos (Real-Time Online Decision Support System for Offsite Emergency Management in Europe) was implied.

Stochastic machine learning modeling for the estimation of some uncertain parameters. Case study: Retardation factor in a radionuclide transport model

In the present work, we define a stochastic model using machine learning techniques to generate random fields of some uncertain parameters. The proposed stochastic model is based on Bayesian inference and aims at reconstituting the parameters of interest and their credible intervals. The main goal of this work is to define a model that estimates the values of the uncertain parameters known only by their distribution probability functions and some observed spatial measurements. We note that this type of parameters may be associated with some mathematical models usually traduced by non-linear differential equations. In our case, we study the uncertainty of the retardation factor in a radionuclide transport model. To achieve a more realistic parameter estimation, Markov сhain Monte Carlo (MCMC) algorithms are applied. We demonstrate that the obtained results confirm the feasibility of our proposed model and lead to a new understanding of contaminants' behavior.

Predictive Modeling of a Simple Field Matrix Diffusion Experiment Addressing Radionuclide Transport in Fractured Rock. Is It So Straightforward?

The SKB GroundWater Flow and Transport of Solutes Task Force is an international forum in the area of conceptual and numerical modeling of groundwater flow and solute transport in fractured rocks relevant for the deep geological disposal of radioactive waste. Two in situ matrix diffusion experiments in crystalline rock (gneiss) were performed at POSIVA’s ONKALO underground facility in Finland. Synthetic groundwater containing several conservative and sorbing radiotracers was injected at one end of a borehole interval and flowed along a thin annulus toward the opposite end. Several teams performed predictive modeling of the tracer breakthrough curves using “conventional” modeling approaches (constant diffusion and sorption in the rock, no or minimum rock heterogeneity). Supporting information, derived from small-scale laboratory experiments, was provided. The teams were free to implement different concepts, use different codes, and apply the transport and retention parameters that they considered to be most suited (i.e., not a benchmark exercise). The main goal was the comparison of the different sets of results and the analysis of the possible differences for this relatively simple experimental setup with a well-defined geometry. Even though the experiment was designed to study matrix diffusion, the calculated peaks of the breakthrough curves were very sensitive to the assumed magnitude of dispersion in the borehole annulus. However, given the very different timescales for advection and matrix diffusion, the tails of the curves provided information concerning diffusion and retention in the rock matrix regardless of the magnitude of dispersion. In addition, although the task was designed to be a blind modeling exercise, the model results have also been compared to the measured experimental breakthroughs. Experimental results tend to show relatively small activities, wide breakthroughs, and early first arrivals, which are somewhat similar to model results using large dispersivity values.

Numerical analysis of safety of a borehole repository for vitrified high-level nuclear waste

We developed a model and carried out transient 3-D computer simulation of radionuclides migration from an underground repository for vitrified high-level radioactive waste (HLW) with long-lived actinides. The repository represents an array of deep boreholes. Canisters with vitrified HLW are disposed one above another in lower parts of the boreholes. The upper parts of the boreholes are sealed. Radionuclides leached from the disposed waste are carried away by thermal convection of groundwater caused by heat generation in HLW. The computer simulation was carried out with use of a refined technique taking into account significant difference between radionuclides concentration in the groundwater in the loaded parts of the boreholes and at the earth surface, dependence of the groundwater properties on pT-parameters, composition of HLW. Values of leaching rate of the waste forms by groundwater depending on time and temperature, as well as role of colloid facilitated transport of radionuclides were determined from previous experiments with Na–Al–P-glass which was used in Russian Federation for HLW vitrification. The results of the calculations allowed us to conclude that distance between containers with HLW and the ground surface of 240 m and more is sufficient for safe disposal of the considered HLW.

Impact of the heterogeneity of the sandy Opalinus clay facies at the Mont Terri underground research laboratory on radionuclide migration

Abstract. The assessment of the safety of a deep geological repository (DGR) for high-level radioactive wastes over assessment time scales of up to 1 million years requires an in-depth understanding of the multi-scale coupled processes that affect the repository system evolution over time, to reduce uncertainties and conservatism in safety analyses. This is in particular required with respect to the challenges of a comparative assessment of different repository concepts in different host rocks within the process of a site selection for a DGR for heat-generating radioactive wastes in Germany. The collaborative project “Integrity of nuclear waste repository systems – Cross-scale system understanding and analysis (iCross)” conducted jointly by five research centres of the Helmholtz Association and co-funded by the Initiative and Networking Fund of the Helmholtz Association and the Federal Ministry of Education and Research (BMBF) has been initiated with the overall objective to improve the understanding of coupled thermal-hydraulic-mechanical-chemical-(micro)biological (THMCB) processes and to develop simulation tools that enable a holistic close to reality description of the long-term evolution of the repository system. Geological formations, such as those foreseen as potential host rocks for DGRs, and their surroundings are heterogeneous on various length scales ranging from nanometers to kilometers. Therefore, the aim of this work in the context of iCross is to evaluate the effects of mineralogical, geochemical and microstructural heterogeneities of repository host rocks on radionuclide transport in the repository far field, using the sandy facies of the Opalinus clay (SF-OPA) from the Mont Terri underground research laboratory (St. Ursanne, Switzerland) as an example. Here, we address in particular the migration behaviour of Ra-226 as an important radionuclide to be considered in safety cases for deep geological disposal of spent nuclear fuel. To assess the impact of the heterogeneities in SF-OPA on radionuclide transport, a complementary approach combining microstructural characterisation methods, experimental techniques for the determination of transport parameters of the rock matrix and the mobility of Ra-226 with innovative developments in reactive transport modelling on the pore and continuum scales was pursued. One of the results was that although the limited clay content in SF-OPA decreases the total amount of Ra bound to the illite phase, the solid solutions of sulphate and carbonate compensate for this and provide a major fixation mechanism.

Combining innovative experimental approaches and cross-scale reactive transport modelling for assessing coupled hydrogeochemical processes at interfaces in deep geological repositories for radioactive waste

Abstract. Deep geological repositories with a multi-barrier concept are foreseen by various countries for the disposal of high-level radioactive waste. A reliable and consistent assessment of the safety of these repositories over time scales of some hundred thousand years requires an advancement of process understanding. Simulation tools need to be developed for a close-to-reality description of repository evolution scenarios. This is especially required to resolve the challenging task of comparing and assessing the safety of different repository concepts in different host rocks within the German site-selection process. The construction of underground galleries and geotechnical barriers in the host rock formation and the emplacement of nuclear waste packages will create perturbations induced by chemical, thermal and pressure gradients at the interfaces of the different barriers, leading to mineral dissolution and precipitation to achieve re-equilibration. Such coupled hydrogeochemical processes generate non-linear responses in transport and mechanical properties of barrier materials and host rocks, which have to be taken into account for a more rigorous assessment of repository system evolution. Reactive transport modeling (RTM) can be applied to investigate these perturbations and processes across temporal and spatial scales, from the micro-scale at interfaces via the repository near field to the entire repository system – information not accessible through experiments alone. Although RTM is capable of addressing highly complex hydrogeochemical phenomena, the application of RTM codes to real systems is impeded by the often simplified description of coupled processes. To enhance the predictive capabilities of reactive transport models and to gain fundamental insights into the coupling between solute and radionuclide transport properties (e.g., permeability and diffusivity) of porous media and dissolution/precipitation processes, we conducted experiments on “simplified” chemical systems combined with pore-scale and continuum-scale reactive transport modelling to study processes in isolation, with the final aim of improving conceptual approaches for process couplings implemented in reactive transport codes. In this context, we investigated the effects of coupled mineral dissolution and precipitation in porous media on changes in permeability using flow-through experiments conducted in a magnetic resonance imaging scanner, which enabled the in situ investigation of porosity evolution in combination with monitoring changes in permeability and mineralogy. Our observations showed that classical implementations in reactive transport codes such as the Kozeny–Carman equation (Carman, 1937) failed to reproduce the changes in permeability and that more sophisticated approaches are required (Poonoosamy et al., 2020a, b). Moreover, we developed a novel “lab-on-a-chip” setup, i.e., micronized counter diffusion reactors with in operando 3D Raman tomography (Poonoosamy et al., 2019, 2020c), which enables evaluation of the alteration in pore architecture and study of the effect of coupled mineral dissolution and precipitation on the diffusive transport of solutes and radionuclides in porous media. Our approach enables the development of process-based theoretical models which allow for improvements in RTM codes and for predicting the evolution of perturbed interfaces in waste repositories, thus building confidence in the predictive capabilities of reactive transport models and reducing uncertainties with respect to future repository evolution.

An interdisciplinary view of the long-term evolution of repository systems across scales: the iCROSS project

Abstract. The interdisciplinary project “Integrity of nuclear waste repository systems – Cross-scale system understanding and analysis (iCROSS)” combines research competencies of Helmholtz scientists related to the topics of nuclear, geosciences, biosciences and environmental simulations in collaborations overarching the research fields energy and earth and environment. The focus is to understand and analyze close-to-real long-term evolutionary pathways of radioactive waste repositories across nanoscales to repository scales. The project is subdivided into work packages dealing with laboratory studies, field experiments in underground research laboratories (URLs), advanced modelling studies and the integration and alignment of data and information using virtual reality methods. In this sense, the project structure aims at a holistic view on relevant processes across scales in order to comprehensively simulate potential repository evolutions. Within the multi-barrier system of a repository for heat-generating radioactive waste, a number of complex reactions proceed, including dissolution, redox processes, biochemical reactions, gas evolution and solid/liquid interface and (co)precipitation reactions. At the same time, thermal and external mechanical stress has an impact on the conditions in a deep geological repository. All those processes are highly coupled, with multiple interdependencies on various scales and have a strong impact on radionuclide mobility and retention. In recent years, substantial progress was achieved in describing coupled thermal-hydro-mechanical-chemical-biological (THM/CB) processes in numerical simulations. A realistic and concise description of these coupled processes on different time and spatial scales is, at present, a largely unresolved scientific and computational challenge. The close interaction of experimental and simulation teams aims at a more accurate quantification and assessment of processes and thus, the reduction of uncertainties and of conservative assumptions and eventually to a close-to-real perception of the repository evolution. One focus of iCROSS is directed to relevant processes in a clay rock repository. In this context, the iCROSS team became a full member of the international Mont Terri consortium and worked in close collaboration with international and German institutions in URL projects. Respective experiments specifically deal with coupled processes at the reactive interfaces in a repository near field (e.g. the steel/bentonite and bentonite/concrete interfaces). Within iCROSS, the impact of secondary phase formation on radionuclide transport is investigated. At Mont Terri, experiments are in preparation to study radionuclide transport phenomena in clay rock formations within temperature gradients and in facies exhibiting significant heterogeneities on different scales (nm to cm). Beside those studies, high resolution exploration methods for rock characterization are developed and tested and the effect of temperature and other boundary conditions on the strength, creep properties and healing of faults within Opalinus clay are quantified. Multiphysics models coupled to reactive transport simulation have been further developed and applied to laboratory and field experiments. Results are digitally analyzed and illustrated in a visualization center, in order to enhance the comprehension of coupled processes in repository systems across scales. The present contribution provides an overview on the project and reports selected results. The impact of considering complex coupled processes in repository subsystems for the assessment of the integrity of a given (generic) repository arrangement is discussed.

Investigation of transport of radionuclide in a thermal stratification test facility using radiotracer technique

A radiotracer investigation was carried out in a Thermal Stratification Test Facility (TSTF) with objectives of investigating the dispersion and diffusion of radionuclide and effectiveness of the thermocline to minimize the radionuclide content in the hot water layer. Technetium-99m (99mTc) as sodium pertechnetate was used as a radiotracer in the investigation. Qualitative analysis showed that a thermocline is formed within the TSTF and is effective in preventing the transport of radionuclide from bottom section to the top section of the facility. It was found that the radiotracer injected at the bottom of the pool took about 17.4 h to disperse from bottom to the top of the facility. The results of the investigation helped in understanding the effectiveness of hot water layer and thus to minimize the pool top radiation levels.

APERTRACK: A particle-tracking model to simulate radionuclide transport in the Arabian/Persian Gulf

A Lagrangian rapid-response model for simulating the transport of radionuclides in the Arabian (or Persian) Gulf is described. The model is based on a tide model including five constituents, which was solved in advance, and baroclinic circulation was obtained from HYCOM operational ocean model. The radionuclide model includes physical transport (advection and diffusion), radioactive decay and geochemical processes (interactions of radionuclides between water and sediments, described in a dynamic way). The model can lead with instantaneous or continuous releases. Some hypothetical releases from a coastal nuclear power plant were simulated. Results show that the moment of release affects the fate of radionuclides due to the temporal variability of baroclinic currents. Also, comparing results for releases of Cs and Pu, it was seen how the geochemical behaviour of the radionuclide clearly affects the further radionuclide distributions. It is easy to setup the model for a particular release and it provides a fast response; thus the present model is an appropriate tool to support decision-making after a nuclear accident.

An Application of Safety Assessment for Radioactive Waste Repository: Non-Equilibrium Transport of Tritium, Selenium, and Cesium in Crushed Granite with Different Path Lengths

Advection-dispersion experiments (ADE) were effectively designed for inadequate transport models through a calibration/validation process. HTO, selenium (Se), and cesium (Cs) transport in crushed granite were studied using a highly reliable, dynamic column device in order to obtain the retardation factors (R) and the dispersion coefficients (D) by fitting experimental breakthrough curves (BTCs) for various path lengths. In order to conduct a safety assessment (SA) of a deep geological repository for high-level radioactive waste, radionuclide transport in rock systems is necessary to clarify and establish a suitable model. A dynamic column with a radiotracer (HTO, Se(IV), and Cs) was applied to 2, 4, and 8 cm path lengths using a STANMOD simulation. The results showed similar results between the BTCs of Se and Cs by fitting a non-equilibrium sorption model due to the retardation effect. In fact, there was a relatively obvious sorption of Se and Cs in the BTCs obtained by fitting a retardation factor (R) value higher than 1. In addition, a two-region (physical) and a two-site (chemical) non-equilibrium model with either the lowest sum of squared residuals (SSQ) or the root mean square error (RMSE) were applied to determine the Se and Cs sorption mechanisms on granite.

Aquatic speciation of Eu(III) with N-(hydroxyl ethyl)iminodiacetic acid (HEIDA), a major degradation product of organic radioactive waste

The organic ligands present in the radioactive waste and it’s vicinity can participate in the transport of radionuclides from waste storage sites into geosphere through ground water by forming aqueous soluble metal-organic complexes. The aquatic speciation studies of Eu(III) with the organic ligands present in the radioactive waste are required to model the transportation of actinides from waste storage sites into the environment. N-(2-hydroxyethyl)iminodiacetic acid (HEIDA), a degradation product of decontaminating agents co-disposed with the waste, is one of the important chelating agents identified at Hanford site waste. A detailed study of aqueous phase speciation of Eu(III) with HEIDA has been carried out to obtain thermodynamic parameters (log K, ∆G, ∆H and ∆S), stoichiometry and spectral properties by potentiometry, calorimetry and luminescence spectroscopy, while the molecular level structural information about the complexes were deduced from 1H and 13C NMR, X ray absorption spectroscopy (XAS) and density functional theory (DFT). Potentiometric and luminescence data showed the formation of ML1 and ML2 complexes of Eu(III)-HEIDA. Calorimetry revealed the endothermicity of complexation with positive entropy. NMR studies confirmed the participation of hydroxyl group of HEIDA along with two carboxylates and nitrogen in direct coordination to Eu(III) in 1:1 complex. The XAS studies revealed the coordinating atoms and bond distances for 1:2 Eu(III)-HEIDA complex. The DFT calculations supported the experimental results. The data can be used to model the environmental fate (migration and transportation) of actinides at waste storage sites as well as for environmentally safe designing and maintenance of nuclear radioactive waste repositories.

Post-Closure Safety Analysis of Nuclear Waste Disposal in Deep Vertical Boreholes

Isolation of spent nuclear fuel assemblies in deep vertical boreholes is analyzed. The main safety features of the borehole concept are related to the repository’s great depth, implying (a) long migration distances and correspondingly long travel times, allowing radionuclides to decay, (b) separation of the repository from the dynamic hydrological cycle near the land surface, (c) stable geological and hydrogeological conditions, and (d) a geochemically reducing environment. An integrated simulation model of the engineered and natural barrier systems has been developed to examine multiple scenarios of the release of radionuclides from the waste canisters, the transport through a fractured porous host rock, and the extraction of potentially contaminated drinking water from an aquifer. These generic simulations include thermal effects from both the natural geothermal gradient and the heat-generating waste, the influence of topography on regional groundwater flow, moderated by salinity stratification at depth, and the role of borehole sealing. The impact of these processes on the transport of select radionuclides is studied, which include long-lived, soluble, sorbing or highly mobile isotopes along with a decay chain of safety-relevant actinide metals. The generic analyses suggest that a deep vertical borehole repository has the potential to be a safe option for the disposal of certain waste streams, with the depth itself and the stable hydrogeological environment encountered in the emplacement zone providing inherent long-term isolation, which allows for reduced reliance on a complex engineered barrier system.

An experimental approach to assess the post-depositional mobility of 134Cs

The partitioning coefficient, Kd, which is defined by the reversible sorption processes between a solid and an aqueous phase at equilibrium, is one of the most important parameters to assess environmental transport and risk. In this study, a series of simple laboratory experiments were conducted to investigate sorption properties of 134Cs on a model sediment under two treatments (shaken vs non-shaken) and with three (small: <75 μm, large: > 75 μm and bulk i.e., composite) particle size fractions. Vertical transport of 134Cs across the water-sediment interface and into sediment was also evaluated. As expected, grain size had the strongest influence on 134Cs Kd values, with the small particle size fraction yielding significantly higher Kd values than the large and bulk fractions. The mean Kd values obtained from the various experiments ranged from 89 ± 13–130 ± 5 L kg−1 (small), 44 ± 10–91 ± 13 L kg−1 (large), 73 ± 3–112 ± 11 L kg−1 (bulk, shaken) and 73 ± 5–110 ± 4 L kg−1 (bulk, non-shaken). Most of the 134Cs partitioning processes occurred rapidly (<2 h) into the experiment. Physical mixing (shaken) did not appear to significantly affect the 134Cs Kd values. In complement, a separate experiment on the vertical penetration of 134Cs into a bulk sediment column showed that 134Cs was able to penetrate up to 5 cm into the sediment column after 88 days (∼0.6 mm d−1) and this flux rate is comparable to natural settings. Adsorption and contact time were found to be key for the 134Cs penetration process. Results from these experiments add to the literature on post-event radionuclide transport studies in marine settings and provide an experimental perspective that can be built upon to complement field observations.

An Adjoint Sensitivity Model for Radionuclide Transport in Heterogeneous Discrete Fractured Porous Formation in Steady‐State Regime

The development of the adjoint state method for the system of partial differential equations (PDEs) describing radionuclide transport in heterogeneous discrete fractured porous media in steady-state regime is presented in this study for both continuous and discrete adjoint approaches. The continuous adjoint approach has its own advantage in the sense that the adjoint system can be derived in terms of PDEs which can be solved analytically for specific performance measures. These PDEs can then be used to obtain a physical interpretation of the adjoint state functions. The discrete adjoint approach permits a simpler mathematical treatment based on the use of the same matrix (or its transpose) of the linear system of the discretized transport equations referring to from hereon as the forward problem. Analytical solutions for both the forward problem and its associated adjoint problem have been derived for radionuclide transport in layered formations including a fracture using an analytical inversion of tridiagonal matrices. A set of 14 formulas of sensitivity coefficients with respect to pertinent system parameters has been derived and presented in integral forms. Analytical results have been compared with numerical results and have shown excellent agreements. The discrete adjoint approach has been coupled with the differential sensitivity analysis method and applied to a real field case involving the performance assessment of a site currently being considered for underground nuclear storage in northern Switzerland, Zürich Nordost. The results show that when a fracture is present in the vicinity of waste repository, the state parameter controlling flow through the fracture, referred to herein as fracture transmissivity, is among the important parameters which should be considered in sensitivity analysis studies.

The Effect of Porosity Change in Bentonite Caused by Decay Heat on Radionuclide Transport through Buffer Material

Bentonite is used as a buffer material in most high-level radioactive waste (HLW) repository designs. Smectite clay is the main mineral component of bentonite and plays a key role in controlling the buffer’s physical and chemical behaviors. Moreover, the long-term functions of buffer clay could be lost through smectite dehydration under the prevailing temperature stemming from the heat of waste decay. Therefore, the influence of waste decay temperatures on bentonite performance needs to be studied. However, seldom addressed is the influence of the thermo-hydro-chemical (T-H-C) processes on buffer material degradation in the engineered barrier system (EBS) of HLW disposal repositories as related to smectite clay dehydration. Therefore, we adopted the chemical kinetic model of smectite dehydration to calculate the amount of water expelled from smectite clay minerals caused by the higher temperatures of waste decay heat. We determined that the temperature peak of about 91.3 °C occurred at the junction of the canister and buffer material in the sixth year. After approximately 20,000 years, the thermal caused by the release of the canister had dispersed and the temperature had reduced close to the geothermal background level. The modified porosity of bentonite due to the temperature evolution in the buffer zone between 0 and 0.01 m near the canister was 0.321 (1–2 years), 0.435 (3–10 years), and 0.321 (11–20,000 years). In the buffer zone of 0.01–0.35 m, the porosity was 0.321 (1–20,000 years). In the simulation results of near-field radionuclide transport, we determined that the concentration of radionuclides released from the buffer material for the porosity of 0.321 was higher than that for the unmodified porosity of 0.435. It occurs after 1, 1671, 63, and 172 years for the I-129, Ni-59, Sr-90, and Cs137 radionuclides, respectively. The porosity correction model proposed herein can afford a more conservative concentration and approach to the real release concentration of radionuclides, which can be used for the safety assessment of the repository. Smectite clay could cause volume shrinkage because of the interlayer water loss in smectite and cause bentonite buffer compression. Investigation of the expansion pressure of smectite and the confining stress of the surrounding host rock can further elucidate the compression and volume expansion of bentonite. Within 10,000 years, the proportion of smectite transformed to illite is less than 0.05%. The decay heat temperature in the buffer material should be lower than 100 °C, which is a very important EBS design condition for radioactive waste disposal. The results of this study may be used in advanced research on the evolution of bentonite degradation for both performance assessments and safety analyses of final HLW disposal.

A complex network theoretic approach for interdependence investigation: An application to radionuclide behavior in the subsurface

The failure of uranium mine tailings dams results in the infiltration and spreading of tailings in the subsurface. The fate and transport of radionuclides in the subsurface depends on several confounding, complex interdependent factors that describe the elements of the integrated system (i.e., meteorological; hydrological; hydrogeological; and, soil, groundwater, and mine tailings chemistry). The factors describing the integrated system have typically been investigated independently; however, their interdependence and resulting collective influence on the subsurface migration of radionuclides are yet to be explored. The current study develops a complex network theoretical approach to analyze these interdependencies. In this respect, a network of factors (NoF) was developed, and its characteristics (e.g., diameter, density, characteristic pathlength, average clustering coefficient, and factor centrality measures) were evaluated to determine the importance of considering these interdependencies when developing radionuclide fate and transport models. A sensitivity analysis was subsequently performed on the NoF to characterize the propagation of uncertainty associated with the factors in the NoF through a fate and transport model. The sensitivity analysis indicated that microorganisms present in the soil and mine tailings, fraction of organic carbon in the soil matrix, infiltration, and transmissivity must be well characterized (i.e., to minimize their uncertainty) when developing an integrated subsurface radionuclide fate and transport model, as uncertainty in these parameters will be amplified in the model output. The NoF developed in this study can be used to allocate data collection resources strategically in order to minimizing uncertainty in fate and transport models. This improves the reliability of fate and transport models, and ultimately leads to better management and remediation strategies to mitigate impacts from UMTD failures.

Consequences for Norway from a hypothetical accident at the Sellafield reprocessing plant: Atmospheric transport of radionuclides

The potential consequences for Norway should a nuclear accident at the Sellafield nuclear site occur, have been of concern for Norwegian authorities for several decades. Meteorological data from a 33-year period and the dispersion model ‘SNAP’ were used to evaluate meteorological conditions for which atmospheric transport of radionuclides from Sellafield to Norway would lead to the most severe impacts. The worst-case meteorological scenario for Norway, was found on 25th June 1989 for a low elevation (0–800 m) release and on 29th June 2001 for a higher elevation (800–1600 m) release. In both cases the western part of Norway was most affected. In general, the probability for depositions (>10 Bq/m2 of 137Cs) increased about 40% during the autumn and winter compared to the spring and summer months. An influence of climate change on the depositions was analysed, but not verified. Results from a number of simulations were also compared to identify how factors such as radioactive particle characteristics and initial release conditions could affect the predicted radionuclide deposition. The impact on predicted total depositions as well as hot-spot depositions by varying particle density and size as well as release elevation in worst-case scenario simulations amounted to about 40%–50%.

Reduced-order modeling of near-field THMC coupled processes for nuclear waste repositories in shale

Performance assessment (PA) of geologic radioactive waste repositories requires three-dimensional simulation of highly nonlinear, thermo-hydro-mechanical-chemical (THMC), multiphase flow and transport processes across many kilometers and over tens to hundreds of thousands of years. Integrating the effects of a near-field geomechanical process (i.e. buffer swelling) into coupled THC simulations through reduced-order modeling, rather than through fully coupled geomechanics, can reduce the dimensionality of the problem and improve computational efficiency. In this study, PFLOTRAN simulations model a single waste package in a shale host rock repository, where re-saturation of a bentonite buffer causes the buffer to swell and exert stress on a highly fractured disturbed rock zone (DRZ). Three types of stress-dependent permeability functions (exponential, modified cubic, and Two-part Hooke’s law models) are implemented to describe mechanical characteristics of the system. Our modeling study suggests that compressing fractures reduces DRZ permeability, which could influence the rate of radionuclide transport and exchange with corrosive species in host rock groundwater that could accelerate waste package degradation. Less permeable shale host rock delays buffer swelling, consequently retarding DRZ permeability reduction as well as chemical transport within the barrier system.

Stable isotope analysis in environmental impact assessment of radioactivity releases from nuclear power plants

The potential use of stable fission products from the decay of radioactive contaminants in recreating the impact assessment of radioactivity releases from nuclear power stations is examined. With the advances in mass spectrometry, Minimum Detectable Limits (MDLs) comparable to radioactive measurements have been achieved in the case of stable isotopes as well. The applications of stable fission products in safeguards and environmental surveillance by the author are mentioned. The limited application of stable fission products in the analysis of Fukushima environmental samples is reviewed. The article examines the possible candidates for such analysis and points out the enormous possibilities. This is a neglected field in environmental impact assessment till now and calls for standardizing measurement procedures and analysis methodologies. The stable fission product signatures can also be used to study the uptake and transport of radionuclides in the environment.

Validation of the correlation-based aerosol model in the ISFRA sodium-cooled fast reactor safety analysis code

ISFRA (Integrated SFR Analysis Program for PSA) computer program has been developed for simulating the response of the PGSFR pool design with metal fuel during a severe accident. This paper describes validation of the ISFRA aerosol model against the Aerosol Behavior Code Validation and Evaluation (ABCOVE) experiments undertaken in 1980s for radionuclide transport within a SFR containment. ABCOVE AB5, AB6, and AB7 tests are simulated using the ISFRA aerosol model and the results are compared against the measured data as well as with the simulation results of the MELCOR severe accident code. It is revealed that the ISFRA prediction of single-component aerosols inside a vessel (AB5) is in good agreement with the experimental data as well as with the results of the aerosol model in MELCOR. Moreover, the ISFRA aerosol model can predict the “washout” phenomenon due to the interaction between two aerosol species (AB6) and two-component aerosols without strong mutual interference (AB7). Based on the theory review of the aerosol correlation technique, it is concluded that the ISFRA aerosol model can provide fast, stable calculations with reasonable accuracy for most of the cases unless the aerosol size distribution is strongly deformed from log-normal distribution.