Nuclide Transport Research Articles

A fractional derivative model for nuclides transport in heterogeneous fractured media

Nuclide transport in fractured media involves the advection, dispersion, adsorption, etc. The dispersion and adsorption properties of the rock matrix have spatial variability, which results in an anomalous transport of nuclides. In this study, a time-fractional advection-diffusion equation (t-FADE) model is utilized to capture the sub-diffusion transport behavior with heavy-tail property, including the breakthrough curves (BTCs) of uranium and thorium transport in granite plates. Moreover, hydrodynamic dispersion of tritiated water, strontium and cesium in granite fractures are also studied. The results indicate that BTCs of nuclides transport in the granite fractures are unimodal and asymmetric. The decrease of the fractional order α reflects the stronger sub-diffusion. Furthermore, small initial velocity enhances sub-diffusion effect of nuclides and lengthens the breakout time of BTCs, which results in obvious heavy-tail phenomena. The analysis results demonstrate that the t-FADE model can accurately describe sub-diffusion behavior of nuclides transport. At last, the advantages of the t-FADE model in prediction and remediation of nuclides contamination are put forward.

Numerical modeling on nuclide transport around a nuclear waste repository under coupled thermo-hydro-mechanical condition

Coupled thermo-hydro-mechanical (THM) processes may raise the uncertainty of nuclide transport behavior in long-term operation of nuclear waste repository. A far-field modeling framework on nuclide transport around a nuclear waste repository under coupled THM conditions was developed and verified. Base on conceptual models of high-level waste repository in fractured rocks, a three-dimensional repository model including a vertical fault and a horizontal fracture was built, and influence of coupled THM processes on nuclide transport is numerically investigated. Effects of repository depth, fracture inclination, and internal aperture heterogeneity were investigated using the developed model. The results show that the coupled THM processes significantly affect nuclide transport behavior, especially in the early stages of nuclide leaching. The thermal stress induced by heat release of nuclear waste causes the reduction in fracture aperture around the repository. The nuclide transport under THM conditions is retarded before 103 years than that under hydraulic condition. The deeper locations of the candidate repository exhibit the lower rate of nuclide transport. The inclined fracture induces opposite flow directions between two fractures, which causes the delaying of nuclide transport. Fractures with heterogeneous apertures have preferential flow paths, which results in early arrivals of nuclide transport than fractures with homogenous apertures.

Visualization research on ENT1/NIS dual-function gene therapy to reverse drug resistance mediated by MUC1 in GEM-resistant pancreatic cancer

PurposeTo use bifunctional target genes to increase the intracellular transport of gemcitabine (GEM) to reverse chemotherapy resistance and to simultaneously use reporter gene imaging to localize therapeutic genes. The therapeutic effect was evaluated by [18F]FLT PET/CT to visualize the effect of gene therapy. MethodsA viral gene vector containing the pancreatic cancer-targeting promoter MUC1 for specific transcription of equilibrative nucleoside transporter 1 (ENT1) and NIS (nuclide transport channel) was employed. [125I]NaI uptake tests and [131I]NaI SPECT imaging were performed to verify the function of NIS and the target function of MUC1. The correlation between [18F]FLT uptake and GEM resistance were assessed, and the influence ENT1 and thymidine kinase 1 (TK1) expression on [18F]FLT micro-PET/CT was measured, which provides a theoretical basis for the use of [18F]FLT micro-PET/CT to evaluate the efficacy of gene therapy. ResultsFirst, functions of gene therapy were confirmed: ENT1 reversed the drug resistance of GEM-resistant pancreatic cancer cells by increasing GEM intracellular transport; MUC1 drove NIS target gene expression in pancreatic cancer; and therapeutic genes could be localized using [131I]NaI SPECT reporter gene imaging. Second, the [18F]FLT uptake ratio was affected by drug resistance and GEM treatment. The mechanism underlying this effect was related to ENT1 and TK1. Increased expression of ENT1 inhibited the expression of TK1 after GEM chemotherapy to reduce the uptake of [18F]FLT. Finally, micro-PET/CT indicated that the SUVmax of [18F]FLT could predict survival time. SUVmax exhibited an increasing trend in resistant pancreatic cancer but a trend of inhibition after upregulation of ENT1, which was more significant after GEM treatment. ConclusionsBifunctional targeted genes can localize therapeutic genes through reporter gene imaging, reverse the drug resistance of GEM-resistant pancreatic cancer and be visually evaluated through [18F]FLT micro-PET/CT.

Sequential Bayesian Updating Module of Input Parameter Distributions for More Reliable Probabilistic Safety Assessment of HLW Radioactive Repository

A Bayesian approach was introduced to improve the belief of prior distributions of input parameters for the probabilistic safety assessment of radioactive waste repository. A GoldSim-based module was developed using the Markov chain Monte Carlo algorithm and implemented through GSTSPA (GoldSim Total System Performance Assessment), a GoldSim template for generic/site-specific safety assessment of the radioactive repository system. In this study, sequential Bayesian updating of prior distributions was comprehensively explained and used as a basis to conduct a reliable safety assessment of the repository. The prior distribution to three sequential posterior distributions for several selected parameters associated with nuclide transport in the fractured rock medium was updated with assumed likelihood functions. The process was demonstrated through a probabilistic safety assessment of the conceptual repository for illustrative purposes. Through this study, it was shown that insufficient observed data could enhance the belief of prior distributions for input parameter values commonly available, which are usually uncertain. This is particularly applicable for nuclide behavior in and around the repository system, which typically exhibited a long time span and wide modeling domain.

A Bayesian approach for probabilistic safety assessment of HLW repository

A Bayesian approach was studied to improve the belief for input parameters for the probabilistic safety assessment of high-level waste (HLW) repository. A GoldSim-based module has been developed by which sequential Bayesian updating of parameter distributions and safety assessment of the repository can be carried out. Updating the distributions for four selected parameters associated both with nuclide transport in the fractured rock medium and abnormal earthquake events was demonstrated through probabilistic safety assessment of the conceptual repository. For illustrative purpose, all the prior, sequential posterior distributions and likelihoods were assumed. Although, with insufficient data, there is a limitation to estimation of the real likelihood function, it sufficiently seems to be possible to offer improved results with enhancement of the belief for parameter values, which are usually uncertain, especially for the nuclide behavior in and around the repository system, which typically has both long time span and wide modeling domain.

Models for the assessment of transport of naturally-occurring nuclides in fractured media

In fractured media, naturally-occurring nuclides are produced by the decay of the natural uranium and thorium present in the rock matrix. Back-diffusion from the matrix into the water bearing fractures and advection along these fractures leads to nuclide concentration values that are representative of groundwater flow and transport processes. Hence, interpretation of measured levels of naturally-occurring nuclides might provide valuable information about these underlying processes. Here, we have provided mathematical models for the simulation of generation and transport of naturally-occurring nuclides in fractured media. The proposed analytical solutions have been applied to study 4He generation and migration in realistic transport pathways. The results of these simulations have shown that, in fractured crystalline media, steady-state conditions are attained after thousands to tens of thousands of years. One of the developed analytical solution have also been used to infer typical values of transport resistance (i.e. the ratio between the groundwater residence time and the fracture half aperture) in the shallow bedrock at Olkiluoto (Finland). The results of this analysis have pointed out the very strong retention capacity of the considered fractured system.

Removal of Co, Sr and Cs ions from simulated radioactive wastewater by forward osmosis

The removal of Co, Sr and Cs ions form simulated radioactive wastewater using forward osmosis (FO) process was investigated. The effect of various factors on nuclide transport was examined, including membrane orientation, NaCl concentration, flow velocity, and the main factors were identified by correlation analysis. The mechanisms of nuclides transfer through membrane were explored. The results indicated that the active layer facing draw solution (AL-DS) had higher nuclide flux than AL-FS. At AL-FS mode, the highest flux of Co, Sr and Cs were only 1.54, 10.22 and 15.63 mg m-2 h-1 respectively by cellulose triacetate with embedded polyester screen support (CTA-ES) membrane. At AL-DS mode, the flux of Co and Cs increased when NaCl concentration and flow velocity increased. Convection, diffusion and electrostatic interactions were found to influence the nuclide transport all together. The Pearson correlation and partial correlation analysis identified that the diffusion coefficient of nuclides and reverse NaCl flux were the most important factors affecting nuclide flux through cellulose triacetate membrane. The water flux, NaCl concentration, flow velocity and partition coefficient were not the main affecting factors for nuclide flux.

Effect of ion exchange membrane on the removal efficiency of continuous electrodeionization (CEDI) during low level radioactive wastewater treatment

The treatment of low level radioactive wastewater (LLRW) has gained more attention with the development of nuclear energy. Continuous electrodeionization (CEDI) process is a promising technology to remove trace nuclides and minimize radioactive wastes. In this study, seven CEDI stacks filled with different ion exchange membranes (IEM) were compared for their performance during the LLRW treatment. The removal ratios of boron ranged from 5% to 16%, and the decontamination factors (DF) of nuclides ranged from 15 to 960. The performance of the CEDI stacks varied from each other. Some stacks had high removal efficiencies on both boron and nuclides, while some had high DF values of nuclides but low removal ratio of boron. Correlation analysis showed that ion exchange capacity of IEMs had no significant correlation with the efficiency of CEDI stacks. The diffusion dialysis coefficient had a negative effect on the removal of nuclides, but had no significant effects on the removal of boron. The transport of strong electrolyte nuclides and weak electrolyte boric acid was quite different, which helps to guide the CEDI stack design for different purposes, such as the separation of boron and nuclides or removal them simultaneously.

A new nuclide transport model in soil in the GENII-LIN health physics code

The nuclide soil transfer model, originally included in the GENII-LIN software system, was intended for residual contamination from long term activities and from waste form degradation. Short life nuclides were supposed absent or at equilibrium with long life parents. Here we present an enhanced soil transport model, where short life nuclide contributions are correctly accounted. This improvement extends the code capabilities to handle incidental release of contaminant to soil, by evaluating exposure since the very beginning of the contamination event, before the radioactive decay chain equilibrium is reached.

Implementation of a Time-Domain Random-Walk Method into a Discrete Element Method to Simulate Nuclide Transport in Fractured Rock Masses

It is essential to study nuclide transport with underground water in fractured rock masses in order to evaluate potential radionuclide leakage in nuclear waste disposal. A time-domain random-walk (TDRW) method was firstly implemented into a discrete element method (DEM), that is, UDEC, in this paper to address the pressing challenges of modelling the nuclide transport in fractured rock masses such as massive fractures and coupled hydromechanical effect. The implementation was then validated against analytical solutions for nuclide transport in a single fracture and a simple fracture network. After that, the proposed implementation was applied to model the nuclide transport in a complex fracture network investigated in the DECOVALEX 2011 project to analyze the effect of matrix diffusion and stress on the nuclide transport in the fractured rock masses. It was concluded that the implementation of the TDRW method into UDEC provided a valuable tool to study the nuclide transport in the fractured rock masses. Moreover, it was found that the total travel time of the nuclide particles in the fractured rock masses with the matrix diffusion and external stress modelled was much longer than that without the matrix diffusion and external stress modelled.

An evaluation of nuclide release from a trench-type LILW repository

A simple and effective model and a GoldSim (GoldSim, 2006) template program, by which a probabilistic safety assessment of a conceptual trench-type repository for low- and intermediate level radioactive waste (LILW) disposal can be carried out under various nuclide release scenarios, have been developed. To quantify the exposure dose rates due to nuclide release from the trench and transport through the various pathways possible in the near- and far-fields of the repository system under a base case and some alternative scenarios, illustrative evaluations for a comparison among the scenarios as well as a sensitivity of shortcut pathways generated due to earthquakes on the nuclide transport are made and demonstrated. To this end, by changing the conservative base case nuclide release scenario under which all portions of the cap of the trench are failed unconditionally and immediately after a closure of the repository, a total of four other alternative scenarios were separately evaluated for the total exposure dose rates to the farming exposure group and then compared to the base case results. Among them, an earthquake scenario shows a dominant behavior almost throughout the whole time span. To see the influence of all shortcut travel pathways that are assumed to be newly generated by this earthquake scenario, their sensitivities to the exposure dose rates to the farming exposure group were also made and compared to each other.

Radionuclide migration through fractured rock for arbitrary-length decay chain: Analytical solution and global sensitivity analysis

This study presents an analytical approach to simulate nuclide migration through a channel in a fracture accounting for an arbitrary-length decay chain. The nuclides are retarded as they diffuse in the porous rock matrix and stagnant zones in the fracture. The Laplace transform and similarity transform techniques are applied to solve the model. The analytical solution to the nuclide concentrations at the fracture outlet is governed by nine parameters representing different mechanisms acting on nuclide transport through a fracture, including diffusion into the rock matrices, diffusion into the stagnant water zone, chain decay and hydrodynamic dispersion. Furthermore, to assess how sensitive the results are to parameter uncertainties, the Sobol method is applied in variance-based global sensitivity analyses of the model output. The Sobol indices show how uncertainty in the model output is apportioned to the uncertainty in the model input. This method takes into account both direct effects and interaction effects between input parameters. The simulation results suggest that in the case of pulse injections, ignoring the effect of a stagnant water zone can lead to significant errors in the time of first arrival and the peak value of the nuclides. Likewise, neglecting the parent and modeling its daughter as a single stable species can result in a significant overestimation of the peak value of the daughter nuclide. It is also found that as the dispersion increases, the early arrival time and the peak time of the daughter decrease while the peak value increases. More importantly, the global sensitivity analysis reveals that for time periods greater than a few thousand years, the uncertainty of the model output is more sensitive to the values of the individual parameters than to the interaction between them. Moreover, if one tries to evaluate the true values of the input parameters at the same cost and effort, the determination of priorities should follow a certain sequence.

질량 이동 모사 프로그램 개발을 위한 골드심 이동 패쓰웨이의 이해와 활용

상용의 GoldSim과 GoldSim 이동 모듈 (GoldSim Transport Module; GTM)을 이용하여 방사성폐기물 처분시스템과 같이 복잡한 질량 이동 시스템을 신뢰성 있고 효율적으로 모사할 수 있다. 그러나 GTM의 특성을 보다 정확하게 이해하여야 이를 사용하여 실제 처분시스템의 안전성 평가 프로그램을 개발할 때 발생할 수 있는 오류를 피할 수 있다는 것을 인지하는 것이 중요하다. 이를 위하여 GTM에서 다양하게 제공되는 요소 (element) 중, 질량 이동 모사에 유용한 Transport pathway의 특징에 대하여 소개하고, 방사성폐기물 처분시스템 안전성 평가를 위해 시스템 내 핵종의 거동과 같은 질량 이동 모사에서 이에 대한 올바른 활용 방안을 제시하였다. It is very important to properly understand such "Transport Pathways" elements as "Pipe" and "Cell" pathways in commercial GoldSim Transport Module (GTM) for developing higer quality models and programs for performance assessment of complex radioactive waste repositories. With an illustrative case under an earthquake scenario, by which an increasement in the groundwater flow rate occurs though the geological medium, ways of avoiding possible modeling errors in the nuclide transport modeling in the radioactive waste repository system for its safety assessment by utilizing such pathways are discussed and a proper usage of the pathways is proposed.

Solute transport in a single fracture involving an arbitrary length decay chain with rock matrix comprising different geological layers

A model is developed to describe solute transport and retention in fractured rocks. It accounts for advection along the fracture, molecular diffusion from the fracture to the rock matrix composed of several geological layers, adsorption on the fracture surface, adsorption in the rock matrix layers and radioactive decay-chains. The analytical solution, obtained for the Laplace-transformed concentration at the outlet of the flowing channel, can conveniently be transformed back to the time domain by the use of the de Hoog algorithm. This allows one to readily include it into a fracture network model or a channel network model to predict nuclide transport through channels in heterogeneous fractured media consisting of an arbitrary number of rock units with piecewise constant properties. More importantly, the simulations made in this study recommend that it is necessary to account for decay-chains and also rock matrix comprising at least two different geological layers, if justified, in safety and performance assessment of the repositories for spent nuclear fuel.

파이로처리 폐기물 처분 시스템 근계 영역 내 핵종 유출 민감도: 제 1 부 확률론적 접근

파이로처리 방사성폐기물 처분장에서 폐쇄 후 처분장의 성능에 영향을 줄 수 있는 근계 영역 내 세가지 주요 설계 관련 요소에 대하여 주요 핵종별로 최종 피폭 선량에 주는 민감도를 확률론적인 접근을 통하여 조사하였다. 농축 피폭 집단에 방사선 피폭을 주는 주요한 핵종들이 처분장에서 유출된 후 처분 시스템 근계 영역 내 다양한 매질을 이동하는 것에 관련되어, 이들 요소가 어느 정도의 영향을 주게 되는지 보기 위하여 처분 용기의 수명, 선원항으로서 처분 용기에서의 연간 핵종 유출률, 그리고 처분장 주요 인공 방벽으로서 완충재의 손실도 등의 변화에 따른 결과를 검토하였다. 처분장에 대한 결정론적, 확률론적 안전성 평가를 병행 수행하여, 이 세가지 설계 요소의 물리적 변화가 통계적 분포를 가지고 일어난다고 가정하는 확률론적 접근 방법에 따른 연구 결과는 제 1 부의 이 연구에, 그리고 세가지 설계 요소가 가질 수 있는 조합을 서로 다른 시나리오로서 비교하는 결정론적인 방법으로 접근한 결과는 별도로 제 2 부에 제시하였다. 두 가지 접근 결과와 함께 고려된 인자들에 대하여 모두 결과에 민감한 것으로 나타나 이러한 결과와 방법론은 향후 처분장 설계에 모범적인 피드백을 줄 수 있을 것으로 기대된다. A parametric sensitivity to the annual exposure dose rate to the farming exposure group has been probabilistically carried out for three principal elements associated with the nuclide transport behavior in the near-field of the pyroprocessed waste repository system. Credit time for both metal and ceramic containers, annual nuclide release rete, and the degree of loss of bentonite buffer around the container are selected as the elements and investigated for important nuclides. All the elements are shown to be sensitive to the results. Methodology studied through this study and the results are expected to make a good feedback to the repository design. As a follow-up study, separated in Part 2, the A-KRS will be deterministically assessed and then compared among each other with the normal, the worst, and the best case scenarios associated with their extreme values these elements could have.

Developing a modelling tool for density-driven flow in complex hydrogeological structures

Beginning in 1995, the codes $$\hbox {d}^{3}\hbox {f}$$d3f (distributed density driven flow) and $$\hbox {r}^{3}\hbox {t}$$r3t (radionuclides, reaction, retardation, and transport) for modeling density-driven groundwater flow and nuclide transport using UG toolbox are developed in the framework of several joint projects. During this time, the codes were substantially extended as well as numerically improved, and the development is still ongoing. Now, $$\hbox {d}^{3}\hbox {f}$$d3f and $$\hbox {r}^{3}\hbox {t}$$r3t are no longer restricted to modeling of porous media, they also may be used for fractured rock. These are powerful tools that are able to handle salt and heat transport, salt concentrations up to saturation and complex hydrogeological structures with high permeability contrasts.

The development of a through-diffusion model with a parent–daughter decay chain

A valid performance assessment of radioactive waste repositories strongly depends on the reliability of nuclide transport parameters, including distribution and diffusion coefficients. To reduce the waste produced and time spent conducting diffusion experiments, a robust model is required to accurately interpret the experiment results. Therefore, we developed a through-diffusion model with parent–daughter nuclide decay chain. We validated our model through comparisons with the Moridis model (Moridis, 1999) and Bharat model (Bharat et al., 2009), assessing our model and these two models using the distribution of parent nuclide concentrations. This strongly supports the rationality and functionality of extending our proposed model to daughter nuclides. In this study, we derived analytical solutions for the parent nuclides of the through-diffusion experiment using the multicompartment (MC) model. We also propose a simplified formula for estimating the apparent diffusion coefficient of parent nuclides based on the analytical solutions. Through numerical experiments, we verified the feasibility of the formula. Our models are useful for determining the apparent diffusion coefficient of daughter nuclides when conducting through-diffusion experiments with parent–daughter nuclide decay chains. Additionally, the proposed models offer the advantages of saving time and reducing experimental waste.

Lateral variation of 134Cs and 137Cs concentrations in surface seawater in and around the Japan Sea after the Fukushima Dai-ichi Nuclear Power Plant accident

A total of 82 surface seawater samples was collected in the Japan Sea and the southwestern Okhotsk Sea before and after the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident. Analysis of 134Cs and 137Cs concentrations using low-background γ-spectrometry revealed that the 137Cs concentration of the samples collected in June 2011 was 1.5–2.8mBq/L, which is approximately 1–2 times higher than the pre-accident 137Cs level, while the 134Cs concentration was less than detectable to 1mBq/L. In addition to 134Cs being clearly detected (∼1mBq/L), 137Cs concentration in water samples from the northeastern Japan Sea (2–2.8mBq/L) was also higher than that from the coast in the southwestern Japan Sea (∼1.5mBq/L). These higher concentrations in the northeastern Japan Sea could be ascribed to the atmospheric transport of nuclides from the FDNPP as aerosols and subsequent transport and dilution after delivery to the sea surface.

Evaluation of nuclide release scenarios for a hypothetical LILW repository

A program for the safety assessment and performance evaluation of a low- and intermediate-level radioactive waste (LILW) repository system has been developed. Utilizing GoldSim (2006), the program evaluates nuclide release and transport into the geosphere and biosphere under various disruptive natural and manmade events and scenarios that can occur after a waste package failure. We envisaged and illustrated these events and scenarios as occurring after the closure of a hypothetical LILW repository, and they included the degradation of various manmade barriers, pumping well drilling, and natural disruptions such as the sudden formation of a preferential flow pathway in the far-field area of the repository. Possible enhancement of nuclide transport facilitated by colloids or chelating agents is also dealt with. We used the newly-developed GoldSim template program, which is capable of various nuclide release scenarios and is greatly suited for simulating a potential repository given the geological circumstances in Korea, to create the detailed source-term and near-field release scheme, various nuclide transport modes in the far-field geosphere area, and the biosphere transfer. Even though all parameter values applied to the hypothetical repository were assumed, the illustrative results, particularly the probabilistic calculations and sensitivity studies, may be informative under various scenarios. ( GoldSim, LILW, Nuclide transport, Safety assessment, Scenario).

Transport and exchange of U-series nuclides between suspended material, dissolved load and colloids in rivers draining basaltic terrains

This study presents uranium and thorium concentrations and activity ratios for all riverine phases (bedload, suspended load, dissolved load and colloids) from basaltic terrains in Iceland and the Azores. Small basaltic islands, such as these, are thought to account for ~25% of CO2 consumed by global silicate weathering, and for ~45% of the flux of suspended material to the oceans. These data indicate that [U] and [Th] in the dissolved and colloidal fractions are strongly controlled by pH, and to a much lesser extent by levels of dissolved organic carbon (which are low in these environments). At high pH, basalt glass dissolution is enhanced, and secondary mineral formation (e.g. Fe-oxyhydroxides and allophane) is suppressed, resulting in high dissolved [U], and low colloidal [U] and [Th], indicating a direct chemical weathering control on elemental abundances. When the dissolved (234U/238U) activity ratio is >~1.3 (i.e. when physical weathering, groundwater contribution or soil formation are high), there is little isotope exchange between dissolved and colloidal fractions. At lower activity ratios, the dissolved load and colloids have indistinguishable activity ratios, suggesting that when chemical weathering rates are high, secondary clay formation is also high, and colloids rapidly adsorb dissolved U. Many of the suspended sediment samples have (234U/238U) activity ratios of >1, which suggests that uptake of U onto the suspended load is important. Identical (230Th/232Th) in suspended, dissolved and colloidal samples suggests that Th, like U, is exchanged or sorbed rapidly between all riverine phases. This particle-reactivity, combined with poorly constrained contributions from groundwater and hydrothermal water, and short-term variations in input to soils (volcanic and glacial), suggests that U-series nuclides in riverine material from such basaltic terrains are unlikely to reflect steady state erosion processes.