Radioactive Waste Disposal Facility Research Articles

Spanish methodological approach for biosphere assessment of radioactive waste disposal

The development of radioactive waste disposal facilities requires implementation of measures that will afford protection of human health and the environment over a specific temporal frame that depends on the characteristics of the wastes. The repository design is based on a multi-barrier system: (i) the near-field or engineered barrier, (ii) far-field or geological barrier and (iii) the biosphere system. Here, the focus is on the analysis of this last system, the biosphere. A description is provided of conceptual developments, methodological aspects and software tools used to develop the Biosphere Assessment Methodology in the context of high-level waste (HLW) disposal facilities in Spain. This methodology is based on the BIOMASS "Reference Biospheres Methodology" and provides a logical and systematic approach with supplementary documentation that helps to support the decisions necessary for model development. It follows a five-stage approach, such that a coherent biosphere system description and the corresponding conceptual, mathematical and numerical models can be built. A discussion on the improvements implemented through application of the methodology to case studies in international and national projects is included. Some facets of this methodological approach still require further consideration, principally an enhanced integration of climatology, geography and ecology into models considering evolution of the environment, some aspects of the interface between the geosphere and biosphere, and an accurate quantification of environmental change processes and rates.

RADIOACTIVE WASTE MANAGEMENT IN HUNGARY: CHANGING APPROACHES AND CONFLICTS

One of the most interesting paradoxes of the application of nuclear energy in Hungary is that although the building, operation, and even the recent lifetime extension of the Paks nuclear power plant has not faced significant social opposition, attempts for siting radioactive waste disposal facilities have been accompanied by a number of social conflicts. The paper reviews the history of radioactive waste management in Hungary from the 1970s until recently. It is found that since the political transition of 1990, top-down technocratic approaches to facility siting have been replaced by the so-called "market approach", which put a strong emphasis on host community support. Most important elements of the latter approach include public information and the provision of generous financial compensation/incentives for host and neighbouring communities. However, analysis of recent Hungarian siting cases shows the weaknesses of the pure market approach, e.g., the lack of transparency and openness regarding national strategies, the lack of accountability, and the focus on financial incentives rather than sustainable development issues.

Synthesis of nanoporous zirconium oxophosphate and application for removal of U(VI)

Uniformly arrayed zirconium-phosphate nanoporous material was synthesized, characterized, and used as an adsorbent for removal of U(VI) in a NaNO 3 solution with varying background conditions including pH, ionic strength, U(VI) concentrations, and carbonate concentrations. Batch U(VI) adsorption results showed that U(VI) adsorption reached steady-state condition within 48 h, and all the dissolved U(VI) (10 −6 M) was removed by this material at neutral pH and closed conditions to atmospheric CO 2(g). The U(VI) adsorption followed a traditional Langmuir adsorption isotherm, and the distribution coefficient ( K d) calculated from the linear region of the Langmuir isotherm was 105,000 mL g −1. Because this phosphate-bearing nanoporous material exhibits high thermal stability and has a very high K d value, it can be applied as an efficient adsorbent for removing U(VI) from various contaminated waste streams, such as those present at the US Department of Energy defense sites and the proposed geologic radioactive waste disposal facility at Yucca Mountain in Nevada.

Multiple predictor smoothing methods for sensitivity analysis: Description of techniques

The use of multiple predictor smoothing methods in sampling-based sensitivity analyses of complex models is investigated. Specifically, sensitivity analysis procedures based on smoothing methods employing the stepwise application of the following nonparametric regression techniques are described: (i) locally weighted regression (LOESS), (ii) additive models, (iii) projection pursuit regression, and (iv) recursive partitioning regression. Then, in the second and concluding part of this presentation, the indicated procedures are illustrated with both simple test problems and results from a performance assessment for a radioactive waste disposal facility (i.e., the Waste Isolation Pilot Plant). As shown by the example illustrations, the use of smoothing procedures based on nonparametric regression techniques can yield more informative sensitivity analysis results than can be obtained with more traditional sensitivity analysis procedures based on linear regression, rank regression or quadratic regression when nonlinear relationships between model inputs and model predictions are present.

Multiple predictor smoothing methods for sensitivity analysis: Example results

The use of multiple predictor smoothing methods in sampling-based sensitivity analyses of complex models is investigated. Specifically, sensitivity analysis procedures based on smoothing methods employing the stepwise application of the following nonparametric regression techniques are described in the first part of this presentation: (i) locally weighted regression (LOESS), (ii) additive models, (iii) projection pursuit regression, and (iv) recursive partitioning regression. In this, the second and concluding part of the presentation, the indicated procedures are illustrated with both simple test problems and results from a performance assessment for a radioactive waste disposal facility (i.e., the Waste Isolation Pilot Plant). As shown by the example illustrations, the use of smoothing procedures based on nonparametric regression techniques can yield more informative sensitivity analysis results than can be obtained with more traditional sensitivity analysis procedures based on linear regression, rank regression or quadratic regression when nonlinear relationships between model inputs and model predictions are present.

A hydrochemical and isotopic case study around a near surface radioactive waste disposal

As part of the site characterisation program for the near surface radioactive waste treatment and disposal facility (RWTDF) at Püspökszilágy, Hungary, water quality and environmental isotope investigations have been carried out. Water samples for major ion chemistry, tritium, 14C and stable isotope ratio measurements (δ18O, δD, δ34S, δ13C) were taken quarterly from the observation wells, the streams and the precipitation during the period 1999–2001. The chemical composition of groundwaters presented a continuous transition from waters situated on one side to waters on the top and on the other slope of the disposal suggesting the mixing of the three hydrochemical “endmembers”. Most of δD and δ 18O data were situated between GMWL and LMWL (δD = 7.2 × δ 18O − 1‰) with Oligocene aquifer presenting recharge of Pleistocene origin and water on the top and the gentle slope of the hill presenting recharge of Holocene origin. δ 34S values of dissolved sulphates varied in a wide range (-14.2‰ to + 5.4‰). The tritium in precipitation varied between 4.4 and 18.1 TU with an annual weighted average of 10 ± 0.3 TU. The streams showed larger fluctuations than the wells, but the changes of δ 18O, δD and T were small compared to those in precipitation (showing seasonal variation). Stable isotope, tritium and radiocarbon data proved that the replenishment of groundwater is slow on the steeper side and the direction of water movement is toward the gentle slope of the hill. It was judged that this path is the one that is most likely to give rise to high doses and, therefore, was used in the hydrological modelling of the safety assessment that followed the present work. The possibility that there may also be transport through the unsaturated zone and systems of perched water tables in layers 1 and 2 to both the Szilágyi and Némedi streams cannot be excluded; the transport along these pathways is likely to be intermittent.

放射性廃棄物処分場の合理的な処理・輸送・処分システム

放射性廃棄物処分場の合理的な処理・輸送・処分システム

Modeling the composition of the pore water in a clay-rock geological formation (Callovo-Oxfordian, France)

The interstitial water contained in the microporosity of highly compact clay-rich media does not obey the classical condition usually used to derive the ionic composition of a solution. This is because the requirement for global electroneutrality of a charged microporous body (one having a significant fraction of pores with dimensions of the same order of size as the diffuse double layer) implies that the net charge density of the pore water must balance the deficiency (or the excess) of electrical charge carried by the solid matrix. In order to determine the solution composition in the micropores of a clay-rock, we first generalize the Donnan equilibrium conditions for the case of a multi-ionic electrolyte, with partitioning of the charge compensating counterions between the Stern and the diffuse layers A material-specific geochemical equilibrium model, incorporating an electrical triple layer model for adsorption reactions, is used to calculate the partition coefficient for the compensating ion charge (i.e., the fraction of charge in the Stern layer). This is then used to calculate the osmotic pressure and ionic composition of the pore water in the micropores. The material considered in this study is the argillite clay-rock sampled from the Callovo-Oxfordian geological formation under consideration in France for a deep geological disposal facility for radioactive waste.

Application of a generic biosphere model for dose assessments to five Europeansites

The BIOMOSA (BIOsphere MOdels for Safety Assessment of radioactive waste disposal)project was part of the EC fifth framework research programme. The main goal of thisproject was to improve the scientific basis for the application of biosphere models in theframework of long-term safety studies of radioactive waste disposal facilities and to enhancethe confidence in using biosphere models for performance assessments. The study focusedon the development and application of a generic biosphere tool BIOGEM (BIOsphereGEneric Model) using the IAEA BIOMASS reference biosphere methodology, andthe comparison between BIOGEM and five site-specific biosphere models. Thesite-specific models and the generic model were applied to five typical locations inEurope, resulting in estimates of the annual effective individual doses to the criticalgroups and the ranking of the importance of the exposure pathways for each ofthe sites. Uncertainty in the results was also estimated by means of stochasticcalculations based on variation of the site-specific parameter values. This paperdescribes the generic model and the deterministic and stochastic results obtainedwhen it was applied to the five sites. Details of the site-specific models and thecorresponding results are described in two companion papers. This paper alsopresents a comparison of the results between the generic model and site-specificmodels. In general, there was an acceptable agreement of the BIOGEM for boththe deterministic and stochastic results with the results from the site-specificmodels.

A Generic Method to Evaluate the Reasonable Upper-Bound Dose from Near-Surface Radioactive Waste Disposal Facilities Through Drinking Water Pathway

A generic method has been developed to evaluate the reasonable upper-bound (RUB) dose from near-surface radioactive waste disposal facilities through a drinking water pathway. This generic method has been developed by applying a safety assessment model to seven near-surface radioactive waste disposal sites in India. The concentrations and effective radiation dose rates due to different radionuclides are evaluated at different distances from the disposal facilities. The peak dose rates received by members of the public at these distances are given per unit nuclear power capacity at the site [mSv/yr per GW(electric)·yr]. The product of these normalized peak dose rates and the total existing or projected nuclear power capacity at a site [GW(electric)·yr] will indicate the RUB dose rates from the near-surface disposal facility through a drinking water pathway at different distances. Results indicate that the sites can be grouped into two categories: (a) sites having groundwater velocity >10 cm/day (category 1) and (b) sites having groundwater velocity <10 cm/day (category 2). The variation in the dose rates between each category of sites is found to be small. Based on this finding, a generic method has been developed to evaluate the RUB dose rates to members of the public from the near-surface radioactive waste disposal facilities as a function of distances and nuclear power capacity. It is observed that the RUB dose rates at 1, 2, and 3 km are ~0.03, 0.02, and 0.01 mSv/yr, respectively, for category 1 sites for a nuclear power capacity of 1 GW(electric). These dose rates are reduced by a factor of 2 for category 2 sites. This generic method is found useful for the screening analysis of proposed low-level radioactive waste disposal sites as it estimates the RUB effective dose rates as a function of distance and nuclear power capacity for different categories of sites.

High-resolution stratigraphic modeling of the vadose zone at the Savannah River Site low-level radioactive waste trenches disposal facility

The principal disposal facility for solid, hazardous, and mixed low-level radioactive wastes is located in the central part of the Savannah River Site. These wastes are generally disposed of in engineered, unlined trenches and excavations. The trenches vary in length and width but are typically constructed in the upper 7–8 m (21–24 ft) of the local sediments and in the vadose zone. Water from the surface must pass through the waste trenches and unsaturated vadose zone to reach the saturated water table and may contribute to groundwater contamination. The vadose zone underlying the study area extends to depths between approximately 20 and 24 m (65 and 78 ft). To comply with regulatory requirements, an investigation program was developed to characterize the vadose zone underlying two active disposal trench areas. A high-resolution vadose zone lithostratigraphic model was required before the installation and placement of experimental equipment to measure soil hydraulic properties. A detailed mathematical model of flow and transport could then use the stratigraphic model and the water flux information from the instrumentation, for example, to constrain a finite-element grid and arrive at the contaminant flow characteristics in the vadose interval. The two high-resolution stratigraphic models were developed and interpreted from field and laboratory data collected from continuous split-spoon borings, Shelby tube soil samples, piezocone penetration soundings using cone penetrometer technology (CPT), and borehole geophysical logs from nearby shallow monitoring wells. Exposed stratigraphy in open trenches and nearby cuts was also used. These models revealed (1) detailed lithostratigraphic and soil moisture conditions in the vadose zone, including three variations in depositional character roughly corresponding to previously defined formational contacts; (2) that indications that the lateral extent of many vadose zone bedforms is on the order of 20 m (66 ft) or less, therefore requiring vadose zone sampling points to be placed on spacings of less than 20 m (66 ft) for modeling; and (3) that once correlation and modeling have been performed, it is possible to use CPT data, matched to the model, to precisely place new sensors and instrument.

A study on the degree of deliberation and meditation of cyber poll respondents for nuclear-related decision making

Public participation is essential in each step of the nuclear-related decision-making process. Recently, the electronic systems using the Internet have become quite popular, and have emerged as a good medium for communicating with the public. In this study, a comprehensive utilization of electronic public participation was used to analyze public opinion on a given nuclear-related decision-making process. The degree of deliberation and meditation of the public participating in an electronic poll survey was evaluated, and the decision-making factors representing the personal characteristics of the poll respondents such as age, income, education, residence, degree of knowledge and concern were carefully incorporated when preparing the poll-survey questionnaire for its evaluation. Fuzzy analysis was used to assess and aggregate the responses to each decision-making factor. As a case study, this procedure was used to analyzing public opinion on the location of a low-level radioactive waste disposal facility on the campus of Seoul National University. The results show that there is a tendency of respondents who are on the negative side of the argument to be more deliberate and meditative in their decision-making process than those on the positive side. Knowledge and residence were found to be important decision-making factors.

Modelling of long-term diffusion–reaction in a bentonite barrier for radioactive waste confinement

Bentonites have been proposed as buffer material for barriers in geological disposal facilities for radioactive waste. This material is expected to fill up by swelling the void between the canisters containing the waste and the surrounding ground. However, the bentonite barriers may be submitted to changes of humidity, temperature variation, fluid interaction, mass transport, etc. This could modify the physico-chemical performance of the barrier, mainly on the interface with the steel container and with the geological barrier. The engineered barrier development necessitates thus the study of the physico-chemical stability of its mineral component as a function of time under the conditions of the repository in the long-term.The purpose of the present study was two-fold. Firstly, it was hoped to simulate the chemical transformations (geochemical and cation exchange reactions) coupled with diffusion of chemical-elements into the engineered barrier under repository conditions by applying a thermokinetic hydrochemical code (KIRMAT: Kinetic Reactions and Mass transport).Secondly, it was hoped to apply a simplified method to estimate the swelling capacity evolution by a volume balance in the fluid-saturated engineered barrier, considering that the decay of swelling capacity is directly proportional on the volume of transformed montmorillonite and, taking into account that it may be partially compensated by the volume of neo-formed swelling clays.The system modelled herein was considered to consist of 1-m thick zone of water-saturated engineered barrier. This non-equilibrated system was placed in contact with a geological fluid on one side, which was then allowed to diffuse into the barrier, while the other side was kept in contact with a source of metallic iron. Reducing initial conditions(PO2≅0; Eh=−200 mV) and a constant reaction temperature (100 °C) were considered.The results showed that the EB in contact with the geological fluid was highly transformed after 10,000 years, whereas the most significant chemical processes were illitization, cation exchange and saponization, extending up to 20 cm into the EB. Chemical transformations of minor importance in the EB were identified as well, such as a neo-formation of silicates (quartz, cristobalite), anhydrite, laumontite, magnetite and chlorite in the system.A simplified method based on volume balance showed that the swelling capacity of the bentonite barrier is slightly affected after 10,000 years of diffusion–reaction (D close to 1) because the volume of neo-formed swelling-clays is almost directly proportional to the volume of transformed Na/Ca-montmorillonite, except for a strong illitization and/or neo-formation of non-swelling clays. In the present study, this simple approach predicted that the decay of swelling capacity of the engineered barrier is drastically affected close to the geological barrier-engineered barrier interface. Out this zone the swelling capacity decay lies between 5% and 11%.

Application of Bayesian network to the probabilistic risk assessment of nuclear waste disposal

The scenario in a risk analysis can be defined as the propagating feature of specific initiating event which can go to a wide range of undesirable consequences. If we take various scenarios into consideration, the risk analysis becomes more complex than do without them. A lot of risk analyses have been performed to actually estimate a risk profile under both uncertain future states of hazard sources and undesirable scenarios. Unfortunately, in case of considering specific systems such as a radioactive waste disposal facility, since the behaviour of future scenarios is hardly predicted without special reasoning process, we cannot estimate their risk only with a traditional risk analysis methodology. Moreover, we believe that the sources of uncertainty at future states can be reduced pertinently by setting up dependency relationships interrelating geological, hydrological, and ecological aspects of the site with all the scenarios. It is then required current methodology of uncertainty analysis of the waste disposal facility be revisited under this belief. In order to consider the effects predicting from an evolution of environmental conditions of waste disposal facilities, this paper proposes a quantitative assessment framework integrating the inference process of Bayesian network to the traditional probabilistic risk analysis. We developed and verified an approximate probabilistic inference program for the specific Bayesian network using a bounded-variance likelihood weighting algorithm. Ultimately, specific models, including a model for uncertainty propagation of relevant parameters were developed with a comparison of variable-specific effects due to the occurrence of diverse altered evolution scenarios (AESs). After providing supporting information to get a variety of quantitative expectations about the dependency relationship between domain variables and AESs, we could connect the results of probabilistic inference from the Bayesian network with the consequence evaluation model addressed. We got a number of practical results to improve current knowledge base for the prioritization of future risk-dominant variables in an actual site.

Application of biosphere models in the BIOMOSA project: A comparative assessment of five European radioactive waste disposal sites

The BioMoSA (Biosphere Models for Safety Assessment of Radioactive Waste Disposal) project was part of the EC fifth framework research program. The main goal of this project was the improvement of the scientific basis for the application of biosphere models in the framework of long-term safety studies of radioactive waste disposal facilities. Another aim of the project was to provide operators and regulatory bodies with guidelines for performance assessments of repository systems. The study focused on the development and application of site-specific models and a generic biosphere tool BIOGEM (BIOsphere GEneric Model), using the experience from National programs and the IAEA BIOMASS reference biosphere methodology. The models were applied to 5 typical locations in Europe, resulting in estimates of the annual individual doses to the critical groups and the ranking of the importance of the pathways for each of the sites. The results of the site-specific and generic models were then compared. Uncertainty in the results was estimated by means of stochastic calculations which allowed a comparison of the overall model uncertainty with the variability across the different sites considered.

Decommissioning strategy selection

Abstract At the end of their useful life nuclear facilities have to be decommissioned. The strategy selection on how to decommission a facility is a highly important decision at the very beginning of decommissioning planning. Basically, a facility may be subject to (a) immediate dismantling; (b) deferred dismantling after a period of “safe enclosure” or (c) entombment where a facility is turned into a near surface disposal facility. The first two strategies are normally applied. The third one may be accepted in countries without significant nuclear activities and hence without disposal facilities for radioactive waste. A large number of factors has to be taken into account when a decision on the decommissioning strategy is being made. Many of the factors cannot be quantified. They may be qualitative or subject to public opinion which may change with time. At present, a trend can be observed towards immediate dismantling of nuclear facilities, mainly because it is associated with less uncertainty, less local impact, a better public acceptance, and the availability of operational expertise and know how. A detailed evaluation of the various factors relevant to strategy selection and a few examples showing the situation regarding decommissioning strategy in a number of selected countries are presented in the following article.

Investigating Pu and U isotopic compositions in sediments: a case study in Lake Obuchi, Rokkasho Village, Japan using sector-field ICP-MS and ICP-QMS

The objectives of the present work were to study isotope ratios and the inventory of plutonium and uranium isotope compositions in sediments from Lake Obuchi, which is in the vicinity of several nuclear fuel facilities in Rokkasho, Japan. Pu and its isotopes were determined using sector-field ICP-MS and U and its isotopes were determined with ICP-QMS after separation and purification with a combination of ion-exchange and extraction chromatography. The observed (240)Pu/(239)Pu atom ratio (0.186 +/- 0.016) was similar to that of global fallout, indicating that the possible early tropospheric fallout Pu did not deliver Pu from the Pacific Proving Ground to areas above 40 degrees N. The previously reported higher Pu inventory in the deep water area of Lake Obuchi could be attributed to the lateral transportation of Pu deposited in the shallow area which resulted from the migration of deposited global fallout Pu from the land into the lake by river runoff and from the Pacific Ocean by tide movement and sea water scavenging, as well as from direct soil input by winds. The (235)U/(238)U atom ratios ranged from 0.00723 to 0.00732, indicating the natural origin of U in the sediments. The average (234)U/(238)U activity ratio of 1.11 in a sediment core indicated a significant sea water U contribution. No evidence was found for the release of U containing wastes from the nearby nuclear facilities. These results will serve as a reference baseline on the levels of Pu and U in the studied site so that any further contamination from the spent nuclear fuel reprocessing plants, the radioactive waste disposal and storage facilities, and the uranium enrichment plant can be identified, and the impact of future release can be rapidly assessed.

Sorption and transport behavior of radionuclides in the proposed low-level radioactive waste disposal facility at the Hanford site, Washington

AbstractA series of batch sorption and column experiments was conducted to investigate sorption and transport behavior of99Tc,129I,79Se, and90Sr on and through borehole sediments collected from the proposed low-level radioactive waste disposal facility at the Hanford Site (200 East Area). Batch sorption experiments were conducted on Hanford sediment using uncontaminated Hanford groundwater and simulated glass leachates spiked with individual radionuclides. Strongest sorption occurred for90Sr, while79Se sorption was intermediate, and129I and99Tc showed the least sorption affinities on Hanford sediment among these radionuclides studied. The results of column experiments that measured transport behavior of these radionuclides through Hanford sediment were similar to the mobility that can be calculated from the batch sorption results, that is high mobility for99Tc and129I compared to the intermediate and strong retardation for79Se and90Sr, respectively. These contaminant sorption data on sediments from below the proposed disposal facility, especially the tests using simulated glass leachate, corroborate values obtained for sediments collected in the past from near by locations and for generic solutions such as regional groundwater. These new data should provide more technical defensibility for past performance assessment predictions that did not use site-specific sediments and leachates. Further, the new data will be incorporated into future performance assessment activities that will update and improve past predictions.

A simplified method to evaluate the swelling capacity evolution of a bentonite barrier related to geochemical transformations

Bentonites have been proposed as buffer material for barriers in geological disposal facilities for radioactive waste. This material is expected to fill up by swelling the void between the canisters containing the waste and the surrounding ground. However, the bentonite barriers may be submitted to changes of humidity, temperature variation, fluid interaction, mass transport, etc. This could modify the physico-chemical performance of the barrier, mainly on the interface with the steel container and with the geological barrier. The engineered barrier development necessitates thus the study of the physico-chemical stability of its mineral component as a function of time under the conditions of the repository in the long-term. The aim of this paper is to apply a simplified method (volume balance in a saturated medium) to evaluate the swelling capacity evolution of a bentonite barrier because of their geochemical transformations by using a thermo-kinetic hydrochemical code (KIRMAT: Kinetic Reactions and Mass Transport “1D”). The system modelled here consists of 1 m thick zone of water-saturated engineered barrier. This non-equilibrated system is placed in contact with a geological fluid on one side, which is then allowed to diffuse into the barrier, while the other side is kept in contact with iron-charged solution (0.001 mol/kg H 2O). The initial reducing conditions ( P O 2 ≅ 0 ; Eh = −200 mV) and a constant reaction temperature (100 °C) were considered. In the current study the decay of swelling capacity was considered directly proportional on the volume of transformed montmorillonite (cation exchange + geochemical transformation), taking into account that it may be partially compensated by the volume of neo-formed swelling-clays. The results showed that the swelling capacity of the engineered barrier is not drastically affected after about 3000 years of reaction and transport because the volume of neo-formed swelling clays is almost directly proportional on the volume of transformed montmorillonite. In fact, a graphical method predicted that the decay of swelling capacity of the engineered barrier lies between 11% and 14% when the montmorillonite is completely transformed (cation exchange + minimal geochemical transformation) in the system.

Change in pore structure and composition of hardened cement paste during the process of dissolution

An understanding about the dissolution phenomena of cement hydrates is important to assess changes in the long-term performance of radioactive waste disposal facilities. To investigate the alteration associated with dissolution, dissolution tests of ordinary Portland cement (OPC) hydrates were performed. Through observation of the samples after leaching, it was confirmed that ettringite precipitation increased as the dissolution of the portlandite and the C-S-H gel progressed. EPMA performed on cross-sections of the solid phase showed a clear difference between the altered and unaltered parts. The boundary between the two parts was termed the portlandite (CH) dissolution front. As the leaching period became longer, the CH dissolution front shifted toward the inner part of the sample. A linear relationship was derived by plotting the distance moved by the CH dissolution front against the square root of the leaching time. This indicated Ca ion movement by diffusion.