For various countries, the direct disposal of high level nuclear fuel wastes is a key option for the backend of the fuel cycle. For HTR/VHTR reactors this is assumed for the introductory phase of this reactor system. However, closed fuel cycles or a separation of spent coated-particles from the graphite moderator and specific treatment, conditioning and disposal of these waste streams are also possible. In the European Community project “RAPHAEL”, fuel waste performance is going to be studied in depth, including post-irradiation fuel characterization, analysis of the stability and failure mechanism of coatings and of fuel kernels and overall performance of waste packages with compact fuel and/or only with fuel particles in geological disposal environments. Different confinement matrices for separated fuel particles (vitrification, SiC, ZrO2) have been adapted to limit release of radionuclides into groundwater at low temperatures over geological time spans. The investigations are limited to Low-Enriched Uranium (LEU) fuel with uranium oxide and uranium oxycarbide kernels that will allow higher burn-up, but may be more susceptible to leaching.

The hydraulic conductivity of the Boom Clay is investigated since many years in Belgium in the framework of the site characterisation programme supporting the performance assessment of potential geological disposal of high-level radioactive waste in this formation. In this paper, the variability of the hydraulic conductivity is assessed in a large part (1100km2) of the deposition area of the Boom Clay. It is based on numerous vertical and horizontal hydraulic conductivity measurements (Kv and Kh) performed with permeameter-type experiments on clay cores from four distant boreholes, i.e. Doel-2b, Zoersel, Mol-1 and Weelde-1. A thorough analysis of the results of the grain-size measurements is performed and its relationship with the hydraulic conductivities is examined through multiple regressions. This approach allows deriving site-specific empirical relationships to predict vertical hydraulic conductivities from the grain-sizes used as soft data. This increases our confidence in the global value of the vertical hydraulic conductivity at the scale of the formation and enhances the overall assessment of the variability of this parameter at the regional scale. Further efforts should be done to evaluate the variability of the hydraulic conductivity with relationships that are suitable for the whole depositional area. This would make the method more attractive for the overall assessment of a potential host-formation for geological disposal.

It is the objective of the EU-funded 'Red-Impact' project to analyse the impact of partitioning, transmutation and waste reduction technologies on the final nuclear waste disposal. The partnership of 25 organisations is originating from European nuclear industry, waste agencies, research centres and universities. The system studies focus on a realistic evolution of P&T technologies and advanced fuel cycles which can be deployed incrementally on an scale as well as on future developments such as reactors of the third and fourth generation (Gen III & Gen IV) and Accelerator Driven Systems (ADS). A comprehensive inventory of all existing and foreseen nuclear fuel cycle facilities in Europe has been performed including a review on worldwide ongoing R&D programs on P&T. Thus, it was possible to select a set of three so-called industrial scenarios, taking into account feasibility of alternate strategies leading to increased actiniae burning and reduced actinide generation based on direct disposal (reference case) or MOXfuel for LWR and plutonium recycle in Sodium Fast Reactors (SFR). R&D needs for the development of processes and technologies have also been addressed. In addition, three 'innovative scenarios ' have been identified allowing multi-recycling of plutonium and minor actinides in SFR and Accelerator-Driven Systems (ADS) as well as GANEX or COEXprocess and PYRO reprocessing technologies. Waste streams have been calculated for all of these scenarios including the transition from the present situation towards new fuel cycle options. These data provide the input to specific analyses on the impact on geological disposal in different host formations such as granite, clay and salt. The results show that advanced fuel cycles influence the required size of the geological repository in case of disposal in clay, salt or hard rock formations. Recycling of all the actinides results in a reduction of the necessary gallery length (depending on geology and design) at least by a factor 3. If additionally cesium and strontium are extracted from the high-level waste for separate decay, the reduction factor will become 10 or more. In the frame of the project, the feasibility and the impact of the Cs or Sr separated management were not assessed or evaluated. Transmutation of the actinides fast neutron spectrum reactors (FR or ADS) results in a limited reduction of the maximum dose because the dose is essentially due to long-lived fission and activation products. On the other hand, reprocessing the spent fuel decreases the maximum dose at the storage with a factor 5 because a considerable fraction of the iodine is separated from the high level waste during reprocessing. The radiotoxicity in the high level waste or spent fuel as well as human intrusion doses after 500 years are drastically reduced by the transmutation of the actinides. Evaluating actinide minimization systems and industrialised P&T in general requires an assessment of relevant nuclear fuel cycles especially with regard to the economic, environmental and societal advantages/disadvantages (i.e. the sustainability of the fuel cycles). Thus, a set of indicators has been derived for each of these areas. The results are analysed using the multi-criterion analysis approach which allows the importance of each of the indicators to be specified.

The European Commission project Red Impact is in a state of conclusion after three years. Within the project, the consequences of P&T on, mainly, geological disposal have been investigated. Six ...

In Europe, formations become more and more important as candidate geological formations (e.g. Boom Clay, Callovo-Oxfordian, Opalinus...) for the deep disposal of High-Level radioactive Waste (HLW). In demonstrating the suitability of a geological site for the disposal of radioactive waste, it is essential to consider the potential mobility of critical radionuclides through the relevant types. This project addresses the migration behaviour of radionuclides, identified as important for the long term safety (U, Se, Pu, Am), in a reducing environment, with special emphasis on the role of the Natural Organic Matter (NOM). In such reducing environments, the solubility limit is considered as the most important mechanism to lower the mobile concentration of these radionuclides and the speciation is most likely neutral or negatively charged so a low retardation is expected. However, the presence of NOM may jeopardise the expected low concentration and sorption: by solubility enhancement due to complexation/colloid formation with NOM or by influencing the sorption behaviour. The objective is to develop and demonstrate a conceptual model for the description of the migration of radionuclides in a reducing, NOM rich environment that can be implemented in performance assessment (PA) models. The main questions are: (i) does the NOM increase the radionuclide concentration in solution due to complex formation and (ii) what is the possible role of mobile NOM as radionuclide carrier? To answer these questions, following research strategy was adopted taking the Boom Clay Formation as a Case Study. Prior to any experimental work, speciation and solubility's for the considered radionuclides were calculated not accounting for NOM. The mechanisms determining the overall behaviour of the radionuclides in the environment are then studied in solution in presence of NOM (solubility, complexation) and with respect to the solid phase (retention by immobilisation, sorption). Modelling is used to interpret and derive interaction constants which can be implemented in a geochemical clay database. Understanding and quantifying the interaction mechanisms in the environment should allow to model and interpret migration experiments with mixtures of the radionuclides and 1 4 C-labelled NOM. Separately, methodologies were developed to identify the source of mobile organic matter. The obtained results are translated into conceptual models which can be used to evaluate the performance of Boom Clay as potential host rock Batch experiments revealed that the solubility of amorphous UO 2 is ∼10 - 8 mol dm - 3 with no effect of NOM complexation. However, it was evidenced that the presence of NOM facilitated the formation of uranium colloids upon dissolution of UO 2 . The formation of colloids, with molecular size between 2 nm and 0.45 μm, accounts for a total uranium concentration, three orders of magnitude higher than the solubility of the amorphous UO 2 . Uranium colloids were also found being dominant in leaching experiments of natural uranium from Boom Clay samples. These colloids, only evidenced in batch experiments, are unlikely to be mobile in the compact natural Boom Clay. Electromigration experiments showed that U(VI) reduces to U(IV) and precipitates. This U(IV) precipitated phase constantly releases positive or neutral charged U-species. Furthermore, classical migration experiments with U(IV) mixed with 1 4 C-labelled NOM showed that U(IV) migrates independently of the NOM through the Boom Clay. The migration of uranium in the Boom Clay is governed by strong retention due to precipitation (solubility limit) and sorption and is not enhanced by the mobile organic matter.

In 2001, ONDRAF/NIRAS completed a second Safety Assessment and Interim Report, SAFIR 2, on the Belgian research and development programme on geological disposal of radioactive wastes. This report presents the state of the art in Belgium about deep disposal of high and medium-level wastes in a clay formation. In SAFIR 2, the focus was very much on the capacity of the Boom Clay to fulfil its role as main barrier. Indeed, in many evolution scenarios it has been shown that the clay host formation plays the dominant role in performance assessment (PA) calculations. Nevertheless, the basic assumptions on which these PA calculations rest often depend much on processes which occur inside or in the vicinity of the engineered barriers system (EBS). In this respect, detailed EBS modelling appears complementary to PA calculations. To ensure the sound transfer of EBS behaviour understanding, a tight collaboration between experimentalists, modellers and PA specialists is required.

To assess the overall safety of a geological repository by means of numerical radionuclide release calculations, specific magnitudes are needed which follow from the calculation and can be compared with reference values. Such magnitudes are often called safety indicators. The most common safety indicator used so far is the effective dose rate. The uncertainty of dose rate calculations, however, increases with the time under consideration, and therefore it seems desirable to have additional indicators to improve the reliability of performance assessments. As long as not the overall safety but the performance of individual barriers is assessed, or the functioning of the system is to be showed, another type of indicators can be used, these are sometimes called performance indicators. The SPIN project identified and tested seven safety and fourteen performance indicators. Safety indicators have been mainly identified by evaluating the open literature, performance indicators through systematic approaches. The indicators have been tested by re-calculating existing performance assessments of disposal systems for high level waste in crystalline formations in Spain, Germany, Finland and Switzerland. The results have been compared and assessed in view of the general applicability of the specific indicators. Although other geological formations than granite have not been tested, the conclusions of the project might be more generally applicable. The effective dose rate is taken as the basic safety indicator. Two other indicators were found to provide significant benefits and may therefore be used to complement the effective dose rate. The three proposed safety indicators and their preferred application to time frames are: - Elective dose rate: most relevant to early time frames - Radiotoxicity concentration in biosphere water: preference for medium time frames - Radiotoxicity flux from geosphere: preference for late time frames For the effective dose rate the data from present regulations were used as a range of reference values. Reference values for radiotoxicity concentration and fluxes were taken from nature, based on the assumption that nature in general is radiologically safe. Widely reported concentrations and fluxes corresponding to crystalline sites were used as reference values. The project concluded that several performance indicators can be used to show different aspects of the functioning of the individual compartments of the multi-barrier system. These indicators and their preferred applications are: - Inventories in compartments: showing where the radionuclides are at different points in time, and the retention of radionuclides from the biosphere - Fluxes from compartments: showing the decreasing release rates from successive compartments, including radioactive decay and ingrowth, and the delayed release - Time-integrated fluxes from compartments: showing decay during delayed transport - Concentrations in compartment water: showing the decrease of concentration by dilution, dispersion and decay in successive compartments - Transport times through compartments: showing the potential importance of individual radionuclides to the release of radiotoxicity by comparing them to their half-lives For investigations relating to the total radionuclide spectrum, performance indicators based on radiotoxicities should be used.

The thermochemical databases of the NEA (TDB) are used in support of the determination of solubility values for performance assessment (PA). PA uses solubility to define the concentration limit at which radionuclides migrate from the near-field to the far-field of the repository. Being different from a thermodynamically defined solubility, a concentration limit can be equal to the solubility of pure phases but can also be imposed by other processes, e.g. formation of colloids or solid solutions. This paper illustrates a way that the TDB is utilised to interpret some migration experiments through geochemical and solute transport modelling. As a result, some meaningful migration parameters are derived based on both experimental observations and the application of the TDB.

Deep disposal is considered a safe solution to the management of high-level radioactive waste. The safety is usually demonstrated by means of a performance assessment. This paper discusses the methodological aspects and some of the results obtained for the performance assessment of the disposal of vitrified high-level waste in a clay layer in Belgium. The calculations consider radionuclide migration through the following multi-barrier components, all of which contribute to the overall safety: (1) engineered barriers and the host clay layer, (2) overlying aquifer, and (3) biosphere. The interfaces between aquifers and biosphere are limited to the well and river pathway. Results of the performance assessment calculations are given in terms of the time evolution of the dose rates of the most important fission and activation products and actinides. The role of the glass matrix in the overall performance of the repository is also discussed.

During the development of the International Thermonuclear Experimental Reactor (ITER) fusion reactor, many efforts are done to minimise the amount of radioactive waste that will arise from the decommissioning of a nuclear fusion power plant. Nevertheless, the most active waste types will have to be disposed in a repository. The impact of possible future human actions on the performance of the repository has to be evaluated. The most active waste types are selected from the inventory of fusion plant model PM-2 with low-activation martensitic steel as main material. After a cooling period of 100 years, the fusion waste is assumed to be disposed off in a repository located in the boom clay layer at the Mol site (Belgium). A systematic approach for the identification of the relevant intrusion scenarios is applied. Three scenarios resulting from borehole drilling are identified as relevant, core inspection, residence and unsealed borehole. This paper will focus on the analysis of the most drastic human intrusion scenario, i.e. the core inspection scenario. The maximum dose is calculated for waste arising from Be-coatings. In the case of a routine inspection, the dose to a geological worker is always under 0.5 Sv. This value, under which serious deterministic health effects are unlikely, can be considered as a reference level for acute exposure. In the case of the very pessimistic close inspection variant a maximum dose of 0.4 Sv is calculated if the intrusion occurs immediately after the disposal. After 60 000 years, the dose becomes lower than 3.5 mSv, which is the average annual background dose. The results of an alternative approach, considering a fusion-specific repository and wells drilling as the main intrusion scenario, are also included in this paper. Finally, a comparison of the consequences due to intrusion scenarios calculated for fusion and fission waste is presented. For the most active fission waste types it lasts 1 million years before the close inspection dose drops under 0.5 Sv. (C) 2001 Elsevier Science B.V. All rights reserved.

Anisotropy of the migration properties is demonstrated for a sedimentary soft clay formation (Boom Clay). The measured values for the hydraulic conductivity and for the apparent diffusion constant in the direction parallel with the stratification of sedimentation are a factor of two higher than the values perpendicular to the stratification.The influence of the anisotropy due to the stratification of the clay formation is not negligible for the calculation of the migration of radionuclides and hence also included in the performance assessment modelling.

Waste and clay characterization in the context of radioactive waste disposal extends over 25 years. For high-level and long-lived waste, no siting procedure has yet been launched in Belgium. The studies on geological disposal are referring to a potential host rock, the Tertiary Boom Clay Formation. At SCKCEN, special attention has been devoted to the radioactive waste characterization, the migration studies in clay, and the geomechanical behavior of this potential host rock. Performance studies help in assessing the level of confidence that must be achieved at the various stages of the characterization in both near and far fields. The waste characterization issue rapidly extended towards the study of compatibility of these waste forms in the near field and geological environment, with a view to define acceptance criteria. Present activities include a research and development programme on spent fuel that is becoming, in parallel with ongoing studies on glass, an important issue in the frame of alternative fuel cycle policies in Belgium. Our capabilities in research on migration are being extended on both experimental and modelling fields by developing electromigration techniques and geochemical coupled transport codes respectively. Other components of the near field, in particular the backfill and sealing materials, remain the subject of extensive in-situ and laboratory testing. Regarding geomechanics, the feasibility of digging large disposal galleries in a deep, plastic clay has been demonstrated at the Mol site. Attention is now devoted to the quantification of the disturbance affecting the host rock during the construction and operation of a repository. These developments are currently covered by the activities of the Economic Interest Grouping (EIG) PRACLAY, aimed at managing the extension of the HADES Underground Research Facility (URF) and the corresponding experimental programme. More efforts in the frame of performance assessments were devoted to a systematic scenario selection and analysis. In complement to the deterministic consequence analysis of the identified scenarios, stochastic calculations are carried out for uncertainty and sensitivity analyses. To build confidence in the assessment, a natural analogue study on the potential host clay formation and a palaeohydrogeological study of the aquifer system are carried out.

A regional hydrogeological model, developed in the context of the performance assessment of geological radioactive waste disposal, has been adapted to take the climatic changes into account. Transport of natural isotopes is simulated from -125 000 years to the present day by considering a sequence of typical climate periods, and concentration results are compared with measurements. The radionuclide transport from a repository system are then simulated similarly for the next 125 000 years based on the expected climatic evolution. The simulation helps to estimate the range of effects of the possible climate changes on radiological consequences.

The main objective of the PHYMOL project (a palaeo-hydrogeological study of the Mot site, Belgium) is to develop a methodology that can be applied in performance assessments of an argillaceous repository system to simulate the ground water flow and transport of radionuclides for the present conditions as well as for conditions corresponding to the expected climatological evolution.The project consists of three work packages:Geochemistry: ground water samples, and possibly also sediments, are collected at SCK.CEN's underground research laboratory and from the regional piezometric network. Isotope and noble gas analyses are carried out on the samples, because these analyses give indications on the recharge conditions and the ground water residence times. Geochemical analyses will provide complementary information on the ground water flow and possible connections between aquifer layers. The measured residence times and distributions of the geochemical concentrations will be examined and interpreted to find indications on the ground water flow at the Mol site during the Weichselian glaciation.Modelling: a supra-regional aquifer model, which extends from southern Sweden to northern Belgium, is applied to simulate the ground water flow in the North Sea basin under the various climatological conditions of a glacial cycle. The results of the supra-regional model will be used as boundary conditions for the regional model. Regional and local hydrogeological modelling will be carried out for present and past conditions to explain and reconstruct the observed residence times and geochemical concentrations in the Boom Clay layer and in the surrounding aquifers.Development of a methodology applicable to performance assessments: conclusions will be drawn on the main effects of the different climatological conditions on the hydrogeological system of the Mol site and on their impact on the performance of an argillaceous repository system.

Single- or multi-layered surface barriers are used for protecting the underlying hazardous or radioactive waste storage facility and for reducing the risk of dispersion of contaminants to the ground water. To assess the effect of variations in net rainfall rate (total rainfall minus evapotranspiration) on the hydraulic response of a multi-layered barrier, a series of flow simulations were performed. Water fluxes in different layers of the barrier were numerically calculated for three net rainfall rates, i.e. 0.0255, 0.0742, and 0.141 cm/day. These rates correspond to dry, average, and wet climatological conditions in Belgium. Results show that for dry conditions, almost 45% of the water infiltrating the top of the barrier is laterally evacuated. When the net rainfall rate increases from 0.0255 to 0.0742 cm/day, more water is being laterally diverted, and only 22% of the applied rainfall percolates through the barrier. If the rainfall rate is increased from 0.0255 to 0.141 cm/day, the percolation becomes 14% of the applied rainfall. This indicates that fluxes leaving the bottom of the repository increase only slightly, although the rainfall rate for wet conditions is approximately 5 times larger than for dry conditions. Under dry conditions a slightly negative pressure develops in the bottom of the low-permeability clay layer. However, this desaturation does not affect the integrity of the clay.

Liquid wastes, resulting from the reprocessing of spent nuclear fuel from the BR-2 Materials Testing Reactor, will be conditioned in a cement matrix at the dedicated cementation facility of UKAEA at Dounreay. In Belgium, the Boom clay formation is studied as a potential host rock for the final geological disposal of cemented research reactor waste. In view of evaluating the safety of disposal, laboratory leach experiments and in situ tests have been performed. Leach experiments in synthetic claywater indicate that the leach rates of calcium and silicium are relatively low compared to those of sodium and potassium. In situ experiments on inactive samples are performed in order to obtain information on the microchemical and mineralogical changes of the cemented waste in contact with the Boom clay. Finally, results from a preliminary performance assessment calculation suggest a non-negligible maximum dose rate of 5 10(-9) Sv/a for I-129.