Studies were carried out on the effect of redox conditions on the release from simulated vitrified HLW of Np-237, Pu-238 and Tc-99 and their migration through columns packed with site-specific porous material representative of clay and salt aquifers. Results obtained under oxic and anoxic conditions are compared. The release rates of Np, Pu and Tc were found to be lower in the absence of air than with oxygenated groundwaters. Speciation studies showed that neptunium and technetium were released in the oxidized forms Np(V) and Tc(VII) even under the investigated anoxic conditions where reduced species are thermodynamically more stable. In spite of that, a stronger interaction was observed when the leachate was passed through the column in the absence of air. A surface reduction process accounts for the observed behaviour. Under more reducing conditions, plutonium appears to be present in the tetravalent state in colloidal form.

The UO2(s)-La(OH)3(s) coprecipitation equilibria have been studied in 0.5 M NaClO4 medium, at 25°C under anoxic conditions. The data obtained in the pH range between 2 and 5, and at different initial La(III)/U(IV) ratios (10%,5%,3%,2% and 1%), is explained by applying the Doerner-Hoskins logarithmic law. The same law can also be used to describe previously collected UO2(s)-Pu(OH)3(s) coprecipitation data. A method is proposed which uses the logarithmic distribution constants and the thermodynamic quantities for the pure phases to describe the solubility of radionuclides under repository conditions.

The sorption of americium on colloidal quartz and montmorillonite has been studied using a batch technique. Americium appeared to be distributed between all available surfaces in the system (particles and vessel walls). Distribution coefficients, defined as the ratio of the amount of americium per mass of colloidal matter and the concentration in the solution phase, were estimated to be of the same order of magnitude as obtained in measurements on crushed material of much larger particle sizes.

In the presence of sorbents (alumina or granite) the removal of americium from the solution was enhanced, either due to the desorption of the americium from the particle phase and resorption on all available surfaces (sorbent plus vessel wall) or the sorption of the whole colloidal aggregates on the sorbent.

No break-through of americium was observed in column experiments (with alumina or granite packing) after the injection of americium pseudocolloids (quartz or montmorillonite) followed by elution with large volumes of aqueous phase (up to 880 column volumes). However, a minor fraction of the americium passed through the column without significant retention, especially under conditions with high flow rate (short hold-up time in the column).

The population balance methodology is described and applied to the transport and capture of polydispersed colloids in packed columns. The transient model includes particle growth, capture, convective transport, and dispersion. We also follow the dynamic accumulation of captured colloids on the solids. The multidimensional parabolic partial differential equation was solved by a recently enhanced method of characteristics technique. This computational technique minimized numerical dispersion and is computationally very fast. The FORTRAN 77 code ran on a VAX-780 in less than a minute and also runs on an IBM-AT using the Professional FORTRAN compiler. The code was extensively tested against various simplified cases and against analytical models.

The packed column experiments by Saltelli et al. were re-analyzed incorporating the experimentally reported size distribution of the colloid feed material. Colloid capture was modeled using a linear size dependent filtration function. The effects of a colloid size dependent filtration factor and various initial colloid size distributions on colloid migration and capture were investigated. Also, we followed the changing colloid size distribution as a function of position in the column.

Some simple arguments are made to assess the likelihood of colloid migration at a potential NTS Yucca Mountain waste disposal site.

A method based on autoradiography was developed to determine the sorption and diffusion of cesium, strontium, cobalt, nickel, iodine and americium into rock matrix. Samples chosen for this study were filled and unfilled natural fracture surfaces and drill cores having a central drilled hole (”). Rock types were mica gneiss, tonalite and rapakivi granite, which were selected to represent the common rocks and minerals in Finnish bedrock. Distribution coefficients (Ka-values) of cesium and strontium determined for fissure surfaces and drill core cups were of the same order of magnitude. After three months contact time the greatest penetration depth for cesium was 2.5 mm, for a natural fissure surface sample of rapakivi granite. For strontium the penetration depths of 11 mm in three months and 35 mm in twelve months were found for filled natural fissure surface samples of rapakivi granite. The range of Da-values was 1.5 × 10−15 - 3.2 × 10−14 m2/s and 1.4 × 10−14 - 2.1 × 10−13 m2/s for cesium and strontium, resp. For cobalt the Da-values of 5 × 1016 m2/s in tonalite was obtained. In six months the penetration depths of nickel, iodine and americium were too low (< 0.5 mm) to allow calculation of Da.

Three circulation experiments have been carried out between two drill-holes in a granite after hydraulic fracturation (Le Mayet de Montagne, France). We have injected together uranyl carbonate and Th, La, Sm, Eu, Dy and Yb EDTA complexes. Amounts of tracers were different in each run. Recovery curves are similar for Th and lanthanides, but have a delayed peak and an increased tailing for uranium. For rare earth elements, recovery rates increase with increasing stability of the complex used as tracer. Recovery rates increase also with increasing amount of tracer. Those features are explained by non linear and non reversible sorption. The importance of complexation is demonstrated. Strong organic complexing agents enhance trace elements mobility.

Adsorption coefficient and penetration behavior of U(VI) in Itado tuff were measured. The migration behavior of U(VI) in the tuff was discussed.

Adsorption coefficient of U(VI) was measured by a batch method using a wafer specimen and a powder specimen in pH region from 2 to 7. Adsorption coefficient was 17000 m1/g at the pH value of 5.5.

Penetration behavior of U2− (VI) in Itado tuff was studied as a function of pH, concentration of CO32− and temperature by contacting the rock specimen with U(VI) solution. The concentration profile and fractional amount of U(VI) penetrating into the rock specimen Mt/M were measured. The penetration behavior of U(VI) was seen to be diffusion accompanied by adsorption/desorption process. In the pH region from 3 to 4, it is likely that the penetration process of U(VI) is not sufficiently reversible but is competitive process consisting of diffusion and adsorption. In the pH region above 6, penetration depth was very thin because of both large adsorption coefficient and large adsorption rate constant.

The sectioning method was found to have an advantage to see the penetration behavior directly. But in the pH region from 6 to 7, the method has a shortage that diffusion coefficient, and adsorption and desorption rate constant could not be determined, because of small penetration depth of U(VI). From this point of view, the measurement of the fractional amount of penetration as a function of time should be carried out as well as that of concentration profile.

Effective diffusion coefficients for selected radionuclides have been measured in ocean floor sediments to provide data for the assessment of barrier effectiveness in subseabed repositories for nuclear waste. The sediments tested include illite-rich and smectite-rich red clays from the mid plate gyre region of the Pacific Ocean, reducing sediment from the continental shelf of the northwest coast of North America, and Atlantic Ocean sediments from the Southern Nares Abyssal Plain and the Great Meteor East region. Results show extremely small effective diffusion coefficients with values less than 10−14 m2s−1 for plutonium, americium, curium, thorium, and tin. Radionuclides with high diffusion coefficients of approximately 10−10 m2s− include the anionic species pertechnetate, TcO4−, iodide, I−, and selenite, SO3−2. Uranyl(VI) and neptunyl(V) ions which are stable in solution, have diffusion coefficients around 10−12m2s−1. The diffusion behavior of most radionuclides is similar in the oxygenated Pacific sediments and in the anoxic sediments from the Atlantic. An exception is neptunium, which is immobilized by Great Meteor East sedi-ment, but has high mobility in Southern Nares Abyssal Plain sediment. Under stagnant conditions a 30 m thick sediment layer forms an effective geologic barrier isolating radionuclides in a subseabed repository from the biosphere.

Analyses of diffusional release of several typical radionuclides in spent fuel from waste packages emplaced in a repository in basalt were conducted to assess the effects of system characteristics and boundary conditions on computed release rates. Radionuclide releases, including spatial and temporal variations that may be present, represent the source term for transport in the geohydrologic setting and are therefore critical to the assessment of repository acceptability.

Two mathematical approaches were utilized to determine radionuclide release rate versus time characteristics; (1) an analytical solution for one-deimensional diffusion based upon a Dirichlet (constant-concentration) boundary at the waste form surface; and (2) a finite-element numerical solution based upon a Neumann (zero-flux boundary at the waste form surface. The latter method is suitable for radionuclides such as 129I, whose total inventory in spent fuel could be quickly depleted from the waste form and dissolved in the pore spaces of the packing material surrounding the waste form and which, therefore, cannot be adequately represented by a constant concentration at the waste form (i.e., container) surface.

The analysis revealed several system characteristics that are not intuitively obvious. For example, strong sorption in the near-field host rock behaves like a strong mass sink and can yield calculated transient release rates exceeding allowable limits. Similarly, a short half-life effectively removes the radionuclide from the host rock, which induces a steep concentration gradient at the host rock/packing interface and thereby increases the diffusional release rate at that boundary.

Typical results for 79Se and 129I are presented to illustrate these effects. The effects of perturbations to key assumptions are shown to indicate the importance of (1) formulating models that accurately represent the physical system and (2) interpreting analytical results carefully to ensure understanding of the capability of the system.

The construction of a H.L.W. repository in a deep granitic formation creates mechanical disturbances in the rock on the scale of the massif and in the nearfield. Amongst all the disturbances noted in the nearfield, this study is concerned with examining the evolution of stresses linked with the excavation of the rock and the rise in temperature in the proximity of the waste packages. Several linear elasticity calculations were made using on the one hand finite element models and on the other simple analytical models. These calculations concern two different storage concepts -“in room” concept and “in floor” concept- whose differences in mechanical behaviour are analysed. A study of sensitivity with regard to the characteristics of the rock and to the initial geostatic stresses is presented. The comparison of the calculated stresses with three-dimensional failure criteria gives a clear indication of the satisfactory behaviour of granite for final storage. However, the need for experimental study and complementary calculation must be emphasised.

The thermal fracturing of rock has been the object of several research projects, notably for initial rock breakage in mining [4] as well as crushing [6] In addition, the process has been studied carefully in regards to the storage of radioactive waste underground where rock fracturing could lead to a loss of radioactivity confinement. The Stripa Project, a project concerning large scale testing of procedures for underground storage of nuclear waste, probably has dealt most thoroughly with this subject by theoretical studies and in-situ heater testing in an attempt to describe the thermal failure process in rock [13]

This project was designed to test the agreement between theoretical and actual rock fracture times of a rock block, loaded with a physical as well as a thermal load. Laboratory testing consisted of physically loading center-drilled cubes of rock, 0.3 m on a side, uniaxially from 0 to 25 MPa. These were then thermally loaded with a nominal 3.7 kW (factory rating) cylindrical heater until failure occurred. This time to failure was recorded for comparison with a direct mathematical and a finite element solution. For both cases, calculations were performed at specific time-steps and an estimated failure time calculated from the compiled results.

With the program system FAST-STEP the temporal step-by-step disposal procedure of a HLW repository and its finite geometry can be taken into consideration in the calculation of the time and space dependent temperature distribution. This will be obtained through the application of a suitable coupling of calculation models treating both far field and near field phenomena.

After the description of the single steps of the program system the influence of both aspects on the temperature distribution will be demonstrated with the aid of a test calculation. The possible fields of application of FAST-STFP will be pointed out at the end.

MELODIE is a computer code developed at the CEA/IPSN for risk assessment of nuclear waste repositories in geological formations. The interactive evolution of the source, geosphere and biosphere is studied for periods of time longer than 100 000 years. In its first version, the code can describe a repository in granite rock located at a specific site.

The code is built in a modular form which allows to use different versions of the sub-system models.

The basic model for radionuclide migration and hydrpgeology is a subroutine version of the METIS code developed by ENSMP. METIS is a 2D finite element code which solves the advection-dispersion equation for porous media with explicit fracture representation. Linear adsorption kinetics is included as well as matrix diffusion and radionuclide decay chains.

The source model developed at CEA/DRDD**** is derived from CONDIMENT which is a 1D finite difference code describing the behaviour of high level waste packages. Four axisymetric layers are individualized: glass matrix, container, bentonite and granite. The glass leaching is modelled as a dissolution and diffusion process of the individual chemical components.

The biosphere model ABRICOT developed at the CEA/DPT** is based on a detailed description of agricultural activities defined in individual systems.

MELODIE is tested by participation to international exercices such as Pagis [1], Intracoin [2] and Hydrocoin [3]. Future developments will include introduction of scenarios constructed from geoprospective studies and algorithms for sensitivity studies.