The aim of this work was to investigate whether neoformed Ni-phyllosilicates can be observed and identified using transmission electron microscopy (TEM) in combination with energy dispersive spectroscopy (EDS). The investigations focused on Ni-phyllosilicates formed from Ni-doped montmorillonite. The reaction conditions (pH 8, [Ni]initial = 660 and 3300 µM, 0.2 M Ca(NO3)2) employed were similar to those used in previous polarized extended X-ray absorption fine structure (P-EXAFS) investigations of neoformed Ni-phyllosilicates in a Ni-montmorillonite system.

The TEM investigations of Ni-doped montmorillonite revealed the presence of small, thin particles consisting of coherent stacks that yielded only three to five lattice fringes with spacings consistent with smectites. These small particles were neoformed phyllosilicates, based on the fact that the small particles were only observed in Ni-doped samples and their Ni content, as determined from EDS analysis, was high (up to 10 wt.% NiO). Furthermore, the particles did not possess the characteristic properties of montmorillonite particles, such as a 2:1 Si to Al ratio; instead these particles were rich in Si (up to 75 wt.% SiO2). Unlike montmorillonite, these particles did not contain any Fe. The particles were also significantly more resistant to electron beam damage than montmorillonite particles, and EXAFS measurements confirmed the presence of neoformed Ni-phyllosilicates.

The TEM study further indicates the presence of a variety of additional minerals (e.g. cristobalite, halloysite) and an X-ray amorphous Si-rich phase. A Ni signal could only be detected in the latter phase at high Ni loadings (403 µmol/g), suggesting that Ni uptake at low metal loadings (<90 µmol/g) is mainly controlled by the neoformation of phyllosilicates.

This presentation gives an overview of the French major research program on nuclear waste and presents some perspectives in the context of the 2006 Act, while focusing on the main CEA contributions. Development of new conditioning processes combined with in-depth studies of the waste-packages long term behaviour studies allows enhancing the expected performance of waste-packages. Geological disposal performances have been demonstrated based in particular on the relevant understanding of the radionuclides chemistry and migration in complex environment, including tracing field-experiments in ANDRA underground research lab. French research and plans for waste from future reactors will be discussed. It is still a very prospective area as fuel materials that will be used in these reactors is not decided. The capacity to recycle most of the fuel component will be a major point in the choice of an integrated concept including reactor and fuel cycle.

The so-called Instantaneous Release Fraction (or IRF) corresponds to the rapid release of radioactivity by the spent fuel rod at the canister breaching time in repository and depends on the spent fuel evolution in a closed system. The effect of He accumulation on the spent fuel pellet microstructure and diffusion processes are here re-assessed in the light of the recent results issued from the projects Near-Field PROcesses and PRECCI. It allows diminishing the conservatism of the former IRF values of key safety relevant radionuclides for PWR UO2 fuels.

European Commission supported a wide research project entitled “Spent Fuel Stability under repository conditions” (SFS) within the 5th FWP, the aim of which was to develop a common understanding of the radionuclides release from spent nuclear fuel in geological disposal and build a RN release model in order to assess the fuel performance. This project achieved by the end of 2004 focuses both on the Instant Release Fraction (IRF) model and the Matrix Alteration Model (MAM).

A new IRF model was developed based on the anticipated performances of the various fuel microstructures (gap, rim, grains boundaries) and the potential diffusion of RN before the canister breaching. However, this model lets the choice to the end-user about the degree of conservativeness to consider.

In addition, fuel alteration has been demonstrated to be linked to the production of radiolytic oxidants by water radiolysis at the fuel interface, the oxidation of the fuel interface by radiolytic oxidants and the subsequent release of uranium under the influence of aqueous ligands. A large set of experimental data was therefore acquired in order (i) to upgrade the current radiolytic kinetic scheme, (ii) to experimentally correlate the fuel alteration rate and the fuel specific alpha activity by performing experiments on alpha doped samples, (iii) to experimentally test the potential inhibitor effect of hydrogen on fuel dissolution. Based on these results, a new MAM was developed, which was also calibrated using the experiments on inactive UO2 samples. This model was finally applied to representative granitic, salt and clayey environment to predict spent fuel long-term fuel performance.

Spent Nuclear Fuel (SNF) source terms are used to define the release rate of radionuclides (RN) in a direct disposal and to assess the performance of this waste form. They classically distinguish between two contributions: (i) the Instant Release Fraction (IRF) of RN which are directly leached when water contacts the fuel, (ii) the slow and long term release of RN which are embedded within the fuel matrix. Recent experimental results bring significant input in our understanding and assessment of both contributions. However, they have not yet been integrated in the definition of the SNF source term. This paper will present the impact on the RN source term of the latest results on the SNF long term evolution and the key remaining scientific issues.

Eu migration in a Callovo-Oxfordian argilite sample was studied using the micro LIBS technique. Quantitative elemental mapping were made by micro LIBS that showed the actual distribution of the various micro areas observed on the Callovo-Oxfordian sample's surface. Calcite, dolomite, alumino-silicates, quartz, pyrite and iron oxides were identified and their statistical distribution was determined. Experimental Eu profiles observed are consistent with diffusion process accompanied by heterogeneous sorption on alumino-silicate surface.

In the framework of the research conducted on the long term evolution of spent nuclear fuel in geological disposal conditions, a source term model has been developed to evaluate the instantaneous release of RN (Instant Release Fraction IRF) and the delayed release of the RN which are embedded within the matrix. This model takes into account all the scientific results currently available in the literature except the hydrogen effect. IRF was assessed by considering the evolution with time of the RN inventories located within the fuel microstructure to which no confinement properties can be allocated on the long term (rim, gap, grain boundaries). It allows to propose some reference bounding values for the IRF as a function of time of canister breaching and burnup. The matrix radiolytic dissolution was modeled by a simple kinetic model neglecting the radiolytic species recombination and the influence of aqueous ligands and radiolytic oxidants were supposed to completely react with the fuel surface. Spent fuel performance was therefore demonstrated to deeply depend on the reactive surface area.

The anticipated long term evolution of spent nuclear fuel as well as the remaining scientific key issues are presented for the various boundary conditions that can be encountered in long term dry storage and geological disposal. Spent fuel is expected to evolve significantly in closed system conditions which are representative of long term dry storage and the first stages of geological disposal. The mechanical evolution of the grain boundaries, the fate of helium and the evolution of the RN location within the pellet are the three major questions to be addressed which could significantly modify the physical and chemical state of the fuel. In addition, mechanisms and kinetics of fuel alteration by water in deep geological repository are still to be more deeply understood, in particular the inventory of the instant release and the radiolytic dissolution processes, to get a robust and reliable source term.

The Instant Release Fraction at container failure time, IRF(t), is here considered as being the sum of (i) the initial labile fraction, corresponding to the sum of gap and grain boundary inventories of certain radionuclides on exit from the reactor, with a further possible contribution from segregation in the rim region and (ii) the time-dependent fraction of radionuclides accumulating at grain boundaries due to a self-irradiation enhanced diffusion through the grains. The initial labile fraction of radionuclides such as 14C, 36Cl, 79Se, 129I, and 135Cs has been estimated based on leaching experiments, post-irradiation fission gas release measurements and studies of solid-state chemistry of spent fuel, along with estimates of fission product segregation in the rim zone. The contribution of the a self-irradiation enhanced diffusion has also been estimated based on a diffusion coefficient decreasing with time proportionally with the volume α-activity of the spent fuel. Its contribution to the IRF is limited for UO2 fuels. The proposed bounding values of the IRF for fuel with a burnup of 55 GWd/tIHM for 14C, 36Cl, 79Se, 129I, and 135Cs are 11 % at t=0 and close to 15 % at a container failure time of 10,000 y.

Under the geological disposal conditions, spent nuclear fuel (SNF) is expected to evolve during the first thousands years while being maintained isolated from the biosphere before water comes in. Under those circumstances, several driving forces would lead to the progressive intrinsic SNF transformations within the rod which would basically modify the physical and chemical state of the fuel and the subsequent release of radionuclides in solution. In this paper, we briefly summarize the mechanisms we estimate to be significant and propose a new framework for the quantitative assessment of the radionuclide (RN) inventory we estimate to be associated to the classically referred to “Instant Release Fraction” (IRF). We hence demonstrate that in this framework, significantly high IRF values have to be expected for the long term due mainly to the presence of athermal diffusion processes.

The research conducted in the framework of the French research project on spent nuclear fuel (SNF) long - term evolution (PRECCI Project) has enlightened the potential significance of spent nuclear fuel intrinsic evolution in closed system for the assessment of radionuclide (RN) source term in long-term storage or geological disposal. Beyond others, alpha self-irradiation enhanced diffusion and evolution of the grain boundaries cohesion are two major processes which have to be accounted for in view of the RN source term models development. Accounting for these processes, operational models are developed, the aim of which is to quantitatively define the RN release rates from SNF in long-term storage or geological disposal. They distinguish basically an instantaneous contribution (IRF in geological disposal) and a time-dependent contribution (matrix oxidation or alteration). RN inventories associated to these two different processes have to be modeled since they are time-dependent due to the RN diffusion within the pellet. The present paper details the models that are developed in France in terms of assumptions, conservatism and robustness. It comes out from this work that for the instant release fraction, we have to consider a much higher instant release fraction than classically assumed (5–6% in geological disposal) in particular for geological disposal.

Under the geological disposal conditions, spent fuel (SF) is expected to evolve during the 10,000 years while being maintained isolated from the biosphere before coming in contact with water. Under these circumstances, several driving forces would lead to the progressive intrinsic transformations within the rod which would modify the subsequent release of radionuclides. The major mechanisms are the production of a significant volume of He within the UO2 lattice, the accumulation of irradiation defects due to the low temperature which avoids any annealing, the slow migration of radionuclides (RN) within the system by (i) the α self-irradiation-induced athermal diffusion and (ii) locally the building-up of internal mechanical stresses which could turn the pellets into powder. However, the current RN source terms for SF have never accounted for this evolution as they are based on existing knowledge of the fresh SF. Two major mechanisms were considered, the leaching of the readily available fraction (one which was supposed to be instantly accessible to water), and the release of RN through alteration of the UO2 grains. We are now proposing a new RN source term model based on a microscopic description of the system in order to also take account of the early evolution of the closed system, the amplitude of which increases with the burnup and is greater for MOX fuels.