C. Degueldre

Concepts for an inert matrix fuel, an overview

The published contributions in this special issue represent an overview of studies that are today integrated into national or international projects initiated since the first Inert MAtrix Workshops held at the Paul Scherrer Institute and elsewhere since September 1995, as well as from other specific actions. The reported R&D work includes studies about specific properties and conditions that the inert matrix fuel must fulfil.

The colloid and radionuclide retardation experiment at the Grimsel Test Site: influence of bentonite colloids on radionuclide migration in a fractured rock

Abstract The colloid and radionuclide retardation (CRR) experiment is dedicated to the study of the in situ migration behaviour of selected actinides and fission products in the absence and presence of bentonite colloids in a water-conducting feature (shear zone) in the Grimsel Test Site (GTS). The technical scenario considers the bentonite backfill/host rock interface as a potential source for colloids. The experiment investigates the migration behaviour of U, Th, Pu, Am, Np, Sr, Cs, I and Tc and the influence of smectitic bentonite colloids by two in situ tracer injections in a well-characterised dipole. The field experiments are supported by an extended laboratory and modelling programme. Colloid breakthrough is determined on-line by a mobile, laser-induced breakdown detection apparatus (LIBD), a mobile photon correlation spectrometer (PCS) and, afterwards in the laboratory, by a single particle counting method (SPC) using a laser light scattering technique. Bentonite colloids generated from bentonite backfill material were found to be stable in the experimental groundwater and the influence of pH and salinity on colloid stability was investigated. The in situ monitored breakthrough of the tri- and tetravalent actinides Am and Pu and of Cs followed the colloid breakthrough indicating some degree of colloid-mediated migration of these radionuclides in the experimental shear zone. But even when no colloids had been added to the tracer cocktail, part of Am(III) and Pu(IV) appears to migrate as colloids. The different colloid detection techniques revealed a colloid recovery between 80 and 90% of the injected bentonite colloids. The CRR experimental results are considered from the perspective of understanding the likely long-term behaviour of a deep geological repository for radioactive waste and as an indicator of the way forward to the next generation of in situ experiments.

Colloid properties in granitic groundwater systems. I: Sampling and characterisation

Colloids are present in all groundwaters. The role they may play in the migration of contaminants in the geosphere must be studied. Colloid sampling and characterisation campaigns have been carried out in Switzerland. On the basis of the results obtained from studies in the Grimsel area (Grimsel Test Site, Transitgas Tunnel), Northern Switzerland (Leuggern, Zurzach) and the Black Forest (Bad Sackingen, Menzenschwand), a consistent picture is emerging. The groundwater colloids in granitic systems are predominantly composed of phyllosilicates and silica originating from the aquifer material. Under constant hydrogeochemical conditions, the colloid concentration does not exceed 100 ng·ml−1 when the calcium concentration is larger than 10−4 M and the sodium concentration is larger than 10−2 M. However, under transient chemical or physical conditions, such as geothermal or tectonic activity, colloid generation may be enhanced and the colloid concentration may reach 10 μg·ml−1 or more if both calcium and sodium concentrations are low (i.e. < 10−4 M and < 10−2 M respectively). The colloid size distribution yields information about their stability. For a representative deep crystalline water ([Ca] = 3.5 × 10−4 M, [Na] = 1.4 × 10−2 M, [TOC] = 3 × 10−6 M), the colloid concentration is < 100 ng · ml−1 and around 10 ng · ml−1 in Zurzach water for sizes ranging from 100 to 1000 nm.

Groundwater colloid properties: a global approach

This study presents and discusses groundwater colloid results from various geological formations ranging from crystalline to sedimentary, from organic rich to organic poor systems and from subsurface to very deep aquifers. Colloid presence and their potential mobility are justified on the basis of colloid stability properties in the investigated groundwaters. The colloid concentration is a function of pH, redox potential, concentrations of Na, K, Ca, Mg and organic carbon, as well as the status of the chemical and physical steady state of the hydrogeochemical system. The colloid properties are discussed with a non-site specific approach.

Uranium colloid analysis by single particle inductively coupled plasma-mass spectrometry

Uranium single particle analysis has been performed by inductively coupled plasma-mass spectrometry (ICP-MS) and the performances are compared with that provided by scanning electron microscopy and single particle counting. The transient signal induced by the flash of ions due to the ionisation of an uranium colloidal particle in the plasma torch can be detected and measured for selected uranium ion masses ((238)U(+), (235)U(+) or (254)[(238)U(16)O](+)) by the mass spectrometer. The signals recorded via time scanning are analysed as a function of particle size or fraction of the studied element or isotope in the colloid phase. The frequency of the flashes is directly proportional to the concentration of particles in the colloidal suspension. The feasibility tests were performed on uranium dioxide particles. The study also describes the experimental conditions and the choice of mass to detect uranium colloids in a single particle analysis mode.

Results of the colloid and radionuclide retention experiment (CRR) at the Grimsel Test Site (GTS), Switzerland: impact of reaction kinetics and speciation on radionuclide migration

Summary The influence of smectite colloids on the migration behaviour of U(VI), Th(IV), Pu(IV), Am(III), Np(V), Sr(II) and Cs(I) is investigated within the Colloid and Radionuclide Retardation experiment (CRR). Two in situ experiments in a well-characterized granitic fracture zone are carried out in presence and absence of bentonite colloids. Radionuclide retardation observed in the field studies increases in the sequence Np(V)∼U(VI)<Sr(II)<Cs(I), where a small fraction of colloid borne breakthrough is only stated for Cs(I) in presence of bentonite colloids. Am(III) and Th/Pu(IV) mainly migrate as colloids without retardation in the presence and absence of smectitic colloids. The radionuclide migration behaviour is discussed on the basis of results obtained in laboratory batch sorption experiments and spectroscopic studies. Consistent with the field observation, laboratory derived Kd values increase in the order Np(V)∼U(VI)<Sr(II)<Cs(I). Significant kinetic hindrance for the sorption to fault gauge minerals is observed for Sr(II) and Cs(I), but notably for Am(III) and Pu(IV). The slow sorption reaction of tri- and tetravalent actinide ions is explained by their kinetically hindered dissociation from colloidal species. In order to explain the colloidal behaviour of tri- and tetravalent actinides even in absence of bentonite colloids, ultracentrifugation and spectroscopic experiments are performed. It is found that up to 60% of Pu(IV) and Am(III) species can be centrifuged off. Adding Cm(III) (5×10-8 mol L-1) into both injection solutions instead of Am(III) allows for a spectroscopic study by using the time resolved laser fluorescence spectroscopy (TRLFS). Peak position and fluorescence lifetimes (λ=604 nm, τ=110-114 μs) together with the fact that Cm(III) can be widely separated by ultracentrifugation, suggest the existence of inner-sphere surface complexes on groundwater and bentonite colloids. Carbon K-edge XANES analysis of the bentonite colloids reveal the presence of natural organic constituents. They are mainly of aliphatic nature containing high fractions of carboxylate groups. A contribution of these organic species towards the bentonite colloid stability and sorption of actinides is assumed to be likely.

Control of civilian plutonium inventories using burning in a non-fertile fuel

Abstract The increasing inventories of plutonium generated by commercial nuclear power production represent a potential source for proliferation of nuclear weapons. To address this threat we propose separating the plutonium from the other constituents of commercial reactor spent fuel and burning it in a non-fertile fuel based on a zirconium dioxide matrix. The separation can be performed by the Purex process currently in use, but we recommend development of a more compact separation technology that would produce less secondary waste than currently used technology and would allow for more stringent accounting of plutonium inventories. The non-fertile fuel is designed for use in conventional light water power reactors and does not require development of new reactor technology.

Basic Properties of a Zirconia-Based Fuel Material for Light Water Reactors

The properties of zirconia cubic solid solutions doped with yttria, erbia, and ceria or thoria are investigated with emphasis on the potential use of this material as an inert matrix for Pu inciner...

Inert matrix fuel for the utilisation of plutonium

Utilising fuel resources responsibly, reducing waste volume and emissions as well as conflict potentials within the international community (non-proliferation, energy demand) are among the principles for the judgment of sustainable development. Utilising and burning plutonium in a light water reactor has been shown to be feasible for the disposition of the large amount of excess plutonium produced in todays power reactors and resulting from the disarmament efforts of the super powers. With regard to material technology aspects, efforts have concentrated on the evaluation of fabrication feasibility and on the determination of the physicochemical properties of a single phase zirconium/erbium/plutonium oxide material stabilised as a cubic solution by yttrium for plutonium and minor actinide incineration and transmutation. Due to the absence of uranium as origin for plutonium build-up, such a nuclear fuel is called Inert Matrix Fuel. Irradiation testing with a dedicated experiment in the material test reactor in Halden, Norway, is underway within the framework of the OECD-Halden Reactor Project.

Colloid properties in granitic groundwater systems. II : Stability and transport study

The groundwater colloid concentrations previously measured in the Grimsel area (Grimsel Test Site, Transitgas Tunnel), Northern Switzerland (Leuggern, Zurzach) and the Black Forest (Bad Sackingen, Menzenschwand) are explained on the basis of the colloid stability properties for their composition and the chemistry of the investigated groundwaters. If reversible attachment of the colloids onto the rock is assumed, colloid transport could take place when the colloid population is stable. The question considered in connection with colloid transport and its modelling is that of colloid attachment. Natural colloids, and the surface of the rock on which they may be collected, generally have negative surface charge so that colloid attachment may be difficult. Since it has been shown that a theory like DLVO is inapplicable because of inherent shortcomings which lead to unrealistic predictions, attachment factors were determined experimentally for systems which correspond as closely as possible to the natural system. Montmorillonite colloids are used as model material because colloids in granitic groundwaters comprise phyllosilicates (Part I). Under constant hydrogeochemical conditions, the colloid concentration in a granitic groundwater was found to be less than 100 ng · m1−1 (107 pt · ml−1 > 100 nm) when the Ca concentration is larger than 10−4M and the Na concentration is larger than 10−2 M. However, when the chemical or physical conditions of the system are not in a steady state, such as with hydrothermal activity or tectonic events, colloid generation may be enhanced. The small colloid concentration in a representative deep crystalline water ( 100 nm) is a consequence of an attachment factor for clay colloids (e.g. montmorillonite) close to 1, and limits the colloids transport.