Mechanical behaviour and diffusion of gas during neutron irradiation of actinides in ceramic inert matrices

Fission of actinides from nuclear waste in inert matrices (materials without uranium) can reduce the period in time that nuclear waste is more radiotoxic than uranium ore that is the rock from which ordinary reactor fuel is made. A pioneering study is performed with the inert matrices: MgO, MgAl2O4, Y3Al5O12, Y2O3 and CeO2-x. These inert matrices contain inclusions of actinide-oxides with a diameter of about 200 μm. UO2 is used as a phase of actinides to simulate the implantation of fission products in inert matrix fuels. After neutron irradiation, inter-particle fracture was present in fuels with matrices that do not form a solid solution with UO2. These cracks are caused by swelling of the actinide phase and insufficient creep of the matrix. The diffusion length of fission gases is significantly reduced by inter-particle fracture and thereby fission gas releases till 50% were measured. This inter-particle fracture may be prevented when inert matrix fuels are fabricated with porous actinide inclusions embedded in inert matrices that are polluted by actinides. The amount of helium generated during neutron irradiation of a few actinides from nuclear waste can be far larger than the amount of fission gases. Parameters and diffusion mechanisms to simulate diffusion of helium in MgAl2O4 were derived by thermal evolution of He concentration profiles, atomistic calculations and numerical analysis using original definitions of diffusion coefficients for inert gas in materials with defects for single crystal as well as polycrystal media. This study shows that, amongst others, the material degradation of MgAl2O4, caused by the generated amount of defects and He during irradiation conditions, is acceptable. But the pioneering study with UO2 as a phase of actinides, shows that the solid state swelling by fission products in MgAl2O4 is about ten times larger than this swelling in 238UO2 that is the matrix in fuel for commercial reactors. The commercial feasibility of reduction of actinides from nuclear waste, by neutron irradiation in inert matrices, requires more research.Interfaculty Reactor Institut

Early stage beneficial effects of cathodic protection in concrete structures

Over the last 25 years, cathodic protection (CP) of reinforced concrete structures suffering from chloride induced reinforcement corrosion has shown to be successful and durable. CP current causes steel polarisation, electrochemical reactions and ion transport in the concrete. CP systems are designed based on experience, which results in conservative designs and their performance is a matter of wait-and-see. CP systems can be designed for critical aspects and made more economical using numerical models for current and polarisation distribution. Previously, principles of numerical calculations for design of CP systems were reported. The results were satisfactory, except in terms of current density for active corroding systems. This was suggested to be due to neglecting beneficial effects of CP current flow. One of the beneficial effects is pH increase at the steel surface due to oxygen reduction. As the pH increases, the corrosion rate decreases and the current demand decreases. A simple model was set up for this transient process, suggesting that such effects should occur on the time scale of hours to days. This model was validated from start up data of a CP field trial system on part of a bridge. Field results confirmed the modelling proposed here. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Hydrogen implantation defects in MgO

Deuterium and hydrogen ions with an energy of 15 keV have been implanted in virgin MgO (100) single crystals and in single crystals containing helium implantation generated microcavities. Doses were varied from 2×10^15 to 2×10^16 cm-2. The samples were annealed from room temperature to 950 K. The defects produced by hydrogen and the trapping of hydrogen at the defects were monitored by photon absorption and positron beam analysis. With this novel technique a depth distribution of defects can be determined for implantation depths from 0 to 2000 nm. The technique is very sensitive for vacancy and vacancy clusters, i.e. sites with low electron density. After 950 K annealing microcavities were observed for the 2×10^16 cm-2 dose but not for the 10 times lower dose. During annealing up to 750 K point defects are mobile but the defect clusters remain small and filled with hydrogen. In samples which contain already microcavities, point defects and deuterium from the deuterium irradiation are accumulated by the microcavities.

Kinetics of selenite interactions with boom clay: Adsorption–reduction interplay

The speciation of selenium (Se) in clay-rich host rocks is important within the framework of geological disposal of radioactive waste since it affects its migration. Removal of selenite from formation water can be caused by reduction and adsorption. Reduction could potentially be inhibited or delayed by adsorption. Here, the interplay of adsorption and reduction of selenite was investigated in batch experiments with Boom Clay and its separated size fractions. In all experiments, dissolved Se concentrations (Seaq) showed a fast initial decrease that was followed by a slower decline until removal was almost complete. X-ray absorption spectroscopy indicated that adsorption of selenite accounted for the fast removal of Seaq followed by slower selenite reduction. Eventually, almost all solid-bound SeIV became reduced to Se0 in all experiments. The progress of Seaq removal and SeIV reduction to Se0 could be described by a kinetic model involving reversible adsorption on clay minerals and reduction by pyrite. This implies that the reduction of selenite to Se0 is not significantly hindered or delayed by selenite adsorption on clay minerals. Pyrite is probably the most relevant reductant for selenite in Boom Clay, although reduction by FeII structurally bound in clay minerals might provide an additional pathway for selenite reduction in clay rocks

Kinetics of selenite interactions with boom clay : Adsorption–reduction interplay

The speciation of selenium (Se) in clay-rich host rocks is important within the framework of geological disposal of radioactive waste since it affects its migration. Removal of selenite from formation water can be caused by reduction and adsorption. Reduction could potentially be inhibited or delayed by adsorption. Here, the interplay of adsorption and reduction of selenite was investigated in batch experiments with Boom Clay and its separated size fractions. In all experiments, dissolved Se concentrations (Seaq) showed a fast initial decrease that was followed by a slower decline until removal was almost complete. X-ray absorption spectroscopy indicated that adsorption of selenite accounted for the fast removal of Seaq followed by slower selenite reduction. Eventually, almost all solid-bound SeIV became reduced to Se0 in all experiments. The progress of Seaq removal and SeIV reduction to Se0 could be described by a kinetic model involving reversible adsorption on clay minerals and reduction by pyrite. This implies that the reduction of selenite to Se0 is not significantly hindered or delayed by selenite adsorption on clay minerals. Pyrite is probably the most relevant reductant for selenite in Boom Clay, although reduction by FeII structurally bound in clay minerals might provide an additional pathway for selenite reduction in clay rocks