Search

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.

Steel is discarded by railway owners as a material for underground structures near railway lines, due to uncertainty over increased corrosion by DC stray currents stemming from the traction power system. This paper presents a large scale field test in which stray currents interference of a sheet pile wall was monitored in realistic conditions. The field test was representative for a sheet pile wall of 220 m length and 10 m depth. Also an anchor was incorporated. A specially designed coupon sensor was used. Measured stray current interference was very low, maximum values were found for the anchor. A conservative interpretation of the measured results showed that additional corrosion due to stray currents was smaller than 10 % of the expected natural corrosion, based on the applicable standards. © RILEM 2013.

Steel structures near DC powered railways are expected to be affected by stray current interference. This causes accelerated corrosion rates. Therefore steel is often not used as a building material in these cases, although certain advantages over the alternative material concrete exist. These advantages can include amongst others costs, shorter building time and less nuisance during construction. In this paper a non-destructive DC stray current monitoring system is described and the measurement results from a large-scale field test are given. The monitoring system consisted of specially developed coupon sensors for sheet pile walls with which stray currents could be measured, and so-called Reference Electrode (RE's), with which electrical potentials were measured. An advanced cost-effective monitoring system was developed, using a zero resistance current measurement and wireless data transmission. A comparison of the experimental results and literature data about corrosion shows that the additional corrosion by stray current interference was negligible compared to natural corrosion. Although quantified in a rather crude manner, the additional corrosion for the situation at hand turned out to be lower than 0.2 mm over a period of 100 year.

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.