Chiral Supersymmetric Gepner Model Orientifolds

We explicitly construct A-type orientifolds of supersymmetric Gepner models. In order to reduce the tadpole cancellation conditions to a treatable number we explicitly work out the generic form of the one-loop Klein bottle, annulus and Moebius strip amplitudes for simple current extensions of Gepner models. Equipped with these formulas, we discuss two examples in detail to provide evidence that in this setting certain features of the MSSM like unitary gauge groups with large enough rank, chirality and family replication can be achieved.Comment: 37 pages, TeX (harvmac), minor changes, typos corrected, to appear in JHE

Supersymmetric Orientifolds of Gepner Models

Supersymmetric orientifolds of four dimensional Gepner Models are constructed in a systematic way. For all levels of the Gepner model being odd the generic expression for both the A-type and the B-type Klein bottle amplitude is derived. The appearing massless tadpoles are canceled by introducing appropriate boundary states of Recknagel/Schomerus(RS). After determining the Moebius strip amplitude we extract general expressions for the tadpole cancellation conditions. We discuss the issue of chirality for such supersymmetric orientifold models and finally present a couple of examples in detail.Comment: 38 pages, TeX harvmac, ref. adde

Phenomenology of retention processes: experimental approaches

Communication par affich

The Kinetics of Nickel Sorption on Pyrophyllite as Monitored by X-Ray Absorption Fine Structure (XAFS) Spectroscopy

This study investigated the effect of reaction time (minutes to months) on the surface coordination environment of Ni sorbed onto pyrophyllite using XAFS. The data suggest the appearance of multinuclear Ni complexes after a reaction time of minutes. As reaction time progresses, these multinuclear Ni complexes increased in size ((NNi-Ni = 1.4-4.5). Ni-Ni bond distances (2.99-3.03 Å) were similar to those for mixed Ni-Al hydroxides (3.03 Å), but distinctively shorter than those for Ni(OH)2(s) (3.09 Å). We propose that the formation of mixed Ni-Al hydroxides may explain the observed bond distances and coordination numbers

Thermal diffusion of chlorine in uranium dioxide studied by secondary ion mass spectrometry and X-ray adsorption spectroscopy

In a nuclear reactor, 35Cl present as an impurity in the nuclear fuel is activated by thermal neutron capture. During interim storage or geological disposal of the nuclear fuel, 36Cl may be released from the fuel to the geo/biosphere and contribute significantly to the ‘instant release fraction'. In order to elucidate the diffusion mechanisms, both irradiation and thermal effects must be assessed. This paper deals with the thermal diffusion of chlorine in depleted UO2. For this purpose, sintered UO2 pellets were implanted with 37Cl at an ion fluence of 1013 cm−2 and successively annealed in the 1175–1475 K temperature range. The implanted chlorine is used to simulate the behaviour of the displaced one due to recoil and to interactions with the fission fragments during reactor operation. The behaviour of the pristine and the implanted chlorine was investigated during thermal annealing. SIMS and μ-XAS (at the Cl–K edge) analyses show that: (1) the thermal migration of implanted chlorine becomes significant at 1275 K; this temperature and the calculated activation energy of 4.3 eV points out the great ability of chlorine to migrate in UO2 at relatively low temperatures, (2) the behaviour of the implanted chlorine which aggregates into ‘hot spots' during annealing before its effusion is clearly different from that of the pristine one which remains homogenously distributed after annealing, (3) the ‘hot spot' and the pristine chlorine seem to be in different structural environments. Both types of chlorine are assumed to have a valence state of −I, (4) the comparison between an U2O2Cl5 reference compound and the pristine chlorine environment shows a contribution of the U2O2Cl5 to the pristine chlorine

Multicomponent transport of sulfate in a goethite-silica sand system at variable pH and ionic strength

In this work, existing models for adsorption of protons and sulfate on goethite and silica were used in combination with a one-dimensional mass- transport model to predict the transport of sulfate at variable pH and ionic strength in a goethite-silica system. The predicted multicomponent transport phenomena are discussed, and an evaluation is made of the sensitivity of the results to different input parameters. This information was used to select optimal conditions for independent experimental testing of the model. Column experiments were carried out in which solutions of known pH, sulfate concentration, and salt concentration were infiltrated into a goethite-coated quartz sand column. The agreement between the experiments and the predictions is very good, especially considering the fact that no fitting of model parameters to the actual experimental system was involved. According to the model calculations, transport behavior of sulfate in a goethite system is very sensitive to relatively small amounts of adsorbed sulfate initially present. This could be an explanation for the remaining differences between the predicted and the measured curves. It also implies that, for the application of such mechanistic models to soils, accurate information on the initial composition of the soil is essential. | In this work, existing models for adsorption of protons and sulfate on goethite and silica were used in combination with a one-dimensional mass-transport model to predict the transport of sulfate at variable pH and ionic strength in a goethite-silica system. The predicted multicomponent transport phenomena are discussed, and an evaluation is made of the sensitivity of the results to different input parameters. This information was used to select optimal conditions for independent experimental testing of the model. Column experiments were carried out in which solutions of known pH, sulfate concentration, and salt concentration were infiltrated into a goethite-coated quartz sand column. The agreement between the experiments and the predictions is very good, especially considering the fact that no fitting of model parameters to the actual experimental system was involved. According to the model calculations, transport behavior of sulfate in a goethite system is very sensitive to relatively small amounts of adsorbed sulfate initially present. This could be an explanation for the remaining differences between the predicted and the measured curves. It also implies that, for the application of such mechanistic models to soils, accurate information on the initial composition of the soil is essential

Thermal diffusion of chlorine in uranium dioxide

In a nuclear reactor, isotopes such as 35Cl present as impurities in the nuclear fuel are activated by thermal neutron capture. During interim storage or geological disposal of nuclear fuel, the activation products such as 36Cl may be released from the fuel to the geo/biosphere and contribute to the "instant release fraction" as they are likely to migrate in defects and grain boundaries. In order to differentiate diffusion mechanisms due to "athermal" processes during irradiation from thermally activated diffusion, both irradiation and thermal effects must be assessed. This work concerns the measurement of the thermal diffusion coefficient of chlorine in UO2.37Cl was implanted at a 1013 at/cm2 fluence in depleted UO2 samples which were then annealed in the 900−1200 °C temperature range and finally analyzed by Secondary Ion Mass Spectrometry (SIMS) to obtain 37Cl depth profiles. The migration process appears to be rather complex, involving mechanisms such as atomic, grain boundary, directed diffusion along preferential patterns as well as trapping into sinks before successive effusion. However, using a diffusion model based on general equation of transport, apparent diffusion coefficients could be calculated for 1000 and 1100 °C and a mean activation energy of 4.3 eV is proposed. This value is one of the lowest values compared to those found in literature for other radionuclides pointing out a great ability of chlorine to migrate in UO2 at relatively low temperatures. In order to unequivocally determine the diffusion behaviour of both implanted and pristine chlorine before and after thermal annealing, the structural environment of chlorine in UO2 was examined using micro X-ray fluorescence (micro-XRF) and micro X-ray absorption spectroscopy (micro-XAS)

Reduction of selenite on corroded iron: a micro-spectroscopic study

Under anoxie conditions zero-valent iron can react with water to produce hydrogen gas and magnetite or green rust, a highly reactive mineral phase that can induce reduction processes and thus control the speciation, the solubility, toxicity and the mobility of redox sensitive elements in (nuclear) waste repositories. In this study micro X-ray fluorescence (micro-XRF) and micro X-ray absorption spectroscopy (micro-XAS) were used to investigate the speciation of selenium that immobilized in the presence of Fe(0) and an anoxie synthetic groundwater solution. The selenium immobilization was accompanied by the formation of a green rust corrosion layer. Micro-XRF revealed that a Se-rich layer is present along the iron surfaces that were exposed to the Se(IV) solution. Micro-XAS experiments at the Se K-edge showed that Se(IV) was reduced to elemental Se(0). Thus, the reactivity of zero-valent and green rust should to be considered in assessing the long-term fate of selenium in nuclear waste repositories.

Chlorine diffusion in uranium dioxide: thermal effects versus radiation enhanced effects

International audienceChlorine is present as an impurity in the UO2 nuclear fuel. 35Cl is activated into 36Cl by thermal neutron capture. In case of interim storage or deep geological disposal of the spent fuel, this isotope is known to be able to contribute significantly to the instant release fraction because of its mobile behavior and its long half life (around 300000 years). It is therefore important to understand its migration behavior within the fuel rod. During reactor operation, chlorine diffusion can be due to thermally activated processes or can be favoured by irradiation defects induced by fission fragments or alpha decay. In order to decouple both phenomena, we performed two distinct experiments to study the effects of thermal annealing on the behaviour of chlorine on one hand and the effects of the irradiation with fission products on the other hand. During in reactor processes, part of the 36Cl may be displaced from its original position, due to recoil or to collisions with fission products. In order to study the behavior of the displaced chlorine, 37Cl has been implanted into sintered depleted UO2 pellets (mean grain size around 18 µm). The spatial distribution of the implanted and pristine chlorine has been analyzed by SIMS before and after treatment. Thermal annealing of 37Cl implanted UO2 pellets (implantation fluence of 1013 ions.cm-2) show that it is mobile from temperatures as low as 1273 K (Ea=4.3 eV). The irradiation with fission products (Iodine, E=63.5 MeV) performed at 300 and 510 K, shows that the diffusion of chlorine is enhanced and that a thermally activated contribution is preserved (Ea=0.1 eV). The diffusion coefficients measured at 1473 K and under fission product irradiation at 510 K are similar (D = 3.10-14 cm2.s-1). Considering in first approximation that the diffusion length L can be expressed as a function of the diffusion coefficient D and time t by : L=(Dt)1/2, the diffusion distance after 3 years is L=17 µm. It results that there is a great probability for the chlorine contained in the UO2 grains to have reached the grain boundaries after 3 years, in the core of the fuel rod as well as at its periphery. Moreover, diffusion and concentration of chlorine at grain boundaries has been evidenced using SIMS mapping. Our results indicate therefore, that, during reactor operation and after, the majority of 36Cl is likely to have moved to grain boundaries, rim and gap. This fraction might then significantly contribute to the rapid or instant release of chlorine. This could have important consequences for safety assessment