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Bentonite is one of the more safety-critical components of the engineered barrier system in the disposal concepts developed for many types of radioactive waste. It is used due to its favourable properties (including plasticity, swelling capacity, colloid filtration, low hydraulic conductivity, high retardation of key radionuclides) and its stability in relevant geological environments. However, bentonite is unstable under alkaline conditions and this has driven interest in low-alkali cements (leachate pH of 10–11). To build a robust safety case, it is important to have supporting natural analogue data to confirm understanding of the likely long-term performance of bentonite. In Cyprus, the presence of natural bentonite in close proximity to natural alkaline groundwaters permits the zones of potential bentonite/alkaline water reaction to be studied as an analogy of the potential reaction zones in the repository. Here, the results indicate minimal volumetric reaction of bentonite, with production of a palygorskite secondary phase.
The Cyprus Natural Analogue Project was carried out due to the requirement to support ongoing laboratory and modelling efforts on the potential reaction of the bentonite buffer with cementitious leachates in the repository engineered barrier system. Although it is known that the higher pH (12.5–13) leachates from ordinary Portland cement will degrade bentonite, it is unclear if this will also be the case for the lower pH (10–11) leachates typical of low alkali cements. Ongoing laboratory and underground rock laboratory programmes, which are currently investigating this, face the obstacle of slow kinetics and the production of short-lived metastable phases, meaning obtaining unambiguous results may take decades. It was therefore decided to implement a focussed natural analogue study on bentonite/low alkali cement leachate reactions to provide indications of the probable long-term reaction products and reaction pathways to provide feedback on the existing short-term investigations noted above and to ascertain if any critical path research and development needs to be instigated now. The results of the analyses presented here, in this short overview of the project, suggest that there has been very limited alkaline groundwater reaction with the bentonite. This is generally supported by both the geomorphological evidence and the natural decay series data which imply groundwater/rock interaction in the last 105 a.
We report a study of pulse shapes of a radiation detector with a sandwich structure fabricated from chemical vapor deposited (CVD) polycrystalline diamond. The pulse shapes were recorded at room temperature using 5.486 MeV alpha particles from 241Am source. Only “fast” component was observed in the electron predominated pulses, whereas both “fast” and “slow” components were obtained in the hole predominated pulses, suggesting that electron charge drift is prompt and no detrapping occurred. In contrast, hole charge drift is slower than expected and trapping-detrapping took place during hole travel process.
Of the four species of Bulinus found on Madagascar, three species: B. obtusispira, B. liratus and B. bavayi are endemic while the fourth, B. forskalii, is probably a recent introduction from the African mainland. The evolutionary relationships of these species with Bulinus species from Africa were studied by phylogenetic analysis of DNA sequence variation at two mitochondrial loci: cytochrome oxidase subunit I (COI) and large ribosomal subunit (LSU) or 16S. The observed levels of nucleotide divergence within Bulinus were substantial but may underestimate the true levels as there was evidence of ‘saturation' of transitional substitutions at both loci. A putative secondary structure model for the sequenced segment of the 16S was developed. Subsequent phylogenetic analysis using transversional changes only for both loci, showed that there were contrasting levels of divergence within the four species groups. B. obtusispira was consistently placed within the B. africanus group, appearing ancestral to this group and was closest to the basal node within Bulinus. Together with B. bavayi, the two species appear to have been isolated on Madagascar for a long time, contrasting with both B. liratus and B. forskalii that appear more recent colonisers; however, estimate of exact times of divergence is problematic. A PCR-RFLP assay was developed to enable identification and discrimination of B. obtusispira and B. liratus using discriminatory variation within the COI. To enable population genetic analysis within B. obtusispira, microsatellite markers were developed using an enrichment method and 8 primer pairs are reported. Laboratory infection experiments using Madasgacan S. haematobium from the Mahabo area showed that certain populations of B. obtusispira, B. liratus and B. bavayi were compatible.
Recent characterization of nuclear ribosomal small subunit (SSU) genes has shown that variant nucleotides within this region could be useful for species and species group identification within the genus Lymnaea (Gastropoda: Lymnaeidae). This study aimed to characterize a range of populations of Lymnaea natalensis Krauss, 1848 on Madagascar, and addressed two related questions. First, is there any evidence of intraspecific variation of the SSU and, if so, what might be its significance? Secondly, might this variation jeopardize the use of SSU for lymnaeid taxonomy and phylogeny? Lymnaea natalensis (n = 212) was collected from 17 sampling localities, spanning the northern and southern ends of the island. Variation within a selected region of the SSU known to vary between species, the V1 and V2, was assayed by polymerase chain reaction (PCR) linked restriction fragment length polymorphism (RFLP) and denaturing gradient gel electrophoresis (DGGE) analysis. The PCR-RFLP profiles indicated a striking dimorphism across populations at two restriction site loci (CfoI & MspI) within the E10-1 helix of the V2 region. The observed RFLP variation was confirmed by direct sequencing and by genomic digestion with subsequent hybridization. Putative heterozygotes were also encountered and in these individuals the SSU arrays composed of two distinct types approximately 1% divergent. A severe departure from Hardy–Weinberg equilibrium with a highly statistically significant (P < 10-5) heterozygote deficiency was found and genetic variation among populations was highly structured (Fst = 0.53). The geographic distribution of the variants was mapped, revealing that one variant was restricted to higher, predominately colder environments and was thought to be an adaptation. The molecular basis of the SSU variation was caused by single nucleotide polymorphisms (SNPs). To test for the possibility of cryptic taxa, an analysis of individuals representative of the SSU variant types with isoenzyme analysis (ISA), randomly amplified polymorphic DNA (RAPDs) and PCR-RFLP analysis of the ribosomal Internal Transcribed Spacer (ITS) was performed. Little variation was revealed and none that correlated to the groups suggested by SSU, confirming that the SSU variation was intraspecific. The levels of intraspecific divergence of the V1 and V2 within Lymnaea were not appreciably different (1%) from interspecific and would therefore question the validity of these data for lymnaeid taxonomy and phylogeny.
A kinetic model recently developed  for the radiolytically induced
oxidative dissolution of the spent fuel matrix is presented. This is based
on experimental studies on the generation and evolution of radiolytic
products in a closed system containing fragments of PWR-fuel . The
outcome of this model is currently being integrated in the present PA
exercise being prepared by SKB. The calibration of the model against various
experimental information and its predictive capabilities for the long term
performance of the spent fuel matrix are presented.
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