Clay Technology (Sweden)

Donnan equilibriumgoverns the distribution of ions in many systemssuch as ion exchange membranes and biological cells in contact withan external electrolyte. Herein, Donnan equilibrium between bulk saltsolution and bihydrated montmorillonite was investigated because sucha system is of great importance for many nuclear waste disposal concepts.Specifically, we used molecular dynamics simulations to determinethe partition coefficient of chloride, which was achieved by calculatingthe free-energy difference of chloride in the interlayer and the bulkusing enhanced sampling methodology. Montmorillonite in equilibriumwith either NaCl or CaCl2 was examined to elucidate thegeneral difference between 1:1 and 2:1 salts. The concentration dependenceof the partition coefficient for each salt was determined using threeand four concentrations for NaCl and CaCl2, respectively.In the case of NaCl, we found that the partition coefficient increasedlinearly with the concentration, while for CaCl2, the increasewas proportional to the square root of the concentration. A derivationof the partition coefficient using general Donnan theory that includesexcess free energy contributions beyond the electrostatic Donnan potentialis also presented. For both salts, the agreement between the partitioncoefficient from the simulations and Donnan theory was excellent.Although Donnan theory is a continuum theory derived without any referenceto atomistic details, the present results justify its applicationto systems with nanoscale pores.

In connection with the storage of spent nuclear fuel in the Swedish crystalline rock a new technology for the grouting of-very narrow — 100 )µm — fractures is being developed., The method is based on bentonite or cement with a low water content as grouting fluid and a very high static pressure — order of 2 MPa — superimposed by a high-frequency oscillating pressure (with a very large amplitude), the latter for facilitating the entrainment process as oscillations of the grouting particles will significantly decrease the viscosity of the grouting fluid. This paper discusses the grouting methodology, some initial laboratory and prototype studies and some initial efforts in describing the entrainment process using pressure transient theory for fluids. This theoretical approach makes it possible to calculate the variation of the vibrational amplitudes of the grouting particles along the fracture and thus the changing viscosity. A special emphasis is put on the determination and description of the viscosity properties of the grouting fluids as the viscosity will strongly affect the entrainment efficiency due to the very small fracture aperture and the high viscosity of the grouting fluids. Field tests performed in the granite in the Stripa mine yield very good results and it is expected that this technology could be used in a number of special applications where a very low hydraulic conductivity is required but also as an appealing alternative to conventional methods in rocks with large-aperture fractures.

The Prototype Repository (PR) was a field test of six, electrically heated, full-scale waste containers resembling the key component of a KBS-3 nuclear waste repository. The design and heat load was similar to the proposed repository at Forsmark, Sweden. In this paper, the thermal, mechanical and thermo-mechanical response of the PR host rock to excavation and to the subsequent heating is assessed. The assessment is carried out using three-dimensional models (numerical and analytical) in combination with monitoring data and visual observations from the excavations. Certain measurements and observations agree well with results from the models. These include temperature measurements during the heating phase. Additional measurements include patterns of low-magnitude acoustic emission events around the deposition holes tracked during the excavation. The spatial distribution of these events coincide with regions of modelled high compressive stresses. Models with a simple fracture network, consisting of planar disks with laboratory-scale properties, appear to give upper bound estimates of the stress disturbances caused by a real fracture network. The magnitude of the modelled stresses around the deposition hole is typically below the spalling strength. The lack of any significant or systematic occurrence of spalling in the deposition hole walls supports the modelling results. Several instruments installed at different positions to monitor stress change, strain and deformation malfunctioned during the nearly 8-year-long monitoring period. Despite this, there is ample evidence to support the overall conclusion that the modelling results and observations are in sufficient agreement to strengthen the confidence in the modelling approach.

Abstract The effect of exposure to repository-like conditions on compacted Wyoming bentonite was determined by comparing the hydraulic, mechanical, and mineralogical properties of samples from the bentonite buffer of the Canister Retrieval Test (CRT) with those of reference material. The CRT, located at the Swedish Äspö Hard Rock Laboratory (HRL), was a full-scale field experiment simulating conditions relevant for the Swedish, so called KBS-3, concept for disposal of high-level radioactive waste in crystalline host rock. The compacted bentonite, surrounding a copper canister equipped with heaters, had been subjected to heating at temperatures up to 95°C and hydration by natural Na-Ca-Cl type groundwater for almost 5 y at the time of retrieval. Under the thermal and hydration gradients that prevailed during the test, sulfate in the bentonite was redistributed and accumulated as anhydrite close to the canister. The major change in the exchangeable cation pool was a loss in Mg in the outer parts of the blocks, suggesting replacement of Mg mainly by Ca along with the hydration with groundwater. Close to the Cu canister, small amounts of Cu were incorporated into the bentonite. A reduction of strain at failure was observed in the innermost part of the bentonite buffer, but no influence was noted on the shear strength. No change in swelling pressure was observed, while a modest decrease in hydraulic conductivity was found for the samples with the highest densities. No coupling was found between these changes in the hydro-mechanical properties and the montmorillonite — the X-ray diffraction characteristics, the cation exchange properties, and the average crystal chemistry of the Na-converted <1 μm fractions provided no evidence of any chemical/structural changes in the montmorillonite after the 5 y hydrothermal test.

Through comparison of pre- and post-explosion images obtained with the Wide Field and Planetary Camera 2 onboard the Hubble Space Telescope, we have isolated a supergiant star prior to explosion at nearly the same position as the high-luminosity SN II-P 2008cn. We provide evidence that this supergiant may well be the progenitor of the SN, although this identification is not entirely unambiguous due mainly to the distance to the host galaxy (NGC 4603), 33.3 Mpc. The progenitor candidate has a more yellow color than generally would be expected and, if a single star, would require that it exploded during a "blue loop" evolutionary phase. Nonetheless, we estimate an initial mass of Mini = 15 +/- 2 Msun for this star, which is within the expected mass range for SN II-P progenitors. The yellower color could also arise from the blend of two or more stars, such as a red supergiant hidden by a brighter, blue supergiant; or a massive, interacting binary system. Finally, if the yellow supergiant is not the progenitor, or is not a stellar blend or binary containing the progenitor, then we constrain any undetected progenitor star to be a red supergiant with Mini < 11 Msun, considering a physically more realistic scenario of explosion at the model endpoint luminosity for a rotating star. (ABRIDGED)

Current intensive care unit performance measures include in-hospital mortality after intensive care unit admission. This measure does not account for deaths occurring after transfer to another hospital or soon after discharge and therefore, may be biased. Determine how transfer rates to other acute care hospitals and early post-discharge mortality rates impact hospital performance assessments using an in-hospital mortality model. Data were retrospectively collected on 10,502 eligible intensive care unit patients across 35 California hospitals between 2001 and 2004. We calculated the rates of acute care hospital transfers and early post-discharge mortality (30-day overall mortality-30-day in-hospital mortality) for each hospital. We assessed hospital performance with standardized mortality ratios (SMRs) using the Mortality Probability Model III. Using regression models, we explored the relationship between in-hospital SMRs and the rates of hospital transfers or early post-discharge mortality. We explored the same relationship using a 30-day SMR. In multivariable models, for each 1% increase in patients transferred to another acute care hospital, there was an in-hospital SMR reduction of -0.021 (-0.040-0.001). Additionally, a 1% increase in early post-discharge mortality was associated with an in-hospital SMR reduction of -0.049 (-0.142-0.045). Assessing hospital performance based upon 30-day mortality end point resulted in SMRs closer to 1.0 for hospitals at high and low ends of in-hospital mortality performance. Variations in transfer rates and potentially discharge timing appear to bias in-hospital SMR calculations. A 30-day mortality model is a potential alternative that may limit this bias.