Uncertainty in the reactive transport model response to analkaline perturbation in a clay formation

The mineral alteration in the concrete barrier and in the clay formation around long-lived intermediate-level radioactive waste in the French deep geological disposal concept is evaluated using numerical modeling. There are concerns that the mineralogical composition of the surrounded clay will not be stable under the high alkaline pore fluid conditions caused by concrete (pH {approx} 12). Conversely, the infiltration of CO{sub 2}-rich groundwater from the clay formation into initially unsaturated concrete, at the high temperature (T {approx} 70 C) produced from the decay of radionuclides, could cause carbonation, thereby potentially affecting critical performance functions of this barrier. This could also lead to significant changes in porosity, which would affect aqueous diffusive transport of long-lived radionuclides. All these processes are therefore intimately coupled and advanced reactive transport models are required for long-term performance assessment. The uncertainty in predictions of these models is one major question that must be answered. A mass-transfer model response to an alkaline perturbation in clay with standard model values is first simulated using the two-phase non-isothermal reactive transport code TOUGHREACT. The selection of input parameters is thereafter designed to sample uncertainties in a wide range of physico-chemical processes without making a priori assumptions about the relative importance of different feedbacks. This 'base-case' simulation is perturbed by setting a parameter to a minimum, intermediate or maximum value or by switching on/off a process. This sensitivity analysis is conducted using grid computing facilities of BRGM (http://iggi.imag.fr). Our evaluation of the preliminary results suggests that the resaturation and the heating of the near-field will be of long enough duration to cause a limited carbonation through all the width of the concrete barrier. Another prediction is the possibility of self-sealing at the concrete/clay interface. Further research is however required to discuss the effect of such evolution on the desirable performance function of both barriers

Simulation of the degradation of a concrete/clay interface:influence of temperature, unsaturated conditions and porosityvariations

For long-lived intermediate-level radioactive waste, the useof concrete as engineering barrier and Callovian-Oxfordian clay asgeological barrier at a depth of 500 m is considered in the Frenchdisposal concept (ANDRA, 2005). Upon emplacement, initially unsaturatedconcrete is expected to experience coupled processes involving heating,re-saturation with groundwater from the clay formation, gas exchanges andgeochemical reactions. After an early period of re-saturation, solutetransport is supposed to be diffusion-controlled because of the extremelylow permeability of the two media. These coupled processes may lead tochanges in the porosity of the concrete or clay barriers. In the presentpaper, a fully coupled Thermo-Hydro-Chemical (THC) response of atwo-phase (gas and solution) mass-transfer model was evaluated and testedby a sensitivity analysis. This study is an extension of a previous modelapplied to an isothermal and fully saturated concrete/clay interface(Burnol et al., 2005); it investigated the coupled effect of temperatureand unsaturated conditions assuming no production of H2(g). The systemwas simulated for a 2000-year period, which covers the most predominantthermal perturbation

Comparing FRACHEM and TOUGHREACT for reactive transport modelingof brine-rock interactions in enhanced geothermal systems (EGS)

Coupled modelling of fluid flow and reactive transport ingeothermal systems is challenging because of reservoir conditions such ashigh temperatures, elevated pressures and sometimes high salinities ofthe formation fluids. Thermal hydrological-chemical (THC) codes, such asFRACHEM and TOUGHREACT, have been developed to evaluate the long-termhydrothermal and chemical evolution of exploited reservoirs. In thisstudy, the two codes were applied to model the same geothermal reservoir,to forecast reservoir evolution using respective thermodynamic andkinetic input data. A recent (unreleased) TOUGHREACT version allows theuse of either an extended Debye-Hu?ckel or Pitzer activity model forcalculating activity coefficients, while FRACHEM was designed to use thePitzer formalism. Comparison of models results indicate that differencesin thermodynamic equilibrium constants, activity coefficients andkinetics models can result in significant differences in predictedmineral precipitation behaviour and reservoir-porosity evolution.Differences in the calculation schemes typically produce less differencein model outputs than differences in input thermodynamic and kineticdata, with model results being particularly sensitive to differences inion-interaction parameters for highsalinity systems

Distinguishing Asthma Phenotypes Using Machine Learning Approaches.

Asthma is not a single disease, but an umbrella term for a number of distinct diseases, each of which are caused by a distinct underlying pathophysiological mechanism. These discrete disease entities are often labelled as asthma endotypes. The discovery of different asthma subtypes has moved from subjective approaches in which putative phenotypes are assigned by experts to data-driven ones which incorporate machine learning. This review focuses on the methodological developments of one such machine learning technique-latent class analysis-and how it has contributed to distinguishing asthma and wheezing subtypes in childhood. It also gives a clinical perspective, presenting the findings of studies from the past 5 years that used this approach. The identification of true asthma endotypes may be a crucial step towards understanding their distinct pathophysiological mechanisms, which could ultimately lead to more precise prevention strategies, identification of novel therapeutic targets and the development of effective personalized therapies

Childhood cancer and nuclear power plants in Switzerland: a census-based cohort study

Background Previous studies on childhood cancer and nuclear power plants (NPPs) produced conflicting results. We used a cohort approach to examine whether residence near NPPs was associated with leukaemia or any childhood cancer in Switzerland

Risk factors for overweight and obesity after childhood acute lymphoblastic leukemia in North America and Switzerland: A comparison of two cohort studies.

BACKGROUND After childhood acute lymphoblastic leukemia (ALL), sequelae include overweight and obesity, yet with conflicting evidence. We compared the prevalence of overweight and obesity between ≥5-year ALL survivors from the North American Childhood Cancer Survivor Study (CCSS) and the Swiss Childhood Cancer Survivor Study (SCCSS) and described risk factors. METHODS We included adult childhood ALL survivors diagnosed between 1976 and 1999. We matched CCSS participants (3:1) to SCCSS participants by sex and attained age. We calculated body mass index (BMI) from self-reported height and weight for 1287 CCSS and 429 SCCSS participants; we then compared those with siblings (2034) in North America and Switzerland (678) siblings. We assessed risk factors for overweight (BMI 25-29.9 kg/m2 ) and obesity (≥30 kg/m2 ) using multinomial regression. RESULTS We found overweight and obesity significantly more common among survivors in North America when compared with survivors in Switzerland [overweight: 30%, 95% confidence interval (CI): 27-32 vs. 24%, 21-29; obesity: 29%, 27-32 vs. 7%, 5-10] and siblings (overweight: 30%, 27-32 vs. 25%, 22-29; obesity: 24%, 22-26 vs. 6%, 4-8). Survivors in North America [odds ratio (OR) = 1.24, 1.01-1.53] and Switzerland (1.27, 0.74-2.21) were slightly more often obese than siblings. Among survivors, risk factors for obesity included residency in North America (5.8, 3.7-9.0); male (1.7, 1.3-2.3); attained age (≥45 years: 5.1, 2.4-10.8); Non-Hispanic Black (3.4, 1.6-7.0); low household income (2.3, 1.4-3.5); young age at diagnosis (1.6, 1.1-2.2). Cranial radiotherapy ≥18 Gray was only a risk factor for overweight (1.4, 1.0-1.8); steroids were not associated with overweight or obesity. Interaction tests found no evidence of difference in risk factors between cohorts. CONCLUSIONS Although treatment-related risk for overweight and obesity were similar between regions, higher prevalence among survivors in North America identifies important sociodemographic drivers for informing health policy and targeted intervention trials

The Mre11-Rad50-Nbs1 complex mediates activation of TopBP1 by ATM

The activation of ATR-ATRIP in response to double-stranded DNA breaks (DSBs) depends upon ATM in human cells and Xenopus egg extracts. One important aspect of this dependency involves regulation of TopBP1 by ATM. In Xenopus egg extracts, ATM associates with TopBP1 and thereupon phosphorylates it on S1131. This phosphorylation enhances the capacity of TopBP1 to activate the ATR-ATRIP complex. We show that TopBP1 also interacts with the Mre11-Rad50-Nbs1 (MRN) complex in egg extracts in a checkpoint-regulated manner. This interaction involves the Nbs1 subunit of the complex. ATM can no longer interact with TopBP1 in Nbs1-depleted egg extracts, which suggests that the MRN complex helps to bridge ATM and TopBP1 together. The association between TopBP1 and Nbs1 involves the first pair of BRCT repeats in TopBP1. In addition, the two tandem BRCT repeats of Nbs1 are required for this binding. Functional studies with mutated forms of TopBP1 and Nbs1 suggested that the BRCT-dependent association of these proteins is critical for a normal checkpoint response to DSBs. These findings suggest that the MRN complex is a crucial mediator in the process whereby ATM promotes the TopBP1-dependent activation of ATR-ATRIP in response to DSBs

A Mechanistic Treatment of the Dominant Soil Nitrogen Cycling Processes: Model Development, Testing, and Application

The development and initial application of a mechanistic model (TOUGHREACT-N) designed to characterize soil nitrogen (N) cycling and losses are described. The model couples advective and diffusive nutrient transport, multiple microbial biomass dynamics, and equilibrium and kinetic chemical reactions. TOUGHREACT-N was calibrated and tested against field measurements to assess pathways of N loss as either gas emission or solute leachate following fertilization and irrigation in a Central Valley, California, agricultural field as functions of fertilizer application rate and depth, and irrigation water volume. Our results, relative to the period before plants emerge, show that an increase in fertilizer rate produced a nonlinear response in terms of N losses. An increase of irrigation volume produced NO{sub 2}{sup -} and NO{sub 3}{sup -} leaching, whereas an increase in fertilization depth mainly increased leaching of all N solutes. In addition, nitrifying bacteria largely increased in mass with increasing fertilizer rate. Increases in water application caused nitrifiers and denitrifiers to decrease and increase their mass, respectively, while nitrifiers and denitrifiers reversed their spatial stratification when fertilizer was applied below 15 cm depth. Coupling aqueous advection and diffusion, and gaseous diffusion with biological processes, closely captured actual conditions and, in the system explored here, significantly clarified interpretation of field measurements

Modeling acid-gas generation from boiling chloride brines

<p>Abstract</p> <p>Background</p> <p>This study investigates the generation of HCl and other acid gases from boiling calcium chloride dominated waters at atmospheric pressure, primarily using numerical modeling. The main focus of this investigation relates to the long-term geologic disposal of nuclear waste at Yucca Mountain, Nevada, where pore waters around waste-emplacement tunnels are expected to undergo boiling and evaporative concentration as a result of the heat released by spent nuclear fuel. Processes that are modeled include boiling of highly concentrated solutions, gas transport, and gas condensation accompanied by the dissociation of acid gases, causing low-pH condensate.</p> <p>Results</p> <p>Simple calculations are first carried out to evaluate condensate pH as a function of HCl gas fugacity and condensed water fraction for a vapor equilibrated with saturated calcium chloride brine at 50-150°C and 1 bar. The distillation of a calcium-chloride-dominated brine is then simulated with a reactive transport model using a brine composition representative of partially evaporated calcium-rich pore waters at Yucca Mountain. Results show a significant increase in boiling temperature from evaporative concentration, as well as low pH in condensates, particularly for dynamic systems where partial condensation takes place, which result in enrichment of HCl in condensates. These results are in qualitative agreement with experimental data from other studies.</p> <p>Conclusion</p> <p>The combination of reactive transport with multicomponent brine chemistry to study evaporation, boiling, and the potential for acid gas generation at the proposed Yucca Mountain repository is seen as an improvement relative to previously applied simpler batch evaporation models. This approach allows the evaluation of thermal, hydrological, and chemical (THC) processes in a coupled manner, and modeling of settings much more relevant to actual field conditions than the distillation experiment considered. The actual and modeled distillation experiments do not represent expected conditions in an emplacement drift, but nevertheless illustrate the potential for acid-gas generation at moderate temperatures (<150°C).</p