Some considerations on the dependence to numerical schemes of Lagrangian radionuclide transport models for the aquatic environment.

Lagrangian models present several advantages over Eulerian models to simulate the transport of radionuclides in the aquatic environment in emergency situations. A radionuclide release is simulated as a number of particles whose trajectories are calculated along time and thus these models do not require a spatial discretization (although it is always required in time). In this paper we investigate the dependence of a Lagrangian model output with the grid spacing which is used to calculate concentrations from the final distribution of particles, with the number of particles in the simulation and with the interpolation schemes which are required because of the discrete nature of the water circulation data used to feed the model. Also, a Lagrangian model may describe the exchanges of radionuclides between phases (liquid and solid), which is done in terms of transition probabilities. The dependence of these probabilities with time step is analyzed as well. It was found that the optimum grid size used to calculate concentrations should be carefully checked, and that temporal interpolation is more significant than spatial interpolation to obtain a more accurate solution. A method to estimate the number of particles required to have a certain accuracy level is proposed. Finally, it was found that for low sediment concentrations and small radionuclide kd, exact equations for the transition probabilities should be used; and that phase transitions introduce a stability condition as in Eulerian models.

Volumetric properties of human islet amyloid polypeptide in liquid water.

The volumetric properties of human islet amyloid polypeptide (hIAPP) in water were studied in a wide temperature range by computer simulations. The intrinsic density rho(p) and the intrinsic thermal expansion coefficient alpha(p) of hIAPP were evaluated by taking into account the difference between the volumetric properties of hydration and bulk water. The density of hydration water rho(h) was found to decrease almost linearly with temperature upon heating and its thermal expansion coefficient was found to be notably higher than that of bulk water. The peptide surface exposed to water is more hydrophobic and its rho(h) is smaller in conformation with a larger number of intrapeptide hydrogen bonds. The two hIAPP peptides studied (with and without disulfide bridge) show negative alpha(p), which is close to zero at 250 K and decreases to approximately -1.5 x 10(-3) K(-1) upon heating to 450 K. The analysis of various structural properties of peptides shows a correlation between the intrinsic peptide volumes and the number of intrapeptide hydrogen bonds. The obtained negative values of alpha(p) can be attributed to the shrinkage of the inner voids of the peptides upon heating.

Fukushima 137Cs releases dispersion modelling over the Pacific Ocean. Comparisons of models with water, sediment and biota data.

A number of marine radionuclide dispersion models (both Eulerian and Lagrangian) were applied to simulate 137Cs releases from Fukushima Daiichi nuclear power plant accident in 2011 over the Pacific at oceanic scale. Simulations extended over two years and both direct releases into the ocean and deposition of atmospheric releases on the ocean surface were considered. Dispersion models included an embedded biological uptake model (BUM). Three types of BUMs were used: equilibrium, dynamic and allometric. Model results were compared with 137Cs measurements in water (surface, intermediate and deep layers), sediment and biota (zooplankton, non-piscivorous and piscivorous fish). A reasonable agreement in model/model and model/data comparisons was obtained.

Intrinsic thermal expansivity and hydrational properties of amyloid peptide Abeta42 in liquid water.

Volumetric and conformational properties of the amyloid beta(1-42) peptide (Abeta(42)) are studied in relation to the properties of hydration water in a wide temperature range by computer simulations. The apparent volume of Abeta(42), which is the change in the total volume of the solution due to the presence of Abeta(42), shows a quite different temperature dependence below and above T approximately 320 K. The apparent thermal expansion coefficient alpha(app)(Abeta(42)) is about 1.5x10(-3) K(-1) at T320 K. By evaluation of the thermal expansivity of hydration water, the intrinsic expansivity of the biomolecule in liquid water was determined for the first time. The intrinsic thermal expansion coefficient of Abeta(42) is found to be negative: alpha(int)(Abeta(42)) approximately -0.8x10(-3) K(-1). The negative thermal expansion coefficient of Abeta(42) can be attributed to its rubberlike (entropic) elasticity and/or to a decreasing number of intrapeptide hydrogen bonds. Upon heating, Abeta(42) transforms from an extended chain with a significant content of alpha-helices to a compact coil with noticeable content of beta-structures. A hydrogen-bonded spanning network of hydration water envelops Abeta(42) homogeneously at low temperatures but breaks into an ensemble of small water clusters upon heating via a percolation transition, whose midpoint is close to the temperature, where the apparent volume of Abeta(42) changes its temperature behavior. The mutual relation between the volumetric properties of Abeta(42), its conformational properties, and the properties of the hydration water is discussed.

The marine kd and water/sediment interaction problem.

The behavior of marine distribution coefficients is analyzed with the help of numerical experiments and analytical solutions of equations describing kinetic models for uptake/release of radionuclides. The difficulties in measuring true kd in a marine environment perturbed by an external radionuclide source are highlighted. Differences between suspended matter and bed sediment kd are analyzed. The performances of different kinetic models (1-step/2step; single-layer/multi-layer) are studied in model/model and model/experiment comparisons. Implications for the use of models to assess radioactive contamination after an emergency are given; as well as recommendations when kd data are compiled in order to create a useful database.

Modelling of marine radionuclide dispersion in IAEA MODARIA program: Lessons learnt from the Baltic Sea and Fukushima scenarios.

State-of-the art dispersion models were applied to simulate (137)Cs dispersion from Chernobyl nuclear power plant disaster fallout in the Baltic Sea and from Fukushima Daiichi nuclear plant releases in the Pacific Ocean after the 2011 tsunami. Models were of different nature, from box to full three-dimensional models, and included water/sediment interactions. Agreement between models was very good in the Baltic. In the case of Fukushima, results from models could be considered to be in acceptable agreement only after a model harmonization process consisting of using exactly the same forcing (water circulation and parameters) in all models. It was found that the dynamics of the considered system (magnitude and variability of currents) was essential in obtaining a good agreement between models. The difficulties in developing operative models for decision-making support in these dynamic environments were highlighted. Three stages which should be considered after an emergency, each of them requiring specific modelling approaches, have been defined. They are the emergency, the post-emergency and the long-term phases.

Thermodynamic properties of hydration water around solutes: effect of solute size and water-solute interaction.

Density, thermal expansion, and heat capacity of hydration water around various model solutes have been studied as a function of temperature and pressure. The radius of spherical structureless solute was varied from 3 to 10 Å, and the water-solute interaction was varied from strongly hydrophobic to strongly hydrophilic. Thermodynamic properties of hydration water around solutes were compared with those near the inner surface of large cylindrical pores with a radius of 25 Å. For all systems studied, the energy of water-water interactions per molecule in the hydration shell is found less negative than in the bulk. This is the result of the missing neighbor effect, which leads to the liquid density depletion even near strongly hydrophilic surfaces. This effect enhances near concave surfaces and diminishes near convex surfaces, which causes an essential increase of hydration water density around small solutes. Liquid density depletion near surfaces is accompanied by an essential increase of the thermal expansion coefficient of hydration water: at low temperatures, it exceeds the bulk value even near strongly hydrophilic surfaces. The constant volume heat capacity of hydration water is close to the bulk value; it is practically not sensitive to water-surface interaction and slightly increases upon decreasing solute size. The constant pressure heat capacity of hydration water increases upon weakening water-surface interaction and is practically not sensitive to solute size. Increase of the constant pressure heat capacity of water near hydrophobic surfaces is found to be the result of the increasing thermal expansion coefficient.

Thermal stability of the hydrogen-bonded water network in the hydration shell of islet amyloid polypeptide.

The effect of temperature on the connectivity of hydrogen bonds in the hydration shells of the islet amyloid polypeptides (IAPPs) is studied by means of computer simulations. The hydrogen-bonded network of hydration water homogeneously envelopes a peptide at low temperature and breaks into an ensemble of small clusters upon heating. This thermal break occurs via a percolation transition, which is not found to be sensitive to the chemical modifications of IAPP (IAPP with and without a disulfide bridge, human and rat IAPP). The radius of gyration of IAPP starts to increase when the hydration water network breaks upon heating. The fluctuations of the number of intra-peptide hydrogen bonds show negative correlation with the fraction of molecules in the largest cluster of hydration water. The thermal stability of the network of hydration water is enhanced upon increasing number of intra-peptide hydrogen bonds, which makes the peptide surface more hydrophobic. The thermal stabilities of the hydrogen-bonded water networks in the hydration shells of IAPPs and of several other biomolecules are found to be rather similar: the network breaks between 300 and 330 K, i.e., in the temperature interval where the biological activity of living organisms is maximal.

Thermal expansivity of amyloid beta(16-22) peptides and their aggregates in water.

Temperature dependence of the volumetric and structural properties of Abeta(16-22) peptides (wild type and pathogenic forms) and their aggregates in water was studied by simulations. The intrinsic thermal expansion coefficient alpha(p) of peptides was evaluated by taking into account the difference between the volumetric properties of hydration and bulk water. Single peptides show mainly positive values of alpha(p) that correlates with the increasing number of intrapeptide hydrogen bonds upon heating. Negative values of alpha(p) found for large peptide aggregates may be attributed to the shrinking of voids inside aggregates with increasing temperature or to their rubber-like elasticity. The peptide surface exposed to water becomes more hydrophobic with increasing aggregate size that appears in decreasing density of hydration water and evidences a hydrophilic character of aggregation.

Effect of confinement on the liquid-liquid phase transition of supercooled water.

We report on an observation of the phase transition between two liquid phases of supercooled confined water in simulations. The temperature of the liquid-liquid transition of water at zero pressure slightly decreases due to confinement in the hydrophobic pore. The hydrophilic confinement affects this temperature in the opposite direction and shifts the critical point of the liquid-liquid transition to a higher pressure. As a result, in a strongly hydrophilic pore the liquid-liquid phase transition becomes continuous at zero pressure, indicating the shift of its critical point from negative to a positive pressure. These findings indicate that experimental studies of water confined in the pores of various hydrophobicity/hydrophilicity may clarify the location of the liquid-liquid critical point of bulk water.

Thermal breaking of spanning water networks in the hydration shell of proteins.

The presence of a spanning hydrogen-bonded network of water at the surface of biomolecules is important for their conformational stability, dynamics, and function. We have studied by computer simulations the clustering and percolation of water in the hydration shell of a small elastinlike peptide (ELP) and the medium-size protein staphylococcal nuclease (SNase), in aqueous solution. We have found that in both systems a spanning network of hydration water exists at low temperatures and breaks up with increasing temperature via a quasi-two-dimensional percolation transition. The thermal breaking of the spanning water network occurs at biologically relevant temperatures, in the temperature range, which is close to the temperature of the "inverse temperature transition" of ELP and the unfolding temperature of SNase, respectively.

Water in nanopores. I. Coexistence curves from Gibbs ensemble Monte Carlo simulations.

Coexistence curves of water in cylindrical and slitlike nanopores of different size and water-substrate interaction strength were simulated in the Gibbs ensemble. The two-phase coexistence regions cover a wide range of pore filling level and temperature, including ambient temperature. Five different kinds of two-phase coexistence are observed. A single liquid-vapor coexistence is observed in hydrophobic and moderately hydrophilic pores. Surface transitions split from the main liquid-vapor coexistence region, when the water-substrate interaction becomes comparable or stronger than the water-water pair interaction. In this case prewetting, one and two layering transitions were observed. The critical temperature of the first layering transition decreases with strengthening water-substrate interaction towards the critical temperature expected for two-dimensional systems and is not sensitive to the variation of pore size and shape. Liquid-vapor phase transition in a pore with a wall which is already covered with two water layers is most typical for hydrophilic pores. The critical temperature of this transition is very sensitive to the pore size, in contrast to the liquid-vapor critical temperature in hydrophobic pores. The observed rich phase behavior of water in pores evidences that the knowledge of coexistence curves is of crucial importance for the analysis of experimental results and a prerequiste of meaningful simulations.

Phase coexistence and dynamic properties of water in nanopores.

The dynamical properties of a confined fluid depend strongly on the (spatially varying) density. Its knowledge is therefore an important prerequisite for molecular-dynamics (MD) simulations and the analysis of experimental data. In a mixed Gibbs ensemble Monte Carlo (GEMC)/MD simulation approach we first apply the GEMC method to find possible phase states of water in hydrophilic and hydrophobic nanopores. The obtained phase diagrams evidence that a two-phase state is the most probable state of a fluid in incompletely filled pores in a wide range of temperature and level of pore filling. Pronounced variations of the average and local densities are observed. Subsequently, we apply constant-volume MD simulations to obtain water diffusion coefficients and to study their spatial variation along the pore radius. In general, water diffusivity slightly decreases in a hydrophilic pore and noticeably increases in a hydrophobic pore (up to about 40% with respect to the bulk value). In the range of gradual density variations the local diffusivity essentially follows the inverse density and the water binding energy. The diffusivity in the quasi-two-dimensional water layers near the hydrophilic wall decreases by 10 to 20% with respect to the bulk value. The average diffusivity of water in incompletely filled pore is discussed on the basis of the water diffusivities in the coexisting phases.