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Ab initio rigid water : effect on water geometry, ion hydration, and thermodynamics

Proposed for publication in Physical Chemistry Chemical Physics (PCCP).

Rempe, Susan

We investigate the liquid structure, ion hydration, and some thermodynamic properties associated with the rigid geometry approximation to water by applying ab initio molecular dynamics simulations (AIMD) with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional at T = 320 K. We vary the rigid water geometry in order to locate a class of practical water models that yield reasonable liquid structure and dynamics, and to examine the progression of AIMD-predicted water behavior as the OH bond length varies. Water constrained at the optimal PBE gas phase geometry yields reasonable pair correlation functions. The predicted liquid phase pressure, however, is large ({approx}8.0 kbar). Although the O-H bond in water should elongate when transferred from gas to the condensed phase, when it is constrained to 0.02, or even just 0.01 {angstrom} longer than the optimal gas phase value, liquid water is predicted to be substantially overstructured compared to experiments. Zero temperature calculations of the thermodynamic properties of cubic ice underscore the sensitivity toward small variations in the O-H bond length. We examine the hydration structures of potassium, chloride, and formate ions in one rigid PBE water model. The results are in reasonable agreement with unconstrained AIMD simulations.

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TYPE Journal Article YEAR 2005
OSTI

Evaluation of microbial activity for long-term performance assessments of deep geologic nuclear waste repositories

Proposed for publication in the ASR'2004 Proceeding in Japan.

Wang, Yifeng

Microorganisms are ubiquitous in subsurface environments and play a major role in the biogeochemical recycling of various elements. In this paper, we have developed a general approach for a systematic evaluation of microbial impact on the long-term performance of the repository. We have demonstrated that data on microbial population alone are not sufficient for the evaluation of microbial impact on repository performance and a sensible approach for such evaluation must be based on the consideration of environmental constraints on microbial reaction pathways. We have applied our approach to both the Yucca Mountain (YM) repository and the Waste Isolation Pilot Plant (WIPP). We have demonstrated that the effect of microbial activity on the near-field chemistry in the Yucca Mountain repository is negligible because of limited nutrient supply and harsh environmental conditions created by waste emplacement. Whereas for the WIPP, we have shown that, due to the presence of a large quantity of organic materials and nutrients in the wastes, a significant microbial activity can potentially be stimulated and its impact on repository performance can be evaluated with carefully designed incubation experiments coupled with performance assessment calculations. The impact of microbial gas generation on disposal room chemistry in the WIPP can be mitigated by introducing MgO as a backfill material.

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TYPE Journal Article YEAR 2005
OSTI
Results 86251–86275 of 99,299
Results 86251–86275 of 99,299