Basic Research Needs for Geosciences - Energetics Meetings and ...

GRAND CHALLENGE: COMPUTATIONAL THERMODYNAMICS OF COMPLEX FLUIDS AND SOLIDSCOMPUTATIONAL THERMODYNAMICS OF COMPLEX FLUIDS ANDSOLIDSABSTRACTNew modeling strategies for geochemical systems at the fundamental scale (atomic to singlepore) are required to analyze the long-term storage of greenhouse gases and nuclear waste. Thechemical complexity of the principal actinide components of spent nuclear fuel (e.g., thepresence of highly correlated and poorly screened f and d electrons) and the complex nature oftheir transport in aqueous formation fluids requires the development of simulation tools based oninteractions calculated directly from the electronic Schrödinger equation (i.e., first-principlesmethods). Further improvement of the efficiency of present methods based on the DensityFunctional Theory (DFT) and new higher level approximations to the solution of the Schrödingerequation will be required to provide quantitative estimates of the geochemical processes involvedin waste mobilization and migration. However, even with optimistic projections ofimprovements in performance of first-principles simulation tools, the time scales required foranalysis of these processes may be several orders of magnitude longer than can be achieved. Forthese situations strategies must be developed to extend the time frames of simulations whileretaining the chemical specificity of the materials involved.For CO 2 sequestration problems it is likely that very high accuracy empirical interactions at themolecular level could be developed from high level theoretical calculations and data.Nevertheless, to achieve the accuracy required for thermodynamic predictions under the extremeconditions expected in applications will require a large simulation effort and development ofhighly scalable free energy or Gibbs ensemble algorithms. Experimental data for mixed systemswill be needed to complement simulation approaches.Even with the development of fast solvers and methods to upscale the calculations, computedand experimental data will have to be incorporated in Equation of State (EOS) models forapplication to real problems (e.g., for use in hydrological simulators). EOS methods need to befurther developed to treat the reactive and multiphase chemistry of complex mixtures. For closedshell inert gas systems (e.g., H 2 O, CO 2 , N 2 , O 2 mixtures), simulation may be used to supplementexperimental data. However, for more chemically complex systems such as the actinides, theprincipal contribution of simulations will be to analyze complex solution speciation andoxidation-reduction chemistry. This information is essential to develop empirical EOS forparameterization with laboratory data. There is a critical lack of data in many systems, and it isessential that this information be expanded to facilitate development of more reliable models.EXECUTIVE SUMMARYStrategies are being developed to sequester both CO 2 and radioactive waste in geologicformations. An unusual feature of these containment strategies is the extraordinary length of timethat the integrity of the storage facilities must be maintained (hundreds of years for CO 2storage/sequestration and 10 6 years for nuclear waste storage). Since these time frames are ordersof magnitude longer than any laboratory simulations and the effects of many unpredictableBasic Research Needs for Geosciences: Facilitating 21 st Century Energy Systems 67