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

GRAND CHALLENGE: COMPUTATIONAL THERMODYNAMICS OF COMPLEX FLUIDS AND SOLIDSsubsurface processes (e.g., seismic events) cannot be simulated in the laboratory, highly accuratemodels of geochemical interactions that span these time scales must be developed.The principal mechanism for the transfer of nuclear waste materials into the biosphere is viaaqueous phases contacting the containment canisters. The transfer of toxic species from the solidwaste form to formation fluids is controlled by processes such as: speciation of radioactivesolutes (e.g., UO 2 2+ (aq) and Pu(IV) hydroxides) in the fluid phase; solubility of associatedmineral phases; sorption of these species onto formation minerals; their sorption and subsequenttransport on colloidal particles in the formation fluid. While the observed behavior of geologicalprocesses is on time scales of hundreds to thousands of years, the processes that are the origin ofthis behavior occur on atomic time and length scales of tenths of picoseconds (10 -12 s) andseveral angstroms (10 -8 cm). In addition, these processes retain the atomic specificity (chemistry)characteristics of their position in the periodic table (e.g., the complex chemistry of uranium andthe transuranic elements). As a relevant example, spent nuclear fuel is 95% UO 2 ; the formaloxidation state of U in this species is +4 (U(IV)). However, the most abundant uranium oxidationstate in aqueous solution species containing U is the formal +6 valence (U(VI) as UO 2 2+ (aq)).The oxidation process that changes the insoluble (immobile) U(IV) species into soluble(transportable) U(VI) solution species is dependent on poorly understood atomic scale processesat the solution/solid interface.The critical challenge of the model development required to insure the safety of waste storagestrategies is both:• To predict the complex chemistry of these system on the fundamental atomic scale• To provide reliable tools to translate these behaviors to the many orders of magnitude largerscales of geological storage problems without loss of their fundamental atomic characterTo reliably predict behavior of the actinide (uranium, thorium, plutonium, etc.) waste products ofnuclear power production over the large range of conditions possibly encountered in a storagefacility requires a theory based at the most fundamental level on the electronic Schrödingerequation. Many evaluations of this equation are required in any method that expands length andtime scales. This is a grand challenge computational problem. However, remarkableimprovements in the methods available to solve the Schrödinger equation, and the computationalpower available to implement such methods, have been made. In addition, new experimentalmethods based on advanced synchrotron light sources and new neutron sources are becomingavailable that will provide highly informative data at the atomic level to calibrate the accuracy ofpredictions. Considerable development still needs to be done to provide the level of accuracyrequired and to develop simulation tools that will be able to fully exploit the computationalpower of the new leadership class of computers soon to be in place.The chemical issues involved with the prediction of the outcome of CO 2 sequestration are moremanageable. The chemistry of these systems is typically much less complex, and interactionpotentials at the molecular level can be developed using existing quantum chemistry methods.However, even for very commonly encountered systems such CO 2 -H 2 O, there are many regionsof pressure, temperature and composition (P-T-X) variables required for storage applications thathave not been explored, and the interactions of these materials with geological formations is68 Basic Research Needs for Geosciences: Facilitating 21 st Century Energy Systems