Precipitation-Front Modeling: Issues Relating to Nucleation and Metastable Precipitation in the Planned Nuclear Waste Repository at Yucca Mountain, Nevada

Precipitation-Front Modeling: Issues Relating to Nucleation and Metastable Precipitation in the Planned Nuclear Waste Repository at Yucca Mountain, Nevada J.A. Apps and E.L. Sonnenthal Lawrence Berkeley National Laboratory, Berkeley, CA, jaapps@lbl.gov. 1 INTRODUCTION The focus of the presentation is on certain aspects concerning the kinetics of heterogeneous reactions involving the dissolution and precipitation of unstable and metastable phases under conditions departing from thermodynamic equilibrium. These aspects are particularly rele- vant to transient thermal-hydrological-chemical (THC) processes that will occur as a result of the emplacement of radioactive waste within the Yucca Mountain Repository. Most impor- tant of these is a phenomenon commonly observed in altering soils, sediments and rocks, where less stable minerals precipitate in preference to those that are more stable, referred to as the Ostwald Rule of Stages, or the Ostwald Step Rule. W. Ostwald (1897) described the phenomenon characterizing his rule (as cited in Schmeltzer et al., 1998), thus: “...in the course of transformation of an unstable (or metastable) state into a stable one the system does not go directly to the most stable conformation (corresponding to the modification with the lowest free energy) but prefers to reach intermediate stages (corresponding to other metastable modifications) having the closest free energy to the initial state”. This phenomenon is so widespread in natural geochemical systems, particularly under hydrothermal or low temperature conditions, that few geochemical parageneses involving the subcritical aqueous phase can be described without invoking the Ostwald Rule of Stages. Commonly observed systems where this phenomenon occurs include carbonates, silica, clay minerals, iron and manganese oxides, iron sulfides and zeolites (Morse and Casey, 1988). Simulations involving natural or anthropogenically modified reactive chemical transport must therefore be consistent with field observations describable by the Ostwald Rule. Geochemists have long been familiar with the Ostwald Rule, but, with one exception (Steefel and Van Cappellen, 1990), have not incorporated the underlying chemical principles justifying the Rule in reactive chemical transport simulations, other than through arbitrary fixes involving the suppression of the thermodynamically more stable phases, and by prohib- iting the re-dissolution of minerals. Another issue relating to mineral metastability is the contribution of interfacial free en- ergy to the total free energy of a geochemical system. The interfacial free energy contribu- tion is trivial for crystal sizes in excess of 1 micrometer. However, the alteration of soils and sediments entails both the dissolution of finely crystalline products of diagenesis and hetero- geneous nucleation and precipitation of new phases. The latter phases are commonly micro- crystalline or amorphous, with substantial contributions of surface free energy to the total Gibbs free energy of the phase. Such contributions must be taken into account when model- ing the chemical evolution of such systems, as they stabilize metastable phases and can mod- ify aqueous species concentrations by up to two orders of magnitude. This condition is espe- cially relevant to anthropogenically driven geochemical processes involving extreme levels of supersaturation where nucleation processes are dominant. Furthermore, by a process known as Ostwald Ripening, larger crystallites, usually possess a lower surface free energy