EQ3/6 software maintenance and support summary

EQ3/6 is a software package for modeling chemical interactions in aqueous systems of geologic and engineering interest, such as water/rock, water/nuclear waste, and water/nuclear waste/rock. It is being used for a broad range of applications for the Yucca Mountain Site Characterization Project (YMSCP), including predictions of mineralogical changes in the altered zone, man-made materials investigations, and calculations of the long-term release of radionuclides from a variety of waste forms. Version 7.2a was the first qualified version of this software (certified on Aug. 17, 1994). Version 7.2b followed on Aug. 18, 1995 and is the most recent qualified version; it differs from version 7.2a only in that defects noted in the qualification report (Kishi, 7/12/94) were resolved. The present report describes the software maintenance and support activities that were carried out for the Version 7 line of the software in FY97. The most important of these activities is maintaining a system for the logging, documenting, and resolving software defects. This is required by the QARD (Supplement 1) in order for the software to remain certified. Other maintenance activities are necessary to retain functionality as computer hardware, operating systems, programming languages, and compilers change. In FY97, 12 software defects were logged and resolved, and two more were logged and awaiting resolution. These ranged in nature from the trivial to the serious. The corrected software will be released as version 7.2c in the first quarter of FY98. A version 8 line of totally rewritten code in modern Fortran, restructured to support new functionality, and with new capabilities for ion exchange modeling, pressure corrections, and redox disequilibrium, was completed to a baseline level in FY95. Subsequent activities required to complete qualification were not funded in FY96 and FY97. However, in FY97, each line of the software (version 7 and version 8) has been checked whenever a defect has been discovered in the other. A beta release of version 8.0 was made available to selected users in FY97. Qualification of version 8.0 may follow in FY98, depending on funding and direction. If this occurs, the certified software in the version 7 line will likely be retired and maintenance and support activities exclusively focused on the version 8 line

Letter report: status on code maintenance (EQ3/6)

EQ3/6 is a software package for geochemical modeling of aqueous systems, such as water/rock or waste/water rock. It is being developed for a variety of applications in geochemical studies for the Yucca Mountain Site Characterization Project. Version 7.2a was the first version of this software to be certified for use in quality- affecting work (originally issued for use in non-quality-affecting work only on 12/28/93; certified on S/17/94). In the past year, the Version 7 line software has been maintained while the new Version 8 line has been developed. In this period, sixteen defect reports have been logged and resolved. Corrected software is being released as Version 7.2b. Defect reporting and resolution for the Version 7 line will continue until all released versions in this line are retired, perhaps six months to a year after Version 8.0 is released later this year. The Version 7 software is written in Fortran 77, technically speaking, but incorporates many aspects of older Fortran. The Version 8 software is written in a much more modern Fortran, technically somewhere between Fortran 77 and Fortran 90. Future code maintenance activities will include a more complete move to Fortran 90, as well as continued maintaining of defect reporting and resolution

The Standard Chemical-Thermodynamic Properties of Phosphorus and Some of its Key Compounds and Aqueous Species: An Evaluation of Differences between the Previous Recommendations of NBS/NIST and CODATA

The aqueous chemistry of phosphorus is dominated by P(V), which under typical environmental conditions (and depending on pH and concentration) can be present as the orthophosphate ions H{sub 3}PO{sub 4}{sup 0}(aq), H{sub 2}PO{sub 4}{sup -}(aq), HPO{sub 4}{sup 2-}(aq), or PO{sub 4}{sup 3-}(aq). Many divalent, trivalent, and tetravalent metal ions form sparingly soluble orthophosphate phases that, depending on the solution pH and concentrations of phosphate and metal ions, can be solubility limiting phases. Geochemical and chemical engineering modeling of solubilities and speciation requires comprehensive thermodynamic databases that include the standard thermodynamic properties for the aqueous species and solid compounds. The most widely used sources for standard thermodynamic properties are the NBS (now NIST) Tables (from 1982 and earlier; with a 1989 erratum) and the final CODATA evaluation (1989). However, a comparison of the reported enthalpies of formation and Gibbs energies of formation for key phosphate compounds and aqueous species, especially H{sub 2}PO{sub 4}{sup -}(aq) and HPO{sub 4}{sup 2-}(aq), shows a systematic and nearly constant difference of 6.3 to 6.9 kJ {center_dot} mol{sup -1} per phosphorus atom between these two evaluations. The existing literature contains numerous studies (including major data summaries) that are based on one or the other of these evaluations. In this report we examine and identify the origin of this difference and conclude that the CODATA evaluation is more reliable. Values of the standard entropies of the H{sub 2}PO{sub 4}{sup -}(aq), HPO{sub 4}{sup 2-}(aq), and PO{sub 4}{sup 3-}(aq) ions at 298.15 K and p{sup o} = 1 bar were re-examined in the light of more recent information and data not considered in the CODATA review, and a slightly different value of S{sub m}{sup o}(H{sub 2}PO{sub 4}{sup -}, aq, 298.15 K) = 90.6 {+-} 1.5 J {center_dot} K{sup -1} mol{sup -1} was obtained

Hydrothermal growth kinetics of Np(IV) oxide

Toulouse, France All previous knowledge leading to this estimate is of NpO2(c) is indirect, based on thermodynamic cycles. The phase itself has heretofore not been observed as a precipitate from aqueous solution. Recent attempts (e.g., Nitsche et al., 1993; Efurd et al., 1996) to establish solubility controls on Np in oxidizing groundwaters (including J-13 groundwater) starting from high concentrations (i.e., supersaturation) have shown the formation of one or both of two Np(V) phases, NaNpO2CO3:3.5H2O(c) (with variable stoichiometry) and Np2O5(c). These are both highly soluble, yielding Np concentrations on the order of 1 x 10 -4 to 1 x 10 -3 molal, in accord with existing thermodynamic data for these phases. No evidence was found of the formation of NpO2(c). Under reducing conditions, experiments (Rai et al., 1987 and references cited therein) have shown the formation of Np(IV) polymer, which may be viewed as a hydrated form of NpO2. It is orders of magnitude less soluble than the Np(V) phases, but still orders of magnitude more soluble than NpO2(c). No undersaturation experiments with NpO2(c) are known to have been performed. However, both NpO2(c) and Np(IV) polymer are known to be difficult to dissolve. We have hypothesized that NpO2(c) is simply a phase that is slow to form at low temperature. In this regard, it would be analogous to such minerals as quartz, dolomite, and hematite. It is well known that it is difficult to impossible to demonstrate the formation of such minerals in low temperature experiments on feasible time scales. However, it is possible to demonstrate their formation and measure the kinetics of the process by conducting experiments at elevated temperatures, generally in the range of 150-300 o C (e.g., Rimstidt and Barnes, 1980; Sibley et al., 1984). By developing kinetic models, it should be possible to estimate the appropriate time scales on which such solids might impose solubility constraints on natural groundwaters. To test our hypothesis, we reacted solutions of 1 x 10 -4 molal NpO2 + (balanced with Cl-) at 200 o C in closed Parr bombs (with air present). Some runs were conducted in teflon-lined reactors, others in titanium reactors with passivated surfaces. The pH was not buffered, and EQ3/6 calculations indicated that the initial solutions were supersaturated with respect to NpO2(c) and undersaturated with respect to other possible Np solids. In the initial runs, the starting 25 o C pH was 6-7. Periodically the reactors were cooled, opened, and the Np concentration and pH were measured. The Np concentration slowly dropped (Fig. 1), and the pH dropped to ~4.2. A very fine brown precipitate was observed. XRD analysis shows an eight-line match with NpO2(c), validating our hypothesis. Other experiments have been conducted with higher initial pH, and in which the original pH has been periodically restored by the addition of small amounts of base. These show a more rapid drop in dissolved Np (Fig. 1), indicating that the rate of precipitation is pH dependent. The precipitates are being characterized by XRD, EXAFS, and SEM. Additional experiments at 150, 250, and 300 o C are currently planned to allow the development of a kinetic model that can be extrapolated to lower temperature. The use of a mixed flow reactor is also planned in future work. Figure 1. Neptunium molality as a function of time in 200 o C experiments. Top curve (squares): initial (25 o C) pH 6.5, no attempt to restore the original pH. Middle curve (circles): initial pH of 8, pH readjusted. Bottom curve (diamonds): initial pH of 10, pH readjusted

Combine Studies Pertaining to the Solubility of Neptunium in Oxidizing Aqueous Systems

The report combines two separate studies dealing with different aspects of the behavior of neptunium in oxidizing aqueous systems. The goal of both studies is to obtain a better understanding of what would control the concentration of neptunium in oxidizing groundwaters upon release from a geologic high-level waste repository. {sup 237}Np has a very long half-life, and consequently tends to appear as a component of concern in long-term dose calculations. Part A examines the formation of NpO{sub 2} [Np(IV) oxide] from aqueous Np(V) under oxidizing conditions, using elevated temperature as a means of accelerating putative slow kinetics. According to thermodynamic data, NpO{sub 2} should be the stable Np solid, even though Np is generally thought to be mainly some form of Np(V) under oxidizing conditions. The first observed precipitation of this solid from aqueous solution under any conditions is reported. This result suggests, but does not prove, that long-term very slow formation of NpO{sub 2} might be an important long-term control on neptunium migration. Even at 200{sup o}C, the kinetics can be sluggish on typical experimental time scales. Though not reported in Part A, a similar situation may exist for Pu, and NpO{sub 2} and PuO{sub 2} may potentially form solid solutions. Under reducing conditions, UO{sub 2} might join in. Part B examines the interaction of Np(V) with the uranium mineral schoepite. Is neptunium incorporated into the crystal structure as a solid-solution component, or does it just sorb onto the mineral surface? The results presented here suggest that it basically just sorbs onto the surface, which would be a less effective process in limiting neptunium migration. This is not a surprising result, as schoepite is a relatively simple solid of U(VI). There is no obvious means to compensate for the charge difference between U(VI) and Np(V) without introducing unfavorable energetics. However, the results reported here do not rule out a possible substitution of Np(VI) in schoepite or other U(VI) minerals. This may be significant especially at higher pH, where the formation of Np(VI) species is more favored relative to Np(V). Both studies reported here suggest that there is much to learn about the actual long-term behavior of Np in the geologic environment