Thermodynamic Modeling of the In-Drift Chemical Environment of a Potential High-Level Nuclear Waste Repository Using OLI Software by Roberto T. Pabalan Center for Nuclear Waste Regulatory Analyses San Antonio, Texas 23rd OLI Users Conference Parsippany, New Jersey October 5 6, 2005
Outline of Presentation Background on geologic disposal of high-level nuclear waste (HLW) Thermodynamic modeling of repository seepage water evaporation Thermodynamic modeling of salt deliquescence Summary 2
Geologic Disposal of U.S. HLW Potential geologic disposal of HLW at Yucca Mountain (YM), Nevada 70,000 MTHM for disposal ~90% commercial spent nuclear fuel ~10% defense HLW, Naval reactor fuel, Department of Energyowned reactor fuel http://www.ocrwm.doe.gov/ymp/about/tour/index.shtml Engineered and natural barriers to provide isolation for at least 10,000 years 3
Engineered Barrier System Potential YM repository design includes waste packages (WP) and drip shields (DS) Constructed of corrosion resistant materials Expected by DOE to last at least 10,000 years Aqueous corrosion expected to be primary degradation mechanism Mode and rate of aqueous corrosion will depend on water chemistry and temperature 4
In-Drift Water Chemistry Water chemistry will be altered by coupled thermal-hydrological-chemical processes Evaporation processes could form potentially corrosive brines and deposit salts on the WP/DS surfaces Inorganic salts in dusts entrained with ventilation air could deposit on the WP/DS surfaces Deliquescence of inorganic salts could form potentially corrosive brines Deliquescence Brine Seepage H 2 O Waste Package Condensed H 2 O Drip Shield 5
Thermodynamic Modeling of In-Drift Chemical Environment Using OLI Software Evaporation of dilute seepage waters Potential range in chemistry of waters contacting DS and WP (corrosive species and corrosion inhibitors) StreamAnalyzer Ver. 1.3 Deliquescence behavior of salts and salt mixtures Time and temperature of brine formation Deliquescence Environmental Simulation Program (ESP) Ver. 7.0, Mixed Solvent Electrolyte (MSE) model H 2 O Brine Evaporation H 2 O Waste Package 6
Thermodynamic Modeling of Evaporation StreamAnalyzer vs. Experimental Data Seawater evaporation study (McCaffrey et al., 1987) Morton solar production facility for table salt, halite (Great Inagua Island, Bahamas ) Chemical compositions of evaporating seawater at the plant were determined Production process results in concentration factor ~40 relative to seawater Chemical compositions at higher concentration factors (to ~73) were determined through laboratory evaporation experiments 7
Thermodynamic Modeling of Evaporation StreamAnalyzer 1.3 vs. McCaffrey et al. Data 100 10 2 10 Na + 10 1 1 Mg 2+ 10 0 Cl - Molality 0.1 K + Molality 10-1 10-2 SO 4 2-0.01 0.001 Ca 2+ 10-3 10-4 HCO 3-0.0001 1 10 100 Degree of Evaporation 14 10-5 1 10 100 Degree of Evaporation 1.0 ph or Ionic Strength (molal) 12 10 8 6 4 2 ph Ionic strength a(h 2 O) calc. 0.8 0.6 0.4 0.2 Activity of Water 0 0.0 1 10 100 Degree of Evaporation 8
Thermodynamic Modeling of Evaporation YM Seepage Water Evaporation Temperature = 110 C; Pressure = 0.85 atm Chemistry of YM unsaturated zone porewaters used as input Assumed seepage water is similar to ambient YM porewaters Neglected interactions with in-drift engineered materials 9
Thermodynamic Modeling of Evaporation YM Seepage Water Evaporation (Cont d) YM porewater chemistry data (+) available from U.S. Geol. Survey (Yang et al. 1996,1998,2003) Only selected compositions ( ) were used (29 out of 156 in database) Supplemented by chemical divide approach 2- SO 4 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 Ca 2+ 0.0 1.0 0.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.0 0.9 0.8 Three brine types: calcium-chloride, neutral, and alkaline 0.7 0.6 0.5 0.4 0.3 0.2 0.1 - HCO 3 2- + CO 3 10
Thermodynamic Modeling of Evaporation Results of YM Seepage Water Evaporation Brine Type 2 4 6 8 10 12 ph 10-12 10-11 10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 10 0 10 1 10 2 Ca 2+ (moles/kg H 2 O) 0 2 4 6 8 10 12 14 16 18 Na + (moles/kg H 2 O) Brine Type 0 5 10 15 20 25 0.001 0.01 0.1 1 10 100 10-7 10-6 10-5 10-4 10-3 10-2 10-1 10 0 Cl - (moles/kg H 2 O) NO 3 - (moles/kg H 2 O) F - (moles/kg H 2 O) 11
Thermodynamic Modeling of Evaporation Results of YM Seepage Water Evaporation (Cont d) Brine Type 10-7 10-6 10-5 10-4 10-3 10-2 10-1 10 0 F - (moles/kg H 2 O) 0 5 10 15 20 25 Cl - (moles/kg H 2 O) Brine Type Window of Susceptibility to Localized Corrosion of Alloy 22 Some brines have high Cl and F concentration Most have high ratio of corrosion inhibitors (NO 3, SO 2 4, HCO 3, CO 2 3 ) to corrosive Cl 10 0 10 1 10 2 10 3 10 4 Cl - /Σ(inhibitors*) *(NO 3,SO 4,HCO 3,CO 3 ) Note: High Cl /NO 3 ratio is due to formation of CaNO 3+ and NaNO 3 aqueous complexes, which have uncertain thermodynamic data Chemistry information abstracted into a performance assessment code as probability distribution functions (PDFs) 12
Deliquescence of Salts Hygroscopic salts will sorb moisture from humid air (deliquesce) and form potentially corrosive brines At YM, deliquescence relative humidity (DRH) could determine the time and temperature during which brines can form on the DS and WP Relative Humidity (%) 100 75 50 25 WP Temp DRH = 50% Drift RH DRH = 30% 0 0 10 1 10 2 10 3 10 4 Time (years) Susceptibility Susceptibility to Aqueous to Aqueous Corrosion Corrosion Calculated In-Drift Relative Humidity and Waste Package Surface Temperature versus Time 200 150 T 2 >T 1 T 1 100 50 Temperature ( o C) 13
Deliquescence of Salts (Cont d) DRH is a function of salt composition and temperature Experimental data available on DRH of single salts (0 to ~100 C; Greenspan, 1977) Sparse data on binary and multicomponent salt mixtures at temperatures relevant to a HLW geologic repository (>80 C) 14
Thermodynamic Modeling of Deliquescence Deliquescence relative humidity, DRH, is given by DRH = ph 2 O sat /ph 2 O o where ph 2 O sat is the vapor pressure of a saturated salt solution and ph 2 O o is the vapor pressure of pure water Values of ph 2 O sat and ph 2 O o calculated using ESP Ver. 7.0 (MSE model) 15
Thermodynamic Modeling of Deliquescence StreamAnalyzer vs. Experimental Data NaCl-NaNO 3 (90 o C) KNO 3 -NaNO 3 (90 o C) Deliq. Relative Humidity (%) 100 80 60 40 20 30 40 50 60 70 80 90 100 Temperature ( C) KCl NaCl NaNO 3 NaCl+NaNO 3 +KNO 3 % Relative Humidity 80 75 70 65 60 55 (a) Calculated (ESP 7.0) Measured (Craig et al., 2004) Liquid NaCl + Liquid NaNO Solid (NaCl + NaNO 3 ) + Liquid 3 50 0.0 0.2 0.4 0.6 0.8 1.0 X-NaNO 3 % Relative Humidity 70 60 50 40 (b) Calculated (ESP 7.0) Measured (Craig et al., 2004) Liquid KNO 3 + Liquid NaNO 3 + Liquid Solid (KNO 3 + NaNO 3 ) 0.0 0.2 0.4 0.6 0.8 1.0 X-NaNO 3 Measured DRH (Yang et al., 2005) vs. ESP 7.0 (MSE) values Measured DRH (Craig et al., 2004) vs. ESP 7.0 (MSE) values. (a) NaCl-NaNO 3, (b) KNO 3 -NaNO 3 16
Thermodynamic Modeling of Deliquescence Salt Mixtures in the System Na +,K + //Cl,NO 3 KCl NaCl 100 72.6 80 80.6 60 25 C 71.3 KCl 70.1 68.9 77.3 75.3 70.967.6 NaCl KCl 100 80 90 C 67.5 NaCl 66.5 40 66.2 KNO 3 64.9 64.0 65.1 62.7 20 63.5 NaNO 3 66.2 0 67.4 0 20 40 60 80 100 KNO 3 NaNO 3 60 KCl 40 61.5 60.4 52.9 57.9 55.452.749.7 49.7 61.5 56.7 NaCl 100 80 20 60 48.3 53.3 54.3 KNO 46.7 51.5 3 49.1 46.3 40.1 43.2 NaNO 3 40.5 0 41.2 40 0 20 40 60 80 100 KCl 140 C KCl 63.5 62.3 60.0 57.7 54.5 51.7 46.4 NaCl NaCl KNO 3 Solubility (mole %) of two-or three-salt mixtures in the quaternary system Na +,K + //Cl,NO3 calculated using ESP 7.0 (MSE). The calculated DRH at selected compositions are superimposed on the curves. NaNO 3 41.3 20 44.5 42.1 40.4 34.9 32.9 42.9 KNO 28.1 37.3 40.0 3 24.0 32.9 25.5 NaNO 0 13.0 3 13.4 0 20 40 60 80 100 KNO 3 17 NaNO 3
Summary The performance of engineered barriers in the potential YM HLW repository is dependent on in-drift water chemistry OLI software has been applied to thermodynamic modeling of in-drift chemical environment Evaporation of seepage waters Deliquescence of salt mixtures 18
Acknowledgment The work presented here was performed by the Center for Nuclear Waste Regulatory Analyses (CNWRA) for the U.S. Nuclear Regulatory Commission (NRC), Office of Nuclear Material Safety and Safeguards, Division of High-Level Waste Repository Safety, under Contract No. NRC 02-02-012. This presentation is an independent product of the CNWRA and does not necessarily reflect the view or regulatory position of the NRC. 19