Chemical Durability of Nuclear Waste Glasses Joe Ryan Pacific Northwest National Laboratory Presented for Prof. Russell J Hand Dept. of Materials Science & Engineering University of Sheffield Joint ICTP-IAEA International School on Nuclear Waste Vitrification Trieste, Italy September 25 th , 2019
The most important criterion Nuclear waste disposal requires: Controlled release of disposed radionuclides Long-term control: 10 5 to 10 6 years To demonstrate long-term durability of glass, Glass artifact images used courtesy of the Corning Museum of Glass we must understand the mechanisms that govern radionuclide release over all time scales Enable the reliance on the native durability of the waste form Mechanisms behind rate laws must be Decrease necessity for engineered barrier systems Well understood Universality to various repository environments Scientifically defensible Increased public confidence and better data for regulatory International consensus desirable approval Strachan et al.. Applied Geochemistry What do we get? 41 , 107-114 (2014) 2
Durability testing for nuclear waste glasses Vienna et al. 2001 What do we want to know? How quickly do species leach out of the glass? What happens physically to the glass during corrosion? How can we assess this? Um… stick it in water? Yes, but for how long? And… What factors do we need to consider? Intrinsic: Extrinsic: Glass composition pH and chemistry of the attacking solution Glass structure / homogeneity Temperature Processing considerations (T f , stress, shape, etc.) Time Internal radiation Experimental conditions (SA/V, mixing, etc.) 3
Glass is not glass is not glass… Waste glasses are designed to meet specific physical, chemical, and regulatory compliance Regulatory Chemical Compliance constraints Durability Phase Glasses are designed specifically for waste Conductivity Stability Viscosity compositions to be immobilized , examples: Loading US tank waste primarily composed of cold chemicals and Cost with high composition variability and low radioactivity Radiation French UOx HLW is primarily fission products with Stability Melter consistent composition and high radioactivity Corrosion US ILAW (Immobilized Low-Activity Waste) is designed for high alkali content and high throughput Some UK glasses need to accommodate high Mg contents Russian (and others) alumino-phosphate systems Vienna 2014 & The Simpsons 4
Yes, but… Boro-alumino-silicate glasses do behave similarly in similar conditions … so let’s put them in some water! 5
How glass corrodes (dilute conditions) Surface Evolution Solution Rates at 90°C and pH 10 Alteration typically ~30 g m -2 d -1 This equates to ~1cm corrosion in only ~30 years But this never happens …thank goodness Stage I Pristine Time Behavior “ “? Glass 6
Ok, maybe not THAT much water… When glass ions accumulate in solution, various “interesting things” begin to occur … so let’s put them in STILL water! Can be REALLY slow... Accelerate the test by Increased surface area (powders) Increased temperature Getters / Complexants Seeds 7
How glass corrodes (static conditions) Transitional Solution Behavior Alteration Interdiffusion Interdiffusion Porous Gel Layer Pristine Glass Stage I Time Behavior 8
How glass corrodes (static conditions) Transitional Solution Stage II Behavior Behavior Secondary Alteration Alteration Products Porous Gel Layer Interdiffusion Stage I Pristine Time Behavior Glass 9
How glass corrodes (static conditions) Stage III Solution Behavior(?) Transitional Stage II Behavior Secondary Behavior Alteration Products Alteration Cracked Porous Gel Layer Interdiffusion Gin et al. GCA, 151, 68–85(2015) Stage I Time Behavior Pristine 10 Glass
So what’s going on? rate = f(precip),f(gel), f(RF), f(dissol), f(IEX), f(Comp) Solution The behavior of glass at all times is generally Precipitation, Near-field, believed to result from a combination of many Cryst. Alt. f(precip), and Stage III: mechanistic processes at work: Products • Dissolution of the network Porous Alteration Alteration Phase • Solid-state diffusion within otherwise Products Transport: f(gel), undisturbed glass (gel layer) • Transformation of glass into gel at the active f(RF), Reaction front: reaction front • Transport limitations in gel/altered regions Interphase f(dissol), Dissolution: • Condensation/Precipitation reactions Ion-exchanged to form alteration phases from solution glass f(IEX) Solid-state Diffusion: Explaining long-term behavior Pristine • . Glass f(Comp) requires a combination of mechanisms! Glass Composition: 11
Dissolution Mechanisms
Example Reaction Rate Model (without transport) Forward dissolution rate, r f = the rate at which glass dissolves into solution at specific values of the T and pH in the absence of back reactions Dissolution rate most likely to be directly impacted by structure and composition of glass 1 r f potential E Q r v k a exp a 1 +other terms i i 0 H RT K g r i = normalized glass dissolution rate E a = apparent activation energy, J mol -1 (based on element i ), g m -2 d -1 R = gas constant, J mol -1 K -1 r f = forward glass dissolution rate, g m -2 d -1 T = absolute temperature, K v i = stoichiometric coefficient for element i in glass Q = ion-activity product of rate controlling species k 0 = intrinsic rate constant, g m -2 d -1 K g = pseudo-equilibrium constant for glass a H+ = hydrogen ion activity σ = reaction order (Temkin coefficient) η = pH power law coefficient (dependent on pH regime) 13
Isolation of Individual Effects Single-pass flow-through test (SPFT, ASTM C1662) can be used to measure effects of individual parameters Measure impacts of pH, T, [H 4 SiO 4 ] and [Al(OH) 4 - ] Avoid feed-back effects by high flow rate/surface area (q/s) Abraitis et al. 2000 Neeway et al. 2017 14
pH and Temperature Impacts Hydrolysis rate depends on: 2 s)] -5 Bond length and bond angle log(Dissolution Rate) [g/(m 2 d)] 1 log(Dissolution Rate) [mol/(m (stretched O-Si-O bonds favor hydrolysis) -6 Site protonation (high or low pH) 90 o C -7 0 Energetics (temperature) 70 o C Arrhenius fit seems best for -8 50 o C temperature profile -1 25 o C -9 Some debate on how best to associate full pH dependence curve -10 -2 2 4 6 8 10 12 pH(T) 𝑆 = 𝑙 � 𝑓𝑦𝑞 −𝐹 �� � � � + 𝑓𝑦𝑞 −𝐹 �� � � + 𝑓𝑦𝑞 −𝐹 ��� � � � 𝑏 � 𝑏 �� 𝑆𝑈 𝑆𝑈 𝑆𝑈 DM Strachan, (2017) Geochimica et Cosmochimica Acta, 219, 111-123 15
Modeling the Data for Individual Glass Log[ k 0 ] = 8.37 ± 0.92 gm -2 d -1 Measure r f of glass with systematic variation in pH and T η = 0.396 ± 0.060 Fit data to linear equation: E a = 81.6 ± 6.1 kJmol -1 R 2 = 0.983 log[ ] e log[ r ] log[ k ] pH E RMSE = 0.141 f 0 a RT 16
Glass Composition Effects on Forward Rate 19 glasses all measured by SPFT with systematic variation in pH (7 to 13) and T (23° to 90°C) Include broad range of compositions (US HLW glasses, US LAW glasses, International glasses) Vienna et al. (2018) npj Materials Degradation 2 , 22 17
Simultaneously Fit r f to pH, T, and Composition Model explaining 90% of variation in log[ r f ] data obtained with no composition effects (R 2 fit = 0.896, R 2 val = 0.894, RMSE = 0.323) Three glasses have noticeably higher log[ r f ] Composition effects only found in log[k 0 ] term Composition effects model shows most significant composition effect is estimated fraction tetrahedra from [4] B ( f [4] B) Effect non-linear, best modeled by step-function change 18
End Result log( ) e below threshold 7.09 0.421 pH 76,200 ( ) T RT 2 1 log[ r ( g m d )] f log( ) e above threshold 7.86 0.421 pH 76,200 ( ) T RT 3 3 3 3 3 90°C 90°C 90°C 2 2 2 2 2 70°C 70°C 90°C 90°C 1 1 1 1 1 70°C 70°C 70°C log[r f , g m -2 d -1 ] log[r f , g m -2 d -1 ] log[r f , g m -2 d -1 ] log[r f , g m -2 d -1 ] log[r f , g m -2 d -1 ] 40°C 40°C 0 0 0 0 0 40°C 40°C 40°C -1 -1 -1 -1 -1 23°C 23°C 23°C 23°C -2 -2 -2 23°C -2 -2 -3 -3 -3 -3 -3 -4 -4 -4 -4 -4 7 7 8 8 9 9 10 10 7 11 11 8 12 12 9 10 11 7 7 12 8 8 9 9 10 10 11 11 12 12 pH(T) pH(T) pH(T) pH(T) pH(T) 19 Vienna et al. 2018
Forward Rate Parameter Correlation � � �� � � � � � Because there are three parameters with only two variables (T and pH), parameters are correlated: [log k 0 , η ] = 30% [log k 0 , E a ] = 81% [ E a , η ] = 30% Although there are some differences in forward rate, the general behaviors are the same for all compositions 20
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