Chemical Evaluation of SrF2 Capsules in Waste Encapsulation Facility

Chemical Evaluation of SrF2Capsules in the Waste Encapsulation and Storage Facility Charles F. Weber (ORNL) and Karen Balo (UCOR) Presented at the 65thAnnual Meeting of the INMM July 23, 2024 Portland OR ORNL is managed by UT-Battelle LLC for the US Department of Energy

Background Waste Isolation and Storage Facility (WESF) Double-walled stainless steel capsules to isolate 137Cs and 90Sr Production reactor waste from 1970 s and 1980 s Stored as CsCl and SrF2 Harborview contamination incident motivation to evaluate Major studies of CsCl capsules C.F. Weber and D. G. Abrecht, Thermodynamic modeling to facilitate safe handling of CsCl sealed sources, J. Radioanal. Nucl. Chem. 331(12), 4905-11 (2022). C.F. Weber and D.G. Abrecht, Engineering Evaluation of Barium Buildup in a Decayed CsCl Sealed Source and Potential Impact for Cesium Release from a Breached Source, ORNL/TM-2022/2384, Oak Ridge National Laboratory (May 2022). K. Kincaid, D. Abrecht, S. Sasser, N. See, and C. Weber, A heat and mass transfer model for evaluation of damaged cesium chloride radiological sources, submitted for publication in Journal of Radioanalytical and Nuclear Chemistry Open slide master to edit 2 2

Concern for Safe Handling of SrF2Capsules Circumstances analogous to CsCl capsules Both CsCl and SrF2capsules stored in building at ORNL Building no longer in use, scheduled to be demolished Use of SrF2contents in Byproduct Utilization Program (BUP)-500 Radioisotope Thermoelectric Generator (RTG) Would require cutting open capsules Open slide master to edit 3 3

The Problem Radioactive Decay Radioactive decay changes stoichiometry of contents Nuclide Sr-90 Y-90 Zr-90 Half-life 28.6 y 64 h stable 90Sr 90Y 90Zr Initial loading is SrF2 Preferred forms of Decay daughters: YF3, ZrF4 Capsules are sealed, and no additional F atoms are available Result: as decay occurs over time, conditions inside the capsules are steadily reducing Exposure to atmospheric oxygen rapid exothermic reaction Open slide master to edit 4 4

Typical Capsule Contents (Storch, 2006) Initial Loading 8 Sr-90 Zr-90 Total Sr Mol in 1 kg Salt 6 Isotope Sr-90 Sr-88 Sr-87 Sr-86 mol in 1 kg salt 4.05 3.22 0.03 0.04 4 2 0 0 10 Time Since Capsule FIlling (y) 20 30 40 50 60 70 80 Capsules also include small impurity content: BaF2, NaF, CeF3, AlF3, FeF2, CaF2, CrF3 Capsules contents ~ 2.8 kg Open slide master to edit 5 5

Equilibrium Chemistry Calculated by FactSage Code using Ftsalt and FactPS databases Equilibria change with time, as more decay occurs Time (years) 0 1 2 5 10 50 Mol in 1 kg salt ZrF4 0 0.09590 0.19052 0.23581 0.23586 0.23611 SrF2 7.3456 7.2487 7.1541 6.8837 6.4745 4.5029 ZrF2 0 0 0 0.2252 0.6344 2.6062 YF3 103 0 1.010 0.986 0.917 0.812 0.308 ZrF2is a metastable species that will only form under such reducing conditions Open slide master to edit 6 6

Equilibrium Chemistry - Impurities Some impurities gradually give up F atoms to Zr Time (y) Fe FeF2 Fe + FeF2Pb Fe Fe Fe Fe Pb PbF2 Cd CdF2 Cd Cd Cd Cd Cd Mn MnF3 MnF2 MnF2 Mn Mn Mn Ni NiF2 Ni Ni Ni3Al Ni3Al Ni3Al Cr CrF3 CrF3 Cr + CrF2 Cr Cr Cr 0 1 2 5 10 20 Pb Pb Pb Pb All other impurities keep their original F atoms Open slide master to edit 7 7

Safety Concern for Handling and Transportation What happens if capsule is breached and air reaches contents? In extensive reduced state, exothermic reaction could occur: 2 ZrF2+ O2 ZrF4+ ZrO2 Oxidation of reduced impurities will also occur Excess heat generated could vaporize some contents, including undecayed Sr-90. Open slide master to edit 8 8

Chemistry Calculations of Air Oxidation Capsule contents subjected to large excess of air: Nominal contents: 7.3456 mol Sr (1 kg salt) Include 21 mol O2, 79 mol N2 Chemical equilibrium calculations run using FactSage code Principal species in fully-oxidized conditions: SrF2, ZrF4, ZrO2 Minor amounts of impurity oxides: BaCrO4, SrCrO4, MnO2 Several impurities oxidize as fluorides: FeF3, CeF3, CdF2, NiF2, PbF2 Open slide master to edit 9 9

Exothermic Oxidation Generates Considerable Heat 500 0 H (kJ/kg salt) -500 -1000 -1500 -2000 0 10 20 30 40 50 60 70 80 Time since capsule filling (years) Early years are endothermic After several decades, generated heat is significant Open slide master to edit 10 10

Possible Consequences For a capsule initially filled with 2.8 kg salt, oxidizing entire capsule after 50 y generates H = -3,386 kJ. Sufficient to vaporize significant amount of remaining SrF2 Consideration of impurities is conservative With no impurities, H = -4,515 kJ. This analysis based solely on equilibrium calculations Does not consider rate processes: reaction rates, oxygen diffusion, heat dissipation Recent rate analysis for CsCl capsules indicated considerable risk remains Open slide master to edit 11 11

Conclusions Analogous to evaluations for CsCl capsules, radioactive decay produces strongly reducing conditions inside SrF2capsules Considerable risk exists if SrF2capsules are breached in air Any dismantling or penetration of capsule walls should only be done in hot cell with inerted atmosphere Validates current procedures for safe handling Open slide master to edit 12 12