Chemical Evaluation of SrF2 Capsules in the Waste Encapsulation and Storage Facility

The Waste Encapsulation and Storage Facility (WESF) in Hanford, Washington, has sequestered both 137Cs and 90Sr in double-walled capsules for long-term storage and possible disposal. A recent study examined the chemical instabilities of cesium chloride capsules due to radioactive decay as well as possible accident scenarios if the capsules were breached. Similar circumstances could also exist for strontium fluoride capsules due to decay of the principal isotope, 90Sr. This analysis indicated that similar reducing conditions develop that would produce exothermic reactions with air if the capsules were breached. The WESF strontium capsules initially contain approximately 55% 90Sr, with the remainder being mostly 88Sr and minor quantities of other stable isotopes. The capsules also contain small amounts of other elements as impurities, chiefly sodium, barium, calcium, and rare earth elements. Over time, 90Zr is produced through radioactive decay: Sr 90 → Y 90 → Zr 90 .  Because the half-life of 90Y is relatively short (64 h), its inventory is quite small, and it is a negligible contributor to the capsule contents. Hence, over time the 90Sr is converted to a sizable inventory of the stable 90Zr. Chemically, the most stable species of zirconium is ZrF4; however, insufficient fluorine is available because of the stoichiometry of the SrF2 source material. Thus, decay produces reducing conditions within the capsule, just as was the case with CsCl capsules. A basic analysis was conducted to evaluate the seriousness of the internal conditions after significant decay has occurred. Ignoring impurities for a capsule initially filled with 55% 90Sr, after 50 y of decay it should contain 45% stable strontium, 16.4% 90Sr, and 38.6% 90Zr. To estimate the preferred chemical species, chemical equilibrium calculations were run using the FactSage code. The results indicate that almost all zirconium exists as ZrF2, a metastable species that could only form in the reducing conditions of the capsule. To assess the issue of safe handling of such sources, a second equilibrium calculation was run to simulate exposure of the contents to a large excess of air. The N2 is completely unreactive, as expected, but the O2 serves to further oxidize the zirconium according to the following reaction: 2 ZrF2 +O2 →ZrF4 +ZrO2 .  The enthalpy change for this reaction at 400 K is H = −389.3 kJ/mol, and speciation of remaining strontium is unchanged. If we consider that each capsule contains about 2.8 kg of SrF2 initially, then oxidizing all of the zirconium after 50 y decay produces approximately H = −4,515 kJ. We have not evaluated the rate of air ingress or reaction, which would undoubtedly take time and dissipate some of the heat generated. However, it is also possible that rapid air ingress would vaporize some contents (including undecayed 90Sr). Additional calculations were conducted to address the uncertainties in this scenario and verified the basic results. This analysis validates the current procedures that these capsules must be handled with care and should only be opened in a hot cell containing an inert atmosphere.