Abstract
Tungsten-188 is in widespread use in 188W(t1/2 = 69 d)/188Re(t1/2 = 16.9 h) biomedical generators. Oak Ridge National Laboratory has been providing this product to the world since 1999. At ORNL, 188W is produced via irradiation in ORNL’s High Flux Isotope Reactor (HFIR). Enriched 186W targets in the form of sintered metallic pellets or rings achieve a compact loading in the irradiation vessel, providing a high yield per unit target. The enrichment of the target is >90% 186W, and this isotope undergoes double neutron capture to produce the desired 188W product. While 188W is produced by neutron bombardment, 191Os(t1/2 = 15.4 d) is simultaneously produced as a by-product and expected to be separated from 188W by postirradiation treatment.
In the current processing pathway, the irradiated W metal rings are first converted into an oxide form of WO3 by heating the irradiated W metal target at 750°C in a quartz reaction vessel inside a vertical furnace under a constant flow of air. During heating, W metal reacts with oxygen in the air to produce WO3, which is soluble in 6 M NaOH for preparation of 188W product. This oxidation process also converts 188Os (the decay daughter of 188W) and 191Os (15.4 d, the irradiation produced byproduct) into OsO4, a highly volatile and toxic gas. The gaseous effluents driven from the quartz reaction vessel are passed through a scrubbing array to remove OsO4 before the air is discharged from the process.
This heterogeneous oxidation method simultaneously achieves goals of (1) converting metal target to a soluble oxide form and (2) removing volatile OsO4 away from the solid WO3 product by air flow and absorbing the harmful Os species by the scrubbing array. But this method has two potential problems as well: (1) O2 reacts with only W metal at high temperatures, not with W alloyed with other elements. The O2–W reaction will be retarded when formation of WRe or WC occurs, or even when a layer of non-W materials on the surface of the irradiated W rings.; (2) 100% absorption of OsO4 of high yield (>90%) from the reaction of Os + O2 is a strict requirement to the OsO4 scrubbing system--so NaOH scrubbers of a redundant size (2x 1.5 L) are in use for safety reasons.
To resolve above two potential problems, direct dissolution of the irradiated W metal target by a selected reagent is a preferred pathway to avoid heating step with generation of tremendous amount of volatile OsO4. Hydrogen peroxide (H2O2) is such a candidate to dissolve W in forms of either metal or alloys, although literature lacks information of solubilities of Re or Os in H2O2.
With experimental results of dissolving non-radioactive W, Re and Os in H2O2 under various conditions, this report illustrates a method of H2O2 dissolution for irradiated W target, with a complete dissolution of W and Re, but ≤10% dissolution of Os (converted into gaseous OsO4 and carried out into a scrubbing for absorption) during processing irradiated W target. The portion of undissolved Os can be separated from W solution by a follow up filtration step. Solubilities of W, Re and Os in H2O2 at a temperature range from 14° to 50°C are presented. And a dissolution rate of W metal per surface area of W metal in H2O2 is calculated based on results of dissolving a W metal cylinder of known surface area in H2O2 at room temperature without stirring.
