Tungsten-188 is in widespread use in the 188W(t1/2 69 d)/188Re(t1/2 16.9 h) biomedical generator. Oak Ridge National Laboratory (ORNL) has been providing this product to the world since 1999. At ORNL, W-188 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 byproduct requiring separation from 188W by post irradiation treatment. In the current processing pathway, the irradiated W metal pellets or rings are first converted into an oxide form of WO3 by heating the irradiated W metal target at 750oC 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 later purification process. This oxidation process also converts 188Os (the decay daughter of 188W) and 191Os (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 two goals: 1). converting metal target to a soluble oxide form and 2). separating volatile OsO4 away from the solid WO3 product, although harmful 191OsO4 is unfavorable but being taken care of by the down road scrubbing array. In the past twenty years the existing OsO4 scrubbing array served well in preventing OsO4 from being released into the environment, until July of 2020 when a minor amount of 191Os was found to have been released into the environment which resulted in a standby of building 4501 from July 2020 to February 2021.
Since October 20 of 2020 a team of researchers from groups of RSTD and NEFD were organized to work for the following tasks:
• Fully understand the root causes of the July incident of 191Os release and create a redesigned 191Os scrubbing array to eliminate the possibility of Os-191 release in future W-188 processes
• In Phase-1 tests, select a correct air (O2) flow rate in W-188 process, that provides sufficient oxygen for the oxidation of tungsten metal, while still allowing sufficient resonance time for 191Os removal by the scrubbers.
• In Phase-2 tests, confirm the efficiency of the redesigned scrubbing array to absorb excessive amount of non-rad OsO4 (generated from Os metal powder) at the selected air flow.
• In Phase-3 tests, simulate the hot cell W-188 process by heating pressed non-rad W-Os pellets in a quartz vessel of new design and the new array under operation parameters selected in Phase-1 and -2.
• Complete a new operation procedure for W-188 process in hot cell and a TM report as a summary of the Os Mitigation Project.
This TM report summarizes the investigation on root causes from technical aspects in July 191Os release and the correspondent improvement towards the redesigned 191Os scrubbing system. The report presents the efforts made on selection of optimal operational parameter with the new scrubbing array and the quartz vessel of new design, based on experiment data obtained in Phase-1, -2 and -3 tests. Further renovations for W-188 process and additional improvement of W target treatment will also be discussed.