I found a document (that can be purchased or read online) that talks about removing U-233 from the salts of the Molten-Salt Reactor Experiment, a liquid-fluoride reactor that was built at Oak Ridge National Lab and operated from 1965-1969.

Evaluation of the U.S. Department of Energy’s Alternatives for the Removal and Disposition of Molten Salt Reactor Experiment Fluoride Salts

I was aware of the issues that they had faced related to the disposition of the MSRE based on conversations I had had with some “old hands” at ORNL. Basically, it came down to the fact that when they shut down the MSRE in 1969, they thought they might be restarting it again at a future date, so they didn’t go through the full uranium decontamination process that they would have done for a complete shutdown. In that process, they would have fluorinated the salt to remove uranium as a gaseous hexafluoride, thus removing all the fissile material from the MSRE salt. They had already done this process previously when they removed the original uranium fuel (a mixture of U-238 and U-235) and replaced it with U-233 for experiments on that fuel.

But they didn’t fluorinate the salt when they shut down the MSRE. It just sat in the drain tanks. While the salt was liquid, any free fluorine that formed from radiolysis was reabsorbed into the salt as fluorides. But when the salt froze and fell below 150 C in temperature, then radiolysis led to the evolution of fluorine gas. That fluorine gas, in turn, acted like its own little fluorinator, liberating uranium as a gaseous hexafluoride. That UF6 then drifted through the pipes over the ensuing decades and became a concern–one that was not easily fixed by simply melting and fluorinating the original salt.

So things got complicated, and they’ve spent a lot of time remediating MSRE. A problem that would have been fixed it they had simply fluorinated the fuel at shutdown. But hindsight is always 20/20.