This isn't about radioactivity at all. It's about the millions of pounds of mercury used at the Y-12 plant at Oak Ridge,[1] resulting in a lot of low-level mercury contamination.

[1] https://ehss.energy.gov/ohre/new/findingaids/epidemiologic/o...

I am quite confused about the mercury contamination being mentioned, to be honest.

There's a huge amount of mercury contamination at Y-12. The DoE wants to clean it up. The DoE asks for proposals for building an on-site plant capable of cleaning it up. And that's somehow... bad?

What's supposed to be the alternative here: shipping thousands of tonnes of contaminated dirt to an offsite processing plant? Dumping it all in the nearest lake? Burying it and pretending it doesn't exist?

There are dozens of examples of government agencies laser-focusing on one specific solution, but I just don't see how this case is bad enough that it warrants being explicitly mentioned as an example of the DoE not allowing for enough alternatives.

Wait the giant deadly ball of mercury that is highly radioactive for neutron studies? Or is this another mercury thing there?

It's highly likely that the waste is mixed waste, meaning rad + mercury. Much tougher to treat than just rad or mercury.