The problem of doing it with fast fission neutrons is that only 1% of them are energetic enough to make 197hg and meanwhile 197au has a large absorption cross section for all the neutrons (especially thermal neutrons). So, the mercury just needs to be distilled often.

I created this before reading the paper. It actually looks legit but needs more time to cool off. In Theory it could be done with a fast fission reactor too (preferably plutonium) with fairly low burn up on the mercury coolant before you distill it

Less than 2700pCi/g