all good ideas come round twice, back in the late '60s and early 70s the idea of a dissolving the fuel into circulating  molten salts was looked at by a team in the UK, though they were mainly looking art extracting heat by pumping molten lead.

(report from AAEW in 1974)

Awkward to start, but easier to keep running than solid fuel rods.
Mike.

In fact the fuel salt won't be circulated in Moltex's design, which is a key difference to the '60s designs.

Moltex foresees having the fuel salt contained in conventional fuel pins, with a significant modification: the fuel pins are vented at the top. This allows most fission products to be contained within the fuel salt, but a few (primarily noble gases) to bubble out through the secondary coolant salt.

In the 60s design with a circulating fuel salt which contained a wild mixture of elements and isotopes circulating through different temperature and pressure regimes, this is extremely difficult to manage so as to avoid unwanted corrosion/plating out/sedimentation/gassing out. Having a secondary coolant salt of defined and stable composition reduces the complexity (and hence safety/licensing challenge) vastly.

In fact the fuel salt won't be circulated in Moltex's design, which is a key difference to the '60s designs.

Moltex foresees having the fuel salt contained in conventional fuel pins, with a significant modification: the fuel pins are vented at the top. This allows most fission products to be contained within the fuel salt, but a few (primarily noble gases) to bubble out through the secondary coolant salt.

In the 60s design with a circulating fuel salt which contained a wild mixture of elements and isotopes circulating through different temperature and pressure regimes, this is extremely difficult to manage so as to avoid unwanted corrosion/plating out/sedimentation/gassing out. Having a secondary coolant salt of defined and stable composition reduces the complexity (and hence safety/licensing challenge) vastly.