This heat exchanger would be able to use high temperature coolants like liquid sodium and fissonable liquid coolants like liquid thorium salt

Just a small thing i would like to see and it may also add some new RBMK desings chanlanges that are interesting

Liquid sodium was only as good as it was because of the RBMK's different cooling implementation, causing sodium to dupe heat. A fixed version would most likely be pretty bad coolant, since it can only cool as much as the boiling/heating point of the fluid is.