Magnox: un-enriched fuel, designed for weapons, low operating temperature.
AGR: too complicated, can't be refuelled without complete shutdown.
PWR has emerged as the only viable design, and even that is too complicated compared to solar+batteries.
Basically, plumbing is the enemy. Every joint is a potential failure. If what's in the pipe is radioactive or corrosive or explosive (in the case of molten sodium, all three), that's extra bad. So there's tremendous cost of inspections.
Molten sodium as a cooling liquid always struck me as a "courageous decision" (to quote Yes Minister). Is there really nothing else that can do the job?
It's an annoying tradeoff: if you want something that's liquid at 600C, it pretty much either has to be a metal, or heavily pressurized (i.e. the PWR, the pressurized water reactor everyone has converged on).
Or you go for a gas. But then you're not able to use water as a moderator, so it either has to be a "fast neutron" design (which the few liquid sodium reactors are) or use a graphite moderator (flammable, lifespan issues with neutron cracking).
The unexplored corner is https://en.wikipedia.org/wiki/Gas-cooled_fast_reactor : plumbing is "safe" apart from being incredibly hot, but you still have to deal with fast neutrons smashing up the reactor from the inside on an atomic level.
Magnox: un-enriched fuel, designed for weapons, low operating temperature.
AGR: too complicated, can't be refuelled without complete shutdown.
PWR has emerged as the only viable design, and even that is too complicated compared to solar+batteries.
Basically, plumbing is the enemy. Every joint is a potential failure. If what's in the pipe is radioactive or corrosive or explosive (in the case of molten sodium, all three), that's extra bad. So there's tremendous cost of inspections.