it takes a few weeks to defuel, reprocess then refuel a reactor. pellet reactors would be down 50-80% of the time. not all parts of the core get same neutron flux. so you need to first put thorium in high n areas to capture them move them around to use them for power, all while messing with how much uranium is where. it's would be a major pain.  why liquid salt is preferred. it's easier to extract mix and refine to correct quantities in an even distribution. mostly has to do with the changing reactivity of thorium as it captures neutrons. with pure uranium you know it will have a nice downward reactivity curve so you can adjust the boron in the water moderator to keep it at about the same thermal output for the fuel rod life. with thorium it's fiddly. we could design a solid reactor to use it but it would either be a pebble design or have automatic robot arms to move stuff around internally and no current pwr has that.
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