It's not the size, it's the maturity. As you correctly criticize, Superphenix was non-functional much of the time: there were major engineering issues which had

It's not the size, it's the maturity. As you correctly criticize, Superphenix was non-functional much of the time: there were major engineering issues which hadn't been solved beforehand. This is the character of a prototype. >Thorium-based reactors are more mature than FBR as far as I understand Thorium reactors are even less mature than FBRs. Although that's a very loose category, you should distinguish between the extremely different reactor types which burn thorium, such as high temperature gas-cooled reactors (including pebble beds), which were the only commercial thorium reactors (for a short time); and the theoretical molten salt reactors, which unlike HTGRs are breeder reactors. For added confusion, some commercial (both LWR and HWR) reactors can run partially on thorium, but are not self-sufficient without uranium or plutonium fuel.