The Economist has an article summarizing a few possible next-generation fission reactor technologies. They include the Supercritical water-cooled reactor (SCWR), the Very High Temperature Reactor (VHTR), the Sodium-cooled fast reactor (SFR), the Gas-cooled fast reactor (GFR), the Lead-cooled reactor (LFR), and the Molten-salt reactor (MSR). Most promise higher efficiency than conventional pressurized water reactors, largely because they run at a higher temperature. Some are also capable of using more esoteric forms of fuel. For instance, the MSR can use thorium once it has been ‘seeded.’
The article doesn’t give too much consideration to the many challenges facing the nuclear industry: cost, chief among them. Given how opaque the costs of nuclear are, it is hard to know whether existing reactor technologies are really cost-competitive with renewables now, much less untested new variants.