All things aren't understood in nuclear science. The theory is well established but the practice i.e. the actual implementations are still second rate. Just look at the history of uranium separation as an example. 100x reduction in energy separation costs from going to centrifuges and probably another 10x reduction by going to laser separation processes which might also be useable to separate plutonium from waste in the future.
As for reactors there was talk of high-temperature reactors to produce hydrogen which is a useful product for producing ammonia for fertilizer and explosives without using natural gas or petroleum but that will probably stop being developed now that NA has a lot of those resources. Most work will probably be in making reactors safer or cheaper. Not exactly exciting work.