I understand that PWR type reactors are ubiquitous because they are submarine reactors at a fundamental level. Shippingport was derived from submarine PWRs in 1957. However, there is another more simple type of fission reactor known as the Boiling Water Reactor. It is the second most common type of energy producing reactor around the world. It is too big for naval use, however. PWR and BWR dominate global nuclear energy production, with any other reactor types accounting for a much smaller sliver of the nuke pie. Is there a better way of fissioning atoms to make heat to boil water for steam? Are there better generation III+, or generation IV reactors that could be built that outperform current PWR and BWR plants? If so, what is preventing their construction? Also, would it ever be possible to skip the Carnot/Rankine cycles and utilize direct nuclear to make electricity?