Design: Pressurized water
How it Works: Like today's reactors, these bathe enriched uranium fuel in water that absorbs heat to make steam.
Promise: Gen III+ pressurized-water reactors add "passive" safety mechanisms that cool the reactor if the plant loses power. For example, in an emergency, water flows from an extra tank above the reactor, driven by gravity.
Problems: Radioactive waste takes years to cool before it can be stored in underground repositories, which still don't exist.
Status: Mitsubishi-Westinghouse, which developed the design, has received approval from the U.S. Nuclear Regulatory Commission and has signed contracts to build six reactors in the U.S. and four in China.