Define "we" and "running". While the US nuclear industry is admittedly a bit moribund at the moment, there are already many gen 3 plants in operation with passive safety features, and several gen 4 in late research and plans to build commercial plants. But the majority of plants in operation are gen 2. We could be doing so much better by just building out gen 3/3.5 plants today.

The big 3 nuclear disasters, Chernobyl (started construction in 1972), Three Mile ('67), and Fukushima ('67), were al really early gen 2, designed in the 60s.