A nuclear reactor has an incident once every 10.000 years. If we divide that with the number of reactors we get an accident every 30 or so years. (Harrisburg, Chernobyl, Fukusima, ligns up well with that)
Every time there is an accident, the public turns against nuclear for 5-10 years. Add to that that it takes a decade to plan and build a reactor, and you realize that once we double or triple the number of reactors, accidents will be so frequent, that the public wont stand for additional expansion within the time it takes to build more reactors.
We need 10x nuclear to solve global warming. Thats a major accident every 3 years or so. Yes, Nuclear can be made safer, but probably not by as much as needed. Even a country like Japan, who has high safety standards, and a historical heritage of nuclear fallout, has had an accident, and most new reactors are likely to be built by China who has less then stellar safety record.