At the time, it was a great design, but now there are ideas that blow it out of the water. The liquid salt reactors that are being prototyped soon are said to be roughly 96% efficient (Popular Science), compared to the 5 - 10% of most reactors still in use. Not only that, they would run on nuclear waste, are are far less likely to melt down. Actually, this design was thought up a few decades ago, and was only recently brought out of the mothballs.