Talk of a large-scale U.S. nuclear renaissance in the post-Three Mile Island era has long been stalled by the high cost of new nuclear power plants, the challenges of safeguarding weapons-grade nuclear material, and the radioactive lifespan of much nuclear waste, which can extend far beyond 10,000 years. But a growing contingent of scientists believe an alternative nuclear reactor fuel—the radioactive metal called thorium—could help address these problems, paving the way for cheaper, safer nuclear power generation. Three to four times more plentiful than uranium, today's most common nuclear fuel, thorium packs a serious energetic punch: A single ton of...