PITTSBURGH -- In the garage of his house, Frank Sanns spends nights tinkering with one of his prized possessions: a working nuclear-fusion reactor.

Mr. Sanns, 51 years old, is part of a small subculture of gearheads, amateur physicists and science-fiction fans who are trying to build fusion reactors in their basements, backyards and home laboratories. Mr. Sanns, who owns a banquet hall here, believes he's on track to make fusion a viable power source.

"I'm...