Princeton University physicists reported last week that they have taken an important step toward building a reactor that can efficiently extract energy from nuclear fusion.

Dale M. Meade of the Princeton Plasma Physics Research Laboratory said the Tokamak Fusion Test Reactor has reached the point at which it can produce as much energy as it consumes.

Unlike the atom-splitting fission reactors currently used in nuclear power plants, fusion is a theoretically safer process that harvests energy by forcing atomic nuclei together to form heavier nuclei.

The Tokamak reactor combines atoms of deuterium, a heavy form of hydrogen, to produce helium atoms, releasing energy in the process.

Deuterium gas inside the reactor's doughnut-shaped vacuum vessel is heated until its atoms break up into plasma -- a mixture of nuclei and free electrons.

Further heating triggers the fusion of pairs of deuterium nuclei. In tests, plasma inside the Tokamak reactor has reached temperatures of 400 million degrees centigrade, a world record.

By 1993, the researchers hope to try to use the reactor to fuse deuterium and tritium nuclei. If the combination is successful, it is expected to boost energy output 300-fold.

They cautioned that commercial use of fusion is still many years away.

Meade reported the results at a conference in Crystal City sponsored by the International Atomic Energy Agency.