The Soviet Union detonated a rare double nuclear explosion today, two underground blasts 7 1/2 seconds apart, the Swedish Seismological Institute reported.

The institute, pinpointed the blast sites in the semipalatinsk area in Siberia.The first, at 11:14 p.m. EDT, measured 5.6 on the open-ended Richter scales, while the second recorded 6.6.

A spokesman said he could not recall whether the institute had ever recorded two such powerful explosions within such a short period.

He declined to speculate on the meaning of the blasts, but said, "They could have taken place at two different but near places."

They were very heavy shocks. They belong to the big group of explosions," said institute researcher Klaus Meyer. "It was the first double explosion in my experience and I checked with the Ministry of Defense because it was so surprising."

The Ritcher scale is a measure of ground motion as recorded by seismographs. Every increase of one number means a tenfold increase in magnitude. In the case of earthquakes, a Richter measurement of 5 means the tremor could cause considerable damage in the local area, and a 6 could mean severe damage. A reading of 7 denotes a "major quake."

An American authority on nuclear explosions said his preliminary guess would bethat the blasts were connected with some earth-moving exercise in Siberia.