The Soviet Union on Wednesday exploded a nuclear device underground at its Semipalatinsk range in Central Asia, the official Tass news agency said. Tass said the test blast had a yield of up to 20 kilotons. "The test was conducted to check the results of the research into the physics of nuclear explosions," it said. "The radiation situation in and outside the test area is normal," the news agency reported. No further information was provided. The atomic bomb dropped Aug. 6, 1945 on Hiroshima, Japan, had an explosive force of 20 kilotons.