The initial program was patterned on the U.S. version, with the same basic concepts being studied. One test, Chagan test in January 1965, has been described as a "near clone" of the U.S. Sedan shot. Like Sedan, Chagan also resulted in a massive plume of radioactive material being blown high into the atmosphere, with an estimated 20% of the fission products with it. Detection of the plume over Japan led to accusations by the U.S. that the Soviets had carried out an above-ground test in violation of the Partial Test Ban Treaty, but these charges were later dropped.