In 1955, the United States, the United Kingdom, Canada, France, and the Soviet Union began negotiations on ending nuclear weapons testing. As negotiators struggled over differences regarding inspections, the Soviet Union and the United States suspended nuclear tests.
The moratorium lasted funtil September 1961.
In August 1961, the Soviet Union announced its intention to resume atmospheric testing, and over the next three months it conducted 31 nuclear tests, It exploding the largest nuclear bomb in history—4,000 times more powerful than the bomb dropped on Hiroshima.
American testing resumed in April 1962.
Source: JFK Library