12 August 1953 – Soviet ‘RDS-6’ test via CTBTO

RDS-6, nicknamed “Joe 4” by the United States, was the fifth nuclear test conducted by the Soviet Union. With a force equivalent to 400 kilotons of TNT (around 30 times the Hiroshima bomb’s yield), it was the country’s largest test until then. RDS-6 was part of the Soviet Union’s efforts to catch up with the United States, which had detonated its first thermonuclear device with the ‘George’ test two years earlier in May 1951.

The weapon used a design known as the Sloika model (Слойка, named after a type of layered puff pastry) in which fission and fusion fuel were “layered”. Soviet lead physicist Yulii Khariton estimated that 15 to 20% of its yield was based on fusion and the rest on fission.

Because the potential yield of the Sloika design was limited to around one megaton (roughly 75 times the yield of the Hiroshima bomb), it was later abandoned. The multi-stage TellerUlam hydrogen bomb design on the side of the United States, in contrast, could be scaled up indefinitely in yield. Soviet propaganda nonetheless proclaimed RDS-6 to be a “true” hydrogen bomb, asserting that it was – contrary to U.S. hydrogen bombs at the time – deliverable by air.

This spurred U.S. scientists to redouble their own efforts, leading to the high-yield test series Operation Castle. This series took place in the spring of 1954 at the Bikini atoll in the Marshall Islands and included the three most powerful tests ever carried out by the United States. The largest, Castle Bravo, yielded 15 megatons and created the worst radiological disaster in U.S. history. Several of the Marshall Islands’ atolls, including one where U.S. servicemen were stationed, were blanketed with fallout. The Japanese fishing vessel ‘Lucky Dragon Number 5’ was also heavily contaminated, causing the death of at least one crewman. This incident created an international uproar and a diplomatic crisis with Japan.

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