12 August 1953 - Soviet 'RDS-6' test

RDS-6 device shown on the left

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.

Sloika (Слойка), the eponym of the bomb’s design

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 Teller-Ulam 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.

Yulii Borisovich Khariton (1904 – 1996) was the Soviet Union’s chief nuclear weapon designer.

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.

One year later, the Soviet Union drew level with its own “true” two-stage hydrogen bomb, codenamed RDS-37. A total of 456 of over 2,000 nuclear tests worldwide were conducted at the Semipalatinsk Test Site in modern-day Kazakhstan. More than a million residents were exposed to radiation. The region has reported a surge in cancer rates and birth defects.

Lake Chagan at the Semipalatinsk test site, result of a 140-kt underground nuclear explosion in 1965.

At the initiative of the Republic of Kazakhstan, the UN General Assembly decided in 2009 to declare 29 August as the International Day Against Nuclear Tests. It was on this day in 1991 that the Semipalatinsk Test Site was closed for good.

The door to nuclear testing, however, remains open. For the 1996 Comprehensive Nuclear-Test-Ban Treaty CTBT and its complete ban on nuclear testing to enter into force, the Treaty must be signed and ratified by all of the 44 Annex 2 States. These were the countries holding nuclear technology at the time of the Treaty’s negotiations. Eight have yet to sign and/or ratify: China, the Democratic People's Republic of Korea, Egypt, India, Iran, Israel, Pakistan and the United States.


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24 August - France goes thermonuclear