1953

RDS-6s

Soviet RDS-6s (“Joe-4”) nuclear test

Despite being called a hydrogen bomb, about four-fifths of Joe-4’s energy came from fission — and that is exactly why American scientists argued about what to call it.

Semipalatinsk, 12 August 1953

The Soviet Union detonated RDS-6s on a tower at the Semipalatinsk test site in the Kazakh SSR on 12 August 1953. The West labelled the test “Joe-4.”

The yield was about 400 kilotons — roughly twenty times the Nagasaki bomb. The blast vaporised the steel tower and left a crater and glassy material around the epicentre.

A layer cake, not a two-stage bomb

RDS-6s was a single-stage, layered Sloika design combining fusion fuel and fissionable material, developed around ideas from Andrei Sakharov. It was not a Teller–Ulam-style two-stage radiation-implosion weapon.

Its distinction from the American Ivy Mike apparatus was deliverability rather than yield: at about 4.5 metric tons, RDS-6s was compact enough to fit a bomb casing and be carried by aircraft.

The argument over the word

Later technical reconstruction attributed approximately 80 percent of the energy to fission and 20 percent to fusion, while still finding a substantial thermonuclear reaction. A US panel under Hans Bethe analysed the fallout and characterised Joe-4 as a high-yield fission weapon aided by thermonuclear boosting rather than a scalable two-stage H-bomb.

Some US scientists therefore denied it was a “true” hydrogen bomb; other historians and designers regard that dismissal as too narrow. The USSR's first tested two-stage radiation-implosion weapon was RDS-37, in November 1955.

The test came four days after Georgy Malenkov publicly asserted that the United States no longer monopolised hydrogen-bomb production, and on 20 August both the US Atomic Energy Commission and the Soviet press acknowledged that thermonuclear reactions had occurred.

Sources

Researched 23 Aug 2026 6 sources 2 audit passes Date disputed

How this was checked

Researched from the web into a fact sheet, rewritten from that sheet, then audited against it by a different model. How the pipeline works →

Date disputed. Sources disagree; the research places it at 1953-08-12.

What the sources leave uncertain

The date, location, device and approximate yield are solid. Confidence is marked disputed because the title's classification—“first Soviet thermonuclear weapon”—depends on terminology.
RDS-6s unquestionably used fusion reactions, but it was single-stage and poorly scalable. Some U.S. scientists therefore denied that it was a “true” hydrogen bomb; other historians and designers regard that dismissal as too narrow.
The USSR's first tested two-stage, radiation-implosion thermonuclear weapon was RDS-37 in November 1955.