Sloika
슬로이카
The Soviet Union's first thermonuclear weapon design concept, devised by Andrei Sakharov in autumn 1948. It arranged alternating spherical layers of fission fuel (uranium-238) and fusion fuel (lithium-6 deuteride) around a fissile core: a single-stage bomb whose Russian name means 'layer cake.' Unlike a true two-stage hydrogen bomb (the Teller-Ulam design), its fusion reactions primarily boosted fission and it could not be scaled to megaton yields, a fundamental limitation.
In depth
Physical Principle
The Sloika employed a spherically symmetric configuration. At the center sat a fissile 'initiator' of uranium-235 or plutonium (yield approximately 50 kt), surrounded by alternating layers of lithium-6 deuteride-tritide (Li6DT) fusion fuel and natural uranium (U-238). Conventional high explosives on the outside compressed the entire sphere uniformly via implosion. The central fission reaction ignited first; the resulting heat and neutron flux then triggered fusion in the adjacent layers. The dense uranium layers (roughly 12 times denser than the high explosive) performed a dual role: slowing expansion and increasing the concentration and temperature of the fusion fuel. Fast 14 MeV neutrons from D-T fusion then fissioned the U-238, further boosting yield. Sakharov's colleagues called this uranium-confinement effect 'sakharizatsiya' (сахаризация).
Development
Sakharov conceived the idea independently in September–October 1948, shortly after first seeing the design of the Soviet Union's first atomic bomb, RDS-1. In March 1949, Vitaly Ginzburg proposed replacing liquid deuterium with lithium-6 deuteride (the 'second idea'), greatly improving fusion efficiency. The design was developed under the code name RDS-6s and tested on August 12, 1953 at the Semipalatinsk test site, yielding 400 kilotons (US designation Joe-4). Only about 15–20% of the energy came from fusion; the rest was fission.
Limitations and Legacy
Although the Sloika was air-deliverable, it required 1,200 g of tritium per bomb, making it prohibitively expensive and limiting its shelf life to roughly six months. More fundamentally, the single-stage implosion principle could not reach megaton yields. The Soviets subsequently moved to the tritium-free RDS-27 (1955, 250 kt) and then to RDS-37 (November 1955, 1.6 Mt), their first true two-stage thermonuclear weapon. Nevertheless, the Sloika served as a crucial experimental stepping stone that helped Soviet physicists understand radiation implosion and make the leap to staged designs.
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Sources
- Wikipedia (RU) Russian Wikipedia: details on the Sloika design, alternating layers, Sakharov's 1948 conception, Ginzburg's lithium deuteride proposal, 400 kt yield, and the fundamental scaling limitation
- Wikipedia (EN) English Wikipedia: description of the Sloika as alternating layers of fission and fusion fuel, its test on 12 August 1953, comparisons to Teller's Alarm Clock and Ivy Mike, and legacy
- physicstoday.aip.org Physics Today (April 2017) by Alex Wellerstein and Edward Geist: detailed analysis of the Sloika's physical geometry, the fission/fusion ratio (~80/20), yield-to-weight ratio, and its role as a stepping stone to the two-stage design
- atomicarchive.com Atomic Archive: confirms Sakharov's independent conception in October 1948, the alternating uranium-deuterium layer principle, and Ginzburg's 1949 lithium-6 deuteride proposal