RDS-37 was the Soviet Union's first two-stage hydrogen bomb, first tested on November 22, 1955. The weapon had a nominal yield of approximately 3 megatons. It was scaled down to 1.6 megatons for the live test. It was a multi-stage thermonuclear device which utilized radiation implosion called Sakharov's Third Idea in the USSR (the Teller–Ulam design in the USA). It utilized a fissile core containing Uranium-235 and synthetic Uranium-233, and a dry lithium deuteride fusion fuel, with some of it replaced with a "passive material" to reduce its total yield. Despite this reduction in yield, because the weapon exploded under an inversion layer much of its shock wave was focused back downward at the ground unexpectedly, causing a trench to collapse on a group of soldiers, killing one, and a building in Kurchatov, 65 km (40 mi) distant, to collapse and kill a young girl. It was air-dropped at Semipalatinsk Test Site, Kazakhstan, making it the first air-dropped two-stage thermonuclear test. The RDS-6s device (Joe-4) exploded in 1953 had one-stage design, and was not scalable into the megaton yield range.