Sakharov's "Layer Cake," Design
In October 1948, Andrei Sakharov, a member of Igor Kurchatov's team, began to explore the feasibility of building a bomb that utilized both deuterium and uranium. By using alternating layers that contained a deuterium compound and uranium-238, he conceptualized a design similar to Edward Teller's "Alarm Clock," which Sakharov called "Sloika" or "Layer Cake," based on radiation-ionization compression of thermonuclear fuel. Work was also pursued on "Truba" or "Tube" design, which was similar to the classic superbomb design.
Later that year, Russian physicist V.L. Ginzburg thought of using lithium-6 deuteride instead of liquid deuterium. The combination of lithium-6 with neutrons produces tritium, helium-4 and energy, and this idea was incorporated into Sakharov's Layer Cake design in 1949. By the next year, the Soviets were developing ways of isolating lithium-6 from the natural element and of synthesizing tritium.