W88

The W88 is a United States thermonuclear warhead, with an estimated yield of 475 kilotons (kt), and is small enough to fit on MIRVed missiles. The W88 was designed at the Los Alamos National Laboratory in the 1970s. In 1999, the director of Los Alamos who had presided over its design described it as "the most advanced U.S. nuclear warhead." As of 2014, the latest version is called the W88 ALT 370 and the first production unit is scheduled for December 2019. The Trident II submarine-launched ballistic missile (SLBM) can be armed with up to 12 W88 warheads (Mark 5 re-entry vehicle) or 12 100 kt W76 warheads (Mark 4 re-entry vehicle), but it is limited to 8 warheads under the Strategic Offensive Reductions Treaty.

Information about the W88 has implied that it is a variation of the standard Teller–Ulam design for thermonuclear weapons. In a thermonuclear weapon such as the W88, nuclear fission in the primary part causes nuclear fusion in the secondary part, which results in the main explosion. Although the weapon employs fusion in the secondary, most of the explosive yield comes from fission of nuclear material in the primary, secondary, and casing.

In 1999, the San Jose Mercury News reported that the W88 had an egg-shaped primary and a spherical secondary, which were together inside a radiation case known as the "peanut" for its shape. Four months later, The New York Times reported that in 1995 a supposed double agent from the People's Republic of China delivered information indicating that China knew these details about the W88 warhead as well, supposedly through espionage (this line of investigation eventually resulted in the abortive trial of Wen Ho Lee). If these stories are true, it would indicate a variation of the Teller-Ulam design which would allow for the miniaturization required for small MIRVed warheads.

...
Wikipedia