The W78 is an American
The W78 was designed at Los Alamos National Laboratory (LANL) starting in 1974. It is thought that the warhead has never been tested at full yield, with the weapon producing a "disappointing" yield shortly before the 1976 Threshold Test Ban Treaty came into force, leading to a redesign of the weapon that was not complete before the treaty came into force. This led to a dispute between LANL and Lawrence Livermore National Laboratory who estimated the weapon's yield below that of LANL. The dispute was resolved by a panel led by former LLNL director John Foster who found in favor of LANL's yield estimate.
The final W62 warhead was not removed from service until March 2010. This leaves the W78 and W87 warheads as the only warheads carried by Minuteman III. Downloading of the Minuteman III missile (reducing the number of warheads carried) was completed in June 2014, reducing the warheads carried by each missile from three to one.
Dimensions of the W78 are unknown, but it fits within the Mark 12A
- W78 Warhead page at nuclearweaponarchive.org