B-41

The Mk/B-41 was the highest yield nuclear weapon ever deployed by the US. It was also the only three-stage thermonuclear weapon ever developed by the US, and it achieved the highest yield-to-weight ratio of any US weapon design. The B-41 was deployed in a a "dirty" version (the Y1, with a U-238 encased tertiary stage) and a "clean" version (the Y2, with a lead encased tertiary stage). It may be that both used a secondary with a lead fusion tamper. There are actually two reported yields for this bomb, "less than 10 Mt" and 25 Mt. It is possible that the 25 Mt yield applies only to the dirty Y1 version, with the clean Y2 version having the lower yield.

The Bassoon device fired in Redwing Zuni (27 May 1956) was 39 inches in diameter, 135.5 inches long, and weighed 12,158 lbs. The predicted yield for Zuni was 2-3 Mt, it achieved 3.5 Mt. This device used a lead fusion tamper and was quite clean, with 85% of the energy coming from fusion, and only 15% from fission. The Bassoon Prime device fired in Redwing Tewa (20 July 1956) was 39 inches in diameter, and 135.5 inches long, and weighed 15,735 lb. The predicted yield for Tewa was was 6-8 Mt, the actual yield was 5 Mt. In contrast to Zuni, Tewa used an uranium fusion tamper and was quite dirty. This was the highest fission proportion of any US thermonuclear test.