<P> In 1956, President Dwight D. Eisenhower announced the testing of a 95% "clean" (2 - stage) fusion weapon, later identified to have been the 11 July Navajo test at Bikini Atoll during Operation Redwing . This weapon had a yield of 4.5 megatons . Previous "dirty" weapons had fission proportions of 50--77%, due to the use of uranium - 238 as a "pusher" around the lithium deuteride (secondary) stage . (The fusion neutrons have energies of up to 14.1 MeV, well exceeding the 1.1 MeV "fission threshold" for U-238 .) The 1956 "clean" tests used a lead pusher, while in 1958 a tungsten carbide pusher was employed . Hans A. Bethe supported clean nuclear weapons in 1958 as Chairman of a Presidential science advisory group on nuclear testing: </P> <P>... certain hard targets require ground bursts, such as airfield runways if it is desired to make a crater, railroad yards if severe destruction of tracks is to be accomplished...The use of clean weapons in strategic situations may be indicated in order to protect the local population .--Dr. Hans Bethe, Working Group Chairman, 27 March 1958 "Top Secret--Restricted Data" Report to the NSC Ad Hoc Working Group on the Technical Feasibility of a Cessation of Nuclear Testing, p 9 . </P> <P> In consequence of Bethe's recommendations, on 12 July 1958, the Hardtack - Poplar shot of the Mk - 41C warhead was carried out on a barge in the lagoon yielded 9.3 megatons, of which only 4.8% was fission, and thus 95.2% "clean". </P> <P> Cohen in 1958 investigated a low - yield "clean" nuclear weapon and discovered that the "clean" bomb case thickness scales as the cube - root of yield . So a larger percentage of neutrons escapes from a small detonation, due to the thinner case required to reflect back X-rays during the secondary stage (fusion) ignition . For example, a 1 - kiloton bomb only needs a case one - tenth the thickness of that required for 1 - megaton . </P>

Shame confessions of the father of the neutron bomb