sentences:  Even with less compression, the larger 117 kg pit of HEU in the Orange Herald Small should have had a roughly similar efficiency, but the observed 720 kiloton yield equals only just over 6 kilotons per kilogram of uranium.Orange Herald was the first British nuclear device to use an external neutron source.Britain rushed the development of these predicted-megaton class weapons because in 1955 it seemed that atmospheric testing could soon be outlawed by treaty.