Union demonstrated the power of the "staging" concept in October 1961 when they detonated the massive and unwieldy Tsar Bomba, a hydrogen bomb which derived almost of its energy from fusion rather than fission—its uranium tamper was replaced with one of lead shortly before firing, in an effort to prevent excessive nuclear fallout. Had it been fired in its "full" form, it would have yielded at around . The weapon was technically deployable (it was tested by dropping it from a specially modified bomber), but militarily impractical, and was developed and tested primarily as a show of Soviet strength. It is the largest nuclear weapon developed and tested by any country. Other countries United Kingdom The details of the development of the Teller–Ulam design in other countries are less well known. In any event, the United Kingdom initially had difficulty in its development of it and failed in its first attempt in May 1957 (its "Grapple I" test failed to ignite as planned, but much of its energy came from fusion in its secondary). However, it succeeded in its second attempt in its November 1957 "Grapple X" test, which yielded 1.8 Mt. The British development of the Teller–Ulam design was apparently independent, but it was allowed to share in some US fallout data which may have been useful. After the successful detonation of a megaton-range device and thus its practical understanding of the Teller–Ulam design "secret," the United States agreed to exchange some of its nuclear designs with the United Kingdom, which led to the 1958 US-UK Mutual Defence Agreement. China The People's Republic of China detonated its first device using a Teller–Ulam design June 1967 ("Test No. 6"), a mere 32 months after detonating its first fission weapon (the shortest fission-to-fusion development yet known), with a yield of 3.3 Mt. Little is known about the Chinese thermonuclear program. Development of the bomb was led by Yu Min. France Very little is known about the French development of the Teller–Ulam design beyond the fact that it detonated a 2.6 Mt device in the "Canopus" test in August 1968. India On 11 May 1998, India announced that it has detonated a hydrogen bomb in its Operation Shakti tests ("Shakti I", specifically). Some non-Indian analysts, using seismographic readings, have suggested that it might not be the case by pointing at the low yield of the test, which they say is close to