On 27 June 1947 President Harry Truman authorized a new test series for weapons development for the following year. Operation Sandstone was conducted at Enewetak Atoll in the Marshall Islands in 1948 to test the first new weapon designs since World War II. The massive operation involved 10,200 personnel. Up until this time, all four implosion bombs that had been exploded (the Trinity Gadget, the Fat Man bomb used on Nagasaki, and the two bombs used during Operation Crossroads ) had all been identical, based on a conservative wartime design. The Sandstone test series intoduced a second generation of weapon design by evaluating several new design principles. In addition a number of design parameters were varied to evaluate their effects on performance. The original Fat Man pit design used a Christy solid plutonium core, surrounded by a close fitting natural uranium tamper. The Sandstone devices all replaced the contiguous tamper-core approach with a "levitated core" in which the core was suspended within a larger hollow space within the tamper so that a gap existed between them. The collision between the tamper and core would create more efficient compression of the core than the explosive-driven shock in the watime design. They apparently retained a solid core however. These devices also abandoned the use of a pure plutonium core since oralloy (uranium hihgly enriched in U-235) production exceeded plutonium production by a factor of over 3-to-1. The first test, X-Ray, used a composite oralloy-plutonium core. Both Yoke and Zebra used an all oralloy core. The pits (tamper plus core) for all three devices weighed about the same. The X-Ray device used a uranium-plutonium composite Type B levitated pit. The uranium-to-plutonium ratio by weight was on the order of 2:1 or greater. Efficiency of utilization of the plutonium in this core was around 35%; uranium utilization was in excess of 25%. Taken together this indicates a core containing about 2.5 kg of Pu, and 5 kt of U-235. This was the highest yield device tested to date.The Yoke device also used an all-oralloy Type B levitated pit. Yoke was the highest yield device tested, a record it kept until 1951. Despite its high yield, it was regarded as an inefficient device.

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