secondary parts of the bombs in separate pieces ("staging"). The next breakthrough was discovered and developed by Sakharov and Yakov Zeldovich, that of using the X-rays from the fission bomb to compress the secondary before fusion ("radiation implosion"), in the spring of 1954. Sakharov's "Third Idea", as the Teller–Ulam design was known in the Soviet Union, was tested in the shot "RDS-37" in November 1955 with a yield of . If the Soviet Union had been able to analyze the fallout data from either the "Ivy Mike" or "Castle Bravo" tests, they could have been able to discern that the fission primary was being kept separate from the fusion secondary, a key part of the Teller–Ulam device, and perhaps that the fusion fuel had been subjected to high amounts of compression before detonation.(De Geer 1991) One of the key Soviet bomb designers, Yuli Khariton, later said: At that time, Soviet research was not organized on a sufficiently high level, and useful results were not obtained, although radiochemical analyses of samples of fallout could have provided some useful information about the materials used to produce the explosion. The relationship between certain short-lived isotopes formed in the course of thermonuclear reactions could have made it possible to judge the degree of compression of the thermonuclear fuel, but knowing the degree of compression would not have allowed Soviet scientists to conclude exactly how the exploded device had been made, and it would not have revealed its design. Sakharov stated in his memoirs that though he and Davidenko had fallout dust in cardboard boxes several days after the "Mike" test with the hope of analyzing it for information, a chemist at Arzamas-16 (the Soviet weapons laboratory) had mistakenly poured the concentrate down the drain before it could be analyzed. Only in the fall of 1952 did the Soviet Union set up an organized system for monitoring fallout data. Nonetheless, the memoirs also say that the yield from one of the American tests, which became an international incident involving Japan, told Sakharov that the US design was much better than theirs, and he decided that they must have exploded a separate fission bomb and somehow used its energy to compress the lithium deuteride. But how, he asked himself, can an explosion to one side be used to compress the ball of fusion fuel within of symmetry? Then it hit him! Focus the X-rays! The Soviet