 Reines participated in a number of nuclear tests , and writing reports on their results . These included Operation Crossroads at <unk> Atoll in 1946 , Operation <unk> at <unk> Atoll in 1948 , and Operation Ranger and Operation <unk> – <unk> at the Nevada Test Site . In 1951 he was the director of Operation Greenhouse series of nuclear tests in the Pacific . This saw the first American tests of boosted fission weapons , an important step towards thermonuclear weapons . He studied the effects of nuclear blasts , and co @-@ authored a paper with John von Neumann on <unk> stem formation , an important aspect of an air blast wave . 
