Although it had been predicted to produce a yield of 6 to 10 megatons , it actually produced a yield of 13.5 megatons , the second-largest ever yield in a U.S. fusion weapon test . Like the Mike , Bravo and Romeo tests , a large percentage of the yield was produced by fast fission of the natural uranium tamper . Of the total yield , 7 megatons were from fission ; the other 6.5 megatons were from fusion reactions . The high fusion yield was due to the enriched fuel and set a U.S. record that stood until Hardtack Poplar in 1958
Hardtack
Operation Hardtack I
Castle Yankee