What was the last high-yield thermonuclear weapon design test conducted by the US at Bikini Atoll?
Castle Bravo
Castle Bravo was the first in a series of high-yield thermonuclear weapon design tests conducted by the United States at Bikini Atoll, Marshall Islands, as part of "Operation Castle". Detonated on March 1, 1954, the device was the most powerful nuclear device detonated by the United States and its first lithium deuteride fueled thermonuclear weapon. Castle Bravo's yield was 15 megatons of TNT, 2.5 times the predicted 6.0 megatons, due to unforeseen additional reactions involving Li, which led to the unexpected radioactive contamination of areas to the east of Bikini Atoll.
USS Pennsylvania (BB-38)
Allocated to the target fleet for the Operation Crossroads nuclear tests in 1946, "Pennsylvania" was repaired only enough to allow her to make the voyage to the test site, Bikini Atoll. She survived both blasts, but was badly contaminated with radioactive fallout from the second test, and so was towed to Kwajalein, where she was studied for the next year and a half. The ship was ultimately scuttled in deep water off the atoll in February 1948.