Why were the crew members of a Japanese fishing boat harmed by Castle Bravo fallout?

The crew of the Japanese fishing boat was harmed because the Castle Bravo test, conducted on March 1, 1954, was significantly more powerful than anticipated. The thermonuclear bomb yielded 15 megatons—more than double the expected 6 megatons—and scattered radioactive debris over a vast area.1

The fishing boat, located 80 miles (129 km) away from the blast site at Bikini atoll, was showered by radioactive ash. This fallout caused the crew to suffer from severe burns and radiation sickness, similar to the effects experienced by inhabitants of neighboring atolls in the Marshall Islands. The catastrophic impact of this test on human life and the environment underscores the profound ethical responsibility to prioritize safety and human welfare over the development of destructive weaponry.1