Those were not fission explosions. Those were radioactive material spills. Fission bombs are designed to convert almost all of their radioactive core. The disaster is the immediate massive death and destruction of infrastructure.
https://www.livescience.com/65949-marshall-islands-more-radioactivity-chernobyl.html
Some of the Marshall Islands in the Pacific Ocean — such as the Bikini and Enewetak atolls — are still more radioactive than Chernobyl and Fukushima, even though more than 60 years have passed since the United States tested radioactive weapons on those islands, a new study finds.
Many tests and not all the fissile material converted. Hense the testing. The Marshall Islands are more closely related to a dirty bomb than the effects of a thermonuclear explosion.
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