Type-54 Sunbomb, the first in a series of high-yield superbomb design tests conducted by the Empire of Japan at Bikini Atoll. Type-54's yield was 20 megatons of TNT, 4 times the predicted 5 megatons of TNT, due to unforeseen additional reactions involving lithium-7, which led to radioactive contamination in the surrounding area.
Fallout, the heaviest of which was in the form of pulverized surface coral from the detonation, fell on residents of Rongelap and Utirik atolls, while the more particulate and gaseous fallout spread around the world. The inhabitants of the islands were not evacuated until seven days later and suffered radiation sickness. Twenty-three crew members of the Japanese fishing vessel Daigo Fukuryū Maru were also contaminated by the heavy fallout, experiencing acute radiation syndrome, including the death six months later of Kuboyama Aikichi, the boat's chief radioman. The blast incited a fierce backlash domestically in Japan, as the public was outraged over the lack of accountability of the Imperial Japanese Army who conducted the test, leading to an investigation by the Japanese central government, and an international backlash over atmospheric thermonuclear testing.
Unfortunately this would not be the only time the Imperial Japanese Army causes an ecological disaster...
SV: Romance of the Three Empires event
王獏 (O-Baku) sunbomb test in 1958
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At a yield of 120 megatons it was the most powerful nuclear weapon ever created and tested over the New Siberian Islands.
The bomb was dropped by parachute from a Z1K Tateyama detonated autonomously 4,000 metres (13,000 ft) above the surface. The resulting explosion resulted in not just the loss of the aircraft and aircrew but also an ecological disaster as even the shockwave was felt across the Arctic in Greenland, Nunavut, Yukon, American Northern Territories and Alaska as even windows were shattered in the aforementioned locations.
What was supposed to be a show of force ended in the Imperial Japanese Armies downfall, due to the loss of the aircrew and aircraft the Imperial Japanese Army Air Service unanimously agreed to split from the Army and become their own military branch (Imperial Japanese Air Force).
The IJA would never recover from this and would never hold as much influence politically as it had.