Fusion powered creatures

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How would a fusion powered creature work?
In my head I have an image of some colossal whale-like creature, huffing in air instead of krill and shitting out iron.
Would it be able to hover/fly due to the volume of lighter-than-air gases inside itself or would it be too heavy?
Would it be able to walk or would it be too large to support itself?
Would it be restricted to the ocean because of its size?
Would it even have bones?
How would it control the heat and radiation of the reaction inside it?
How would it turn the heat and radiation into energy to power its brain and muscles should it have them?
What would be the best size for one of these beasts? I wouldn't want one to die and destroy the planet.
Could plant life or algae live on a creature?
What about a creature that does the opposite, turning Iron to hydrogen?