Microwave radiation is non-ionizing.
The way it cooks food is by forcing the molecules in the food to change alignment to the field it’s generation which is constantly moving as the waves bounce around the inside of the box and get absorbed into the food.
A microwave without a door from this hypothetical explosion wouldn’t produce enough energy to harm you or even enough to make you notice. It wouldn’t damage you at all.
As a child, I always had an irrational fear that microwaves and computer screens would explode for no reason. Probably because I was ignorant of the science and because everything explodes in 90s video games and movies.
Op never mentioned instantaneous death only instantaneous impalement.
The way it cooks food is by forcing the molecules in the food to change alignment to the field it’s generation which is constantly moving as the waves bounce around the inside of the box and get absorbed into the food.
A microwave without a door from this hypothetical explosion wouldn’t produce enough energy to harm you or even enough to make you notice. It wouldn’t damage you at all.