I think a fission reaction would still explode. An explosion is actually just the sudden release of a massive amount of energy in short period of time. Nuclear reactor meltdown usually result in explosion because the energy inside the reactor builds up due to not being cooled by the cooling system. So internal energy builds up. Since the reactor cannot release internal energy by heat then it releases by work, which translates to forced expansion inside the reactor.
Cutting a reactor while it's undergoing fission would probably result in an explosion, just relatively smaller than a built-up meltdown.
That is all according to my understanding.
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