why is there no explosion during decay?
The short answer is here:
http://www.atomicarchive.com/Fission/Fission3.shtml
There are two types of 'nuclear' reactions - fission and fusion.
Decay is ALWAYS a fission reaction. The wikipedia article on "nuclear fission" is just flat assed wrong when it attempts to call fission just ONE of a number of different types of decay processes.

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