If you look it up in a good thesaurus, you will find the annoying word "Mushrooming", which is hardly satisfying when you want a big bang. Thinking outside the thesaurus a bit, I reckon a reasonable word would be "Megablast" to denote the extreme amount of energy suddenly released by a megaton bomb, or alternatively the disastrous result experienced the next day after eating a meal consisting of onion Bharjis and red chillis.
Synonyms of 'Explosion' which begin with the letter 'B' include; blast, burst, blow up, blow out, bang and bark.
A bombing?
The word 'boom' can be used as a word for explosion that starts with a b.
Another word for addition which starts with the letter N is New.
letter
Another word for water that starts with the letter 'O' is oxygen dihydrogen.
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Atop.
advertise
astamp
Amazing
Astonishing.