A Fizzle

1. A spluttering or hissing sound.
2. (military) Failure of an exploding nuclear bomb to meet its expected yield during testing.
3. An abortive effort; a flop or dud.
4. A state of agitation or worry.

In the context of nuclear testing, a fizzle (an explosion with inadequate yield) is distinguished from a dud (a failure to explode at all).