This word has been in common use for decades. I’ve never once seen it misspelled. But so very many people mispronounce it, and always in the same way.
Look at the word:
First, where exactly are you getting this magical extra syllable hidden between the C and the L? Second, what’s the rationale for the sound you’re putting at the end? But no, I’m getting ahead of myself. Let’s begin at the beginning.
Happily, everyone seems to get that the first bit goes ‘new’. That’s an excellent start, and we can build on that. Having got this part banked, we can look at the rest of the word:
Read that again. See if you can think of a very common word that might contain that string of letters. Perhaps a five-letter word. Got one? Excellent. Try to hold in your mind how it sounds.
Now say them together: New. Clear. Now faster: new-clear. Good.
Easy, isn’t it? So why the common mistake? All I can think is that maybe ‘nuclear’ doesn’t feel like a proper adjective in the way that, say, ‘molecular’ (of or pertaining to molecules) does. Perhaps. But what, then, what the hell is a ‘nucule’???
Nuclear: of or pertaining to the nucleus of an atom. (I could understand people wanting to pronounce it ‘new-clee-ar’, but nobody ever does.)
Phew. Sorry. Rant over. Just as long as we’re clear about that. Are youcular?