Yes, of course there is, especially military technology. For example, the finer points on weaponizing plutonium or uranium to create atomic weapons is very highly classified. The general principles can be looked up (e.g. the Teller-Ulam design), sure, but the exact how to - that's highly classified.

I almost feel like this is too easy of an answer and that I'm missing something here in your question, OP. Is this the sort of thing you were looking for, or did you have something else in mind?

One thing still classified is the design of electric detonation initiators used on implosion atomic weapons, such as the Fat Man.

For this atomic bomb to work, the many initiators had to detonate the explosives within a synchronized time tolerance of less than a millionth of a second. All had to start the detonation in the explosive material at exactly the same moment.

This ensured that the entire sphere of high explosives went off in a symmetrical manner, compressing the plutonium. If it was not perfect, the bomb would have failed.