Most of the USSR nuclear arsenal was acquired by its successor state (Russia), some devices were held by former soviet states like Ukraine. Who gave them up after signing the Budapest memorandum. Belarus and Kazakhstan followed the same with the Lisbon Protocol.

The rest of the arsenal was kept by Russia and was not lost. Yet no one is 100% sure that all were accounted for; notably the USSR suitcase bombs. However it is not a stretch to say that if North Korea had acquired former USSR nuclear weapons that would have tested it or at least showed it to the world as a means of deterrence or saber rattling.