[nl]

Allegedly (ie, according to Wikipedia) the details are on pages 475 & 476 of a book called "The KGB in Europe" by former KGB officer Vasili Mitrokhin.

I don't know if that device was nuclear or not - I suspect not.

Edit: some details about other caches:

A former Soviet spy testified at a congressional hearing in Los Angeles on Monday that Russian intelligence operatives placed weapons and communications caches--perhaps even small nuclear devices--in California and other states as part of a plan to destabilize the United States through sabotage

http://articles.latimes.com/2000/jan/25/local/me-57346