This looks like the one he’s talking about:
*October 23, 1975, Yucca Flats, Nevada
A canister containing a nuclear weapon’s fissile core fell 40 feet to the bottom of a shaft during preparations for an underground nuclear test at the Nevada Test Site. The warhead had a yield of less than 20 kilotons. Although the warhead did not detonate and there was no leakage of radioactive material, 11 Nevada Test Site workers were injured. The device was to be detonated as part of a series of underground tests code-named “Peninsula.”
http://www.cdi.org/Issues/NukeAccidents/Accidents.htm
Sounds about right, from what i can tell, thank you.