Just Human ferrets out the truth, posts on Telegram:

“As pointed out in the thread I shared earlier, the CH-53E has an In-Flight Blade Inspection System (IBIS) which uses a small amount of Strontium-90. That is a possible explanation for the initial report of “nuclear material onboard.” ” 

Previously…

“Military Plane With ‘Nuclear Material’ Crashes Near Highway 78.

USAF Boeing CMV-22B Osprey SCRCH50 currently active over the zone.”

Seems more likely that the downed aircraft is a CH-53E Super Stallion and not an Osprey. Thread on this here: 

USMC Sikorsky CH-53E Super Stallion 16-3077