We knew at Vault-Co they had built this thing, primarily out of fear of Reagan's increased spending in the '80's on his Star Wars programs.
We suspected it was still active. It would be the cheapest system to maintain when Russia went through her cash crunch years. We were right. It was never deactivated.
By the way, the doomsday device is only doomsday for those who have not prepared wisely. For Vault dwellers it's just an expected inconvenience. Like water off a duck's back.
If enough cobalt is lofted and dispersed to produce 10,000 rads an hour worldwide in almost any place the wind will reach, this will mean fresher breath and whiter teeth for Vault inhabitants during their five year stay. These kinds of bombs only kill people trying to shelter in their basement in between suitcases for two weeks.
Aren't you glad that Vault-Co has always advocated preparation with the Death Shroud firmly in mind? If you were listening, you are prepped to a sufficient level to survive it now.