Due to the fact that the NBS devices are designed to prevent radiation leakage, they hold up very well in fires. Our Baton Rouge East Coker suffered a severe fire in 1993, which destroyed the structure, and the NBS devices were not breached. There was no special fire proofing in place.
What caused your fire? How extensive was the damage on the top deck? Can you identify yourself?
Also the source container should be manufactured in accordance with either the ANSI (? number escapes me) standard or ISO 7205. Either way the classification number and reference to the standard will give you the temperature rating of the unit you have. I think the number will be stamped on the container flange or nameplate.