There isn’t much need to extract isotopes if you have an actual working reactor, is there? Just use that as-is to cause whatever damage.

From a strictly red team threat analysis perspective, if you have an extremely safe working reactor that can't be made to melt down, no, the reactor can't be used to hurt much that is in the same location as the reactor. If you are able to get the isotopes out and start spreading them around or making a dirty bomb type thing (where the explosion just serves the purpose of throwing the isotopes around), that could be pretty catastrophic.