Just as some of them had stepped through the door, Joe quickly asked "Oh, one thing before we separate: Is it ok if I close the door behind us? We do not know the nature of the radioactive material, so it is best to keep it as contained as possible for the moment.", after almost having forgotten.
"If you need to quickly get out and I am not available, blast through as a last resort. A hole is harder to close than an open door. Otherwise, wait there and keep in mind the radiation and the ground will interfere with our comms."
With that, Joe headed for the central reactor, while also keeping a close eye on the geiger-counter. Too much radiation exposure wouldn't only hurt him, but a high dose has the potential to short electronics, rendering almost any device inoperable.
As radioactive material had already leaked and at least partially gotten near the entrance of the base, sealing the reactor directly had a slightly lower priority than finding out the nature of the radioactive material, as the base's door already acted as a seal. If it was just radioactive gas, it was relatively easy to deal with(just let it escape into the atmosphere, if the amount is small enough, it will disperse and thus its effect will weaken to the point of being much smaller than the natural background radiation), airborn dust was much harder, simply because it could be washed out of the air by rain, concentrating the material on the ground. Liquids and solids were similarly difficult to deal with.