It's a few grams of plasma, and it's inside of a huge metal contraption. If you "lose containment" the plasma just expands, contacts the walls of the reactor, and cools off. It's kinda bad for the reactor because high temperature plasmas tend to chew through materials, but fusion reactions stop, it's a non-event outside the reactor. In fact, losing containment is typical, modern fusion reactors can't contain plasmas very long, only a few seconds. The normal operation of a reactor is magnetic containment for a few seconds, then loss of containment, followed by later "shots" of fusion plasma being contained and heated for another few seconds, and so on.
