Loss of containment is expected, and is more or less trivial. The result is that the fusion plasma simply expands or runs into the side of the reactor wall. This may sound dramatic but it's not. We're talking about at most a few grams of hot gas/plasma. In reality it's not any more significant than running a welding torch or a plasma cutter. As for the fusion reactions, they require the high pressure and high temperature conditions of containment in order to operate. Once containment breaks down the fusion reactions stop.

Don't let ignorance fuel fear of technological advancement.