Fusion reactors are fundamentally different from fission reactors. You have to get a tenuous wisp of hydrogen (the reactor in this particular experiment was running at 10 mTorr, or 0.0013% atmospheric pressure) very, very hot, and keep it away from solid matter, which is millions of degrees of colder than the fuel plasma, and will suck all of the energy out of the reaction.
You shut off the containment coils, and tremendously hot plasma ions leap away from each other, instantly stopping the reaction. If the reaction somehow "runs away", which it absolutely can't, then it brushes the walls of the vacuum chamber, poisoning it with cold metal ions, like injecting lead shavings that have been chilled to absolute zero directly into your heart.
If our magical "runaway reaction" somehow overcomes this, and melts a hole in the vacuum chamber, then the atmosphere rushes in, both freezing cold and at intolerably high pressure, like the North Sea flooding into the hull of a submarine resting on the ocean floor.
Fusion reactors don't melt down, or explode. At all. It just can't happen, much like how a Yugo rear-ending a garbage truck in Brooklyn doesn't instantly consume all of New York City in a gasoline fireball.
It is indicative of the great difficulty of both starting and sustaining a fusion reaction that the only fusion reactions which produced more energy than they consumed, that have ever taken place on Earth were powered by atomic bombs, and still only ran for fractions of a second.