| in a thermonuclear fashion. As I understand it, they can't. The only reason a thermonuclear bomb makes such a big BOOM is that all the hydrogen contained in it is persuaded to fuse at exactly the same instant.
Only tiny amounts of fuel would be fed into the reactor at a time. In the case of a constant reaction reactor, the fuel would be fused almost immediately, making a build-up of fuel (and thus the sudden ignition of a large quantity) impossible.
For a pulse reactor, tiny fuel pellets would be ignited briefly to generate power (a whole series of these would be chained to form a system like a multicylinder car engine). Even if multiple pellets somehow ended up in a chamber at one time, it would still be a simple multipication of power. It would destroy the chamber, and possibly ruin the power plant as a whole, but there wouldn't be an "Earth-shattering KABOOM" that most people envision destroying entire cities or states.
Keep in mind that the reaction would cool off extremely quickly once it started expanding outside of the reactor containment vessel. Also, the structure of the vessel itself would absorb a great deal of the energy that the fusion reaction would need to sustain itself.
As a result of all this, about the worst you're going to get from a loss of containment and coolant in a fusion reactor is a hosed power plant, a fire, and maybe a couple lumps of radioactive metal.
Contrary to what science fiction tells us, fusion power would be cheap, safe, and almost limitless. | 
