You would think so, wouldn't you. However, a real fusion reactor would not explode... it wouldn't even utter a "tiny puff of smoke".
The fusion happens in a cloud of plasma contained in a magnetic field inside a vacuum chamber. "Plasma" is another word for "hot air" (for sufficiently high values of "hot"). As soon as the magnetic containment field collapses, the plasma will expand to fill out the vacuum, thereby making the already miniscule reaction mass density even smaller. In the process of expansion, it cools off significantly. Then the plasma comes into contact with the reactor walls. Due to the need for containing an absolute vacuum and the sheer mass of the required electromagnets, these walls are freaking massive. Even if the entire amount of reaction mass hit a single small spot, it wouldn't hurt it significantly. But no, the plasma has already expanded to fill the entire chamber and thus touches all available wall area at once. It will cool itself to room temperature without even causing the cooling circuits of the electromagnets to register even the briefest change in temperature.
In case of a hull breach, you wouldn't get an explosion either. Instead, you're going to get a mass of air rushing /into/ the vacuum chamber, cooling the plasma and snuffing out the fusion in the process.
TL;DR: a RL fusion reactor will just whiff out like a candle in the wind instead of exploding. In that respect the GregTech implementation is actually fully realistic