Fusion processes cannot go "runaway" (unless you manage to recreate an entire star in the lab by accident, which requires a lot more mass than we have).

If a fusion reactor is breached, the plasma will likely dump it's thermal energy into the air (likely this will cause a minor detonation, it's strength depends on the energy in the reactor and the amount of fuel). Additionally it'll leak some short lived isotopes and maybe create a few long live ones.

All in all, a detonated fusion reactor is likely save to walk in the same year it exploded, if not significantly earlier.