The alternator rotor is coupled to the main turbine but is not in direct contact with the steam path and therefore the housing is not as resistant. The speed is, of course, the same and therefore a tremendous amount of mechanical energy spinning that. Without more details everything is speculative.
Well, the alternator housing is not a pressure vessel that's true, but there's a steel stator laminations and heavy copper bars a two or three feet thick surrounding the rotor on all sides.
The turbine housing on the other hand is made of high-grade steel and is a couple of inches thick. On the other hand, those low pressure blades are close to five feet long and weigh on the order of a hundred pounds each. One of those suckers flies off at 3600 rpm and you've got a problem.
The alternator rotors are solid steel, three or four feet in diameter (for 3600 rpm anyway). They weigh something like fifty or sixty tons.