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.
The rotors come in larger sizes and can present problems if someone is not careful handling them. Here’s what happened this week just to the South of me.
http://www.platts.com/RSSFeedDetailedNews/RSSFeed/ElectricPower/21894074
***Well, the alternator housing is not a pressure vessel that’s true,****
Actually it is. There is approximately 50 lb of pure hydrogen in the generator to cool it. If it went there would have been a massive explosion followed by the sealing oil fire. Very nasty!
This does not sound like a hydrogen gas explosion but more like the relief diaphrams on the LP turbines went, signifying a loss of condenser cooling water.
If it did, it would trip the unit and the main steam valves would slam shut, the reheat relief valve would open and dump reheat steam through the Intermediate turbine into the condenser.
The main Pop valves also went due to the sudden buildup of pressure when the main steam valves closed.
The “smoke” mentioned in the article looks more like water vapor leaking out of the various openings caused by the trip off and diaphram openings.
Other than that, I don’t know much about it. ;-)