I would not class the China Syndrome as a disaster movie. I would call it more of a conspiracy thriller, based loosely on factual events. For a start, it did not contain any actual disaster, and very little in the way of effects sequences or set pieces.
Excellent film, all the same.
Also not sure I would agree that movies like Towering Inferno and Poseidon Adventure were well written - better written perhaps than todays efforts, but largely they were usually just packed with big star names and usually contained plenty of action and effects, plenty of incident. But they were still fairly cliched affairs.
Maybe we didn't notice the cliched back then when we were younger.
That said, I still think TW is the daddy of disaster films. Newman, McQueen, Wagner, Dunaway, Astaire, Chamberlain...great cast. Best effects for my money too, and a lot of them looked physically dangerous, with the cast being unfeasably close to the action a lot of the time.
Recently picked up the blu ray release of this, and it is an excellent transfer.