An improved steam generator for a pressurized water nuclear reactor, said steam generator comprising:
- a vertically oriented heavy wall cylindrical shell means, a thin wall wrapper means disposed within said shell means and closely spaced therefrom to form an annulus between the outside of said wrapper and the inside of said shell means, a plurality of U-shaped tubes disposed within said wrapper means to form a tube bundle, a tubesheet means for supporting said tubes proximate the ends which are opposite the bent sections thereof, a plurality of generally parallel tube support members longitudinally spaced within said wrapper means and having a plurality of apertures through which said tubes pass, and a plurality of individual radial support means associated with said shell means and said wrapper means for radially constraining said tube support members, said thin wall wrapper means extending from a location between said tubesheet means and said tube support member nearest said tubesheet means vertically away from said tubesheet means beyond said tube support member farthest from said tubesheet means, said thin wall wrapper means comprised of steel having a predetermined coefficient of thermal expansion,
- wherein the improvement comprises providing said tube support members of highly corrosion resistant alloy having a predetermined coefficient of thermal expansion which is greater than the coefficient of said thin wall wrapper means and
- providing elongated thin expansion slot means extending through said wrapper means proximate at least the majority of said radial support means to provide additional flexing capabilities for said thin wall wrapper means during steam generator operation to compensate for the differences in thermal coefficients between said thin wall wrapper means and said alloy tube support members to reduce the thermally induced stresses in said tube support members and said wrapper means while maintaining a structural load path between said tube support members and said shell means, thereby preventing localized yielding of said wrapper means, said thin expansion slot means being spaced from said radial support means to provide uninterrupted wrapper material immediately proximate said radial support means for structural protection during seismic occurrences, said slot means being of a predetermined very small width to minimize comingling of cold water flowing through said annulus and heated water flowing within said wrapper means while permitting flexing of said uninterrupted wrapper material immediately proximate said radial support means.