The following principal requirements were taken into account during choice of structural materials for a cassette type straight-tube steam generator:

1) sufficient level of mechanical properties at operating temperatures taking account of long term operation;

2) corrosion resistance in primary and secondary working media;

3) good weldability and provision of required properties of the welded joints.

Titanium alloys are used as structural materials for steam generating elements, tube sheets and module covers, feedwater tubes, block-section shrouds. Titanium alloys developed in Russia for once-through steam generators possess the abovementioned properties and have lower temperature stresses in comparison with steels under equal operating conditions due to lower values of linear elongation coefficient and modulus of elasticity.

The validity of utilization of titanium alloy as structural materials for cassette steam generator has been confirmed by positive operation experience of a large number of numerous steam generators made of titanium alloys in Russia ship plants with PWRs.

Stainless steel (type 18% Cr, 10% Ni) is used as the structural material for block — section headers and section of feed water tubes and steam tubes welded to them.

Titanium alloy tubes are connected to stainless steel tubes using special strong and leak-tight adapters.