In the primary components of a pressurized water reactor (PWR) systems, e.g. 1300 MWe a KWU four-loop unit, the supplied feed water  evaporates within four steam generators with the help of pressurized water heated up in one reactor pressure vessel. 

Steam generators consist mainly of a pressure-retaining enclosure for the primary and secondary media, the tube bundle for feedwater evaporation, and various other internals, such as facilities for feed water supply and distribution, as well as for steam/water separation and steam drying.

The entire thick walled pressure-retaining enclosure is made of seamless forgings, joined together by circumferential welds. All its internal surfaces, which make contact with the primary coolant are given a corrosion-resistant weld-cladding.

Welding is therefore the main manufacturing procedure for primary nuclear components, the importance of which has to be taken into account in all decisive steps of structural design and calculation, material selection, manufacturing and testing.

For this reason in the Federal Republic of Germany, the materials for such nuclear facilities - both base materials and welding filler metals - have been selected and optimized on the basis of their behavior in welding and to subsequently repeated heat treatments for stress relieving.

Safety Standard KTA 3201.1 from the Nuclear Safety Standard Commission defines the safety related requirements for the manufacture of the materials and product forms for pressure retaining components of the reactor coolant of light water reactors.

Materials for the pressure-retaining enclosure

Seamless forgings made from heat resistant quenched and tempered (Q&T) fine-grained steel are exclusively used for the pressure-retaining enclosure of primary components - RPV and SG.

MnMoNi reactor steel type was given priority in comparison with the previously used NiMoCr type.

An optimized composition has been developed for the MnMoNi steel assuring excellent mechanical properties, best weldability and special insensitivity to stress-relief cracking and radiation brittleness.

The weld heat-affected zone (HAZ) of the steel should not be rated lower in terms of ductility than the unaffected base material.

As a result of the extensive work in the line of development of the pressure boundary of the reactor coolant including the steam generator pressurized shell, steel 20 MnMoNi 5 5 of an optimized composition has been obtained. 

Through proper selection of filler metals and auxiliaries in conjunction with the respective welding process it is possible without difficulty to match the properties of the weld metal with those of the base metal. However, it is necessary to restrict the weld metal analysis in line with extensive experiences gained in industrial practice.

The remarkable mechanical properties of submerged-arc (SA) weld metal have resulted in a development envisaging the manufacturing of complete components from pure weld metal. These semi-finished products, fabricated by the so-called shape-welding technology, satisfy all demands placed on them in respect of quality, isotropy, homogeneity, testability, as well as assured processing capability.

Corrosion-resistant materials

The entire primary circuit in contact with the medium is protected against corrosion, erosion, and wear. This measure is aimed at minimizing circuit contamination through loose particles. All ferritic steel surfaces are coated with austenitic CrNi steel, with the exception of the tube zone of the SG's tube sheet, which receives a cladding of a nickel-base alloy. All claddings are made using weld-cladding processes, mainly by submerged-arc welding with strip electrode, and manual welding with coated electrodes. A special process has been developed for the internal cladding of relatively small diameter nozzles - the so called SA thin wire cladding. Particular care was dedicated to the manual arc welding of austenitic claddings with covered electrodes because of its increased risk of crack occurence in the buttering layers.

For corrosion-resistant materials used for tube bundles, internals and weld-claddings additional selection criteria must be taken into consideration. Because of safety reasons, the risk of stress corrosion cracking is rated as the highest of all the different types of corrosion. In the case of tubes, a NiCrAlTi alloy has proved to be highly resistant to intercrystalline stress corrosion cracking.

Concerning the tube/tube sheet connection in SGs, great importance attaches to the weld cladding of the tube sheet which must have sufficiently high enough nickel content to permit porosity-free and crack-free welds. Nickel contents of over 70% and chromium contents commensurate with those of the tube material have proved successful. The interface between the nickel-rich cladding and the niobium-stabilized 18/8 Cr/Ni steel weld-cladding is produced outside the tube/tube sheet zone by a specially qualified fabrication step. 

Quality assurance measures during fabrication demand production weld test coupons (PWTC's) of weld overlays and joint welds as additional evidence for the quality of product welds.