3 1 Steam generator design

In steam generator design for integral reactors, the primary objective is to develop a compact steam generator to enable locating them inside the reactor pressure vessel, eliminating the possibility of a large loss of coolant accident (LOCA) and making efficient use of the space available This implies a high power density in the tube bundle

The main candidates for tube material are titanium alloys and Inconel, mainly 690 and incaloy 800 The choice would be related to national experience Titanium alloys have the following advantages

• Low coefficient of linear elongation, about forty percent lower than stainless steel

• Less sensitivity to thermal loads, as a consequence of a low module of elasticity

Inconel has the advantage of larger thermal conductivity Experience with its use has been mainly in the present generation loop type reactors

Incoloy 800 exhibits excellent properties for heat transfer and corrosion resistance and the allowable stress is larger than stainless steel Experience with Incoloy 800 as a tube material is quite large especially in France and Germany

It could be said that all three materials show good mechanical properties and have demonstrated good corrosion resistance as a tube material for steam generators The basic arrangement in integral reactors is to locate the SG within the RPV, in the annular region above the core level Consideration, however, has to be given to the distance between the lower part of the SG and the core, to prevent secondary water activation and radiation damage to the component

With regard to tube support, straight tube SG allows a simple support systems, using simple spacers, since flow is parallel to the tube, while m helical steam generators the flow crosses the tube Tube support in the latter case is more complicated Corrosion and build-up at tube supports with the secondary working fluid on the vessel side are more pronounced Primary coolant outside the tubes has the following advantages

• a high resistance to stress corrosion cracking,

• safety advantages in case of tube failure and

• a reduced risk of crud accumulation at the tube plate connection

In all cases, a provision for the possibility of in-service inspection of the complete bundle is recommended. The design must allow tube plugging, component removal and replacement. Steam generators that are not once-through type are more suitable for load following due to the water inventory but this option does not appear to be followed in any of the Member States.

Some reactor designs prefer secondary water boiling inside the tubes to reduce the reactivity effect in the core in case of a steam line break. Some designs place the secondary outside the tubes to reduce hydraulic losses, especially if the design uses natural circulation.

Hydraulic stability of parallel tubes is one of the most important design aspects to be considered. Experience shows that instabilities can be controlled through careful design. One of the features currently adopted is the introduction of orifices at the tube inlet to increase the secondary side pressure losses in the liquid phase. To avoid operational problems, the chemistry of the secondary coolant must be maintained at a high quality level for this type of SGs, to avoid crud deposition. Even in the case of some crud deposition, adequate experience exists regarding washing it away. RPV penetration for feed water and steam outlets can be optimized in number according to the needs of diameter limitations and to the specific design requirements.