The most important considerations determining the form of the steam generator are the nature of the joints at the ends of the tubes, and the accommodation of differential expansion between the tubes and the shell. If the tubes are welded to tubeplates that form part of the boundary of the secondary sodium circuit the welds are exposed to sodium or sodium vapour, and the frequency of even very small leaks from the steam side must be very low. In some older reactors the tubes were in the form of a U so that the welds at the ends of each tube were positioned above the level of the sodium in the shell and protected from it by a layer of argon. This helped to reduce the thermal stresses at the joints and accommodated relative thermal expansion of the tubes and the shell.

Thermal expansion stresses can be minimised if the tubes are not straight. One such configuration features helical tubes wound in suc­cessive layers in an annular cylindrical bundle. This has the advantage that it can accommodate very long individual tubes in reasonably com­pact units, but the flow pattern on the sodium side is very complex so that it is difficult to ensure uniform heat transfer conditions, and steam generators of this form are expensive to manufacture. An alternative “hockey-stick” design, utilising tubes that are straight over most of their length but with a single bend near one end, has been proposed.

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Figure 4.11 Tube-to-tubeplate welds for a straight-tube steam generator.

If straight tubes are used there has to be an expansion bellows in the shell, and care has to be taken in design to ensure that the flow patterns on both sodium and water sides are uniform to minimise temperature differences between the tubes.

In a straight-tube steam generator the tubes can be welded at both ends to spigots machined on the tubeplates. At one end the holes in the tubeplate and the spigots have to be large enough so that, during manufacture, the tubes can be passed through them and welded with an offset weld at one end and a butt weld at the other. The arrangement is shown in Figure 4.11. These welds can be inspected from inside the tubes.

An alternative design that avoids exposing welds to sodium alto­gether makes use of “thermal sleeves”. The tubes pass through indi­vidual nozzles in the shell that contains the sodium and are welded to a header positioned outside. The nozzles are in the form of sleeves

welded to the shell and brazed to the tubes. The brazes and the tube-to — header welds can all be inspected from the outside. The thermal sleeve design has the advantage that the joints between the sleeves and the tubes, and those between sleeves and the shell, are not stressed by the steam pressure, and a leak in neither allows water or steam to come into contact with sodium. The disadvantage is that in a large heat exchanger the manifold, which has to connect many hundreds of tubes, is very complicated and expensive. The arrangement is shown in Figure 4.12.

The problems of steam leaks can be avoided almost completely by the use of double-walled heat exchangers with some means of detecting leakage of either steam or sodium into the space between the walls. The disadvantage is that the heat transfer coefficient is bound to be much worse than in a single-walled heat exchanger, and the cost is higher.

If a single-walled design is chosen the risk of leaks has to be accep­ted. A leak puts the heat exchanger in which it occurs out of action

until it is repaired. The effect on the availability of the plant can be reduced by having a large number of separate small heat exchangers or heat exchanger modules, any one of which can be shut down for maintenance or repair without reducing the power output of the whole plant by much. On the other hand it is cheaper to build a small number of large heat exchangers. In the end a choice has to be made between better plant availability and lower capital cost. The design provisions that have to be made to ensure the safety of the plant in the event of a leak in a steam generator are described in section 5.3.3.

Useful discussions of steam generator design are given by Hayden (1976) and Lillie (1978) and of the performance of tube-to-tubeplate welds by Broomfield and Smedley (1979).