4.3.1 Steam Generator Design

The secondary sodium gives up its heat to raise steam in steam gen­erators that are normally shell-and-tube heat exchangers with water or steam in the tubes. These have to be larger than the intermediate heat exchangers because of the poorer heat transfer on the steam side. They also differ in that they are stressed by the high-pressure steam as well as by thermal expansion. The overriding concern in design and operation is to prevent leaks, because of the consequences of the chemical reaction between water and sodium.

Many older fast reactors had evaporators and superheaters in sep­arate vessels, and some had separate sodium-heated reheaters as well. The flow of hot secondary sodium was divided between superheater and reheater and then recombined before flowing to the evaporator. Austenitic stainless steel cannot be used in evaporators because of the risk of chloride stress corrosion. Although the chloride concentra­tion in the feedwater can be controlled by ion-exchange units there is a danger of an accidental increase, particularly if the condenser is cooled with seawater. Evaporator tubes can be made of ferritic steel, such as 2.25 Cr 1 Mo with about 0.4% niobium added to stabilise the carbon, or of a steel with a higher chromium content such as 9 Cr 1 Mo that resists decarburisation. Separate superheaters and reheaters can be made either of austenitic steel (provided they can be kept free from droplets of water from the evaporators) or of a ferritic steel.

Figure 4.9 Steam plant with recirculating boilers.

Some reactors used a Lamont-type boiler in which the evapor­ator produced a mixture of water and steam that were separated in a steam drum, the water being recirculated to the evaporator and the saturated steam being passed to the superheater. Figure 4.9 shows a steam plant with recirculating evaporators and sodium-heated super­heaters and reheaters. The main advantage of this arrangement is that the evaporator tube walls are always covered with water. The flow of the two-phase mixture of water and steam is either “bubbly” (near the water inlet and where the steam is in bubbles dispersed throughout the water) or “annular” (near the outlet where the steam bubbles coalesce to form a continuous vapour region in the centre of the tube and most of the water is in a film on the wall). The boiling water on the wall gives a high heat transfer coefficient so that the wall temperature stays close to the saturation temperature. It also has the advantage that the mass of steam and water in the steam drums tends to decouple the reactor and sodium coolant circuits from rapid changes in demand for steam caused by fluctuations in the electrical load, so that control of

Figure 4.10 Steam plant with once-through boilers.

the plant is easier. But it has the disadvantage of being complex and expensive.

A “once-through” steam generator as shown in Figure 4.10 is much simpler and cheaper because it does not require steam drum or boiler circulating pumps. The feedwater enters a single heat exchanger in which it is heated to saturation, evaporated and then superheated. The disadvantage is that somewhere along the tube the wall ceases to be covered with water. (This is either the point of “departure from nucleate boiling” (DNB), where nucleate boiling gives way to film boiling, or the “dryout” point, where annular flow gives way to dis­persed flow (Collier 1972).) At this point the heat transfer coefficient falls substantially (see section 4.3.3).

It is very difficult to engineer sodium-heated reheaters with a once — through steam generator. The absence of reheat by sodium is a disad­vantage, both because, both because it reduces the thermal efficiency of the plant by reducing the mean temperature at which heat is trans­ferred to the steam, and also because the wetness at the low-pressure end of the turbine is increased. There is little that can be done about
the reduced efficiency, but the wetness can be reduced by means of moisture separators between the turbine stages or by employing bled — steam reheat as shown in Figure 4.9. Some of the steam is taken from the high-pressure turbine and used to heat the main flow of steam after further expansion. The bled steam is partly condensed in the reheater but is still hot enough to be used in a feed heater. There is a loss of thermal efficiency because of the entropy increase in the reheater, but this may be offset by the increased efficiency of the final turbine stages due to the lower wetness.