The water which is typically used as coolant in light water reactors can be used both in a single-circuit system or — to prevent contamination — in a multiple-circuit system via heat exchangers. Light water reactors are known as pressurised water reactors (PWRs) or boiling water reactors (BWRs), depending on whether the water in them is pressurised or boiling.

In a pressurised water reactor, the water in the reactor pressure vessels is at extremely high pressure, around 150 bar, so the water does not boil, even at the design operating

6.

Generator 11. Cooling water treatment

7. Transformer plant

8. Condenser 12. Cooling water pump

9. Heat exchanger 13. Power station basin

10. Feed-water pump 14. Cooling tower

Fig. 2.7 Nuclear power plant with pressurised water reactor (PWR) temperature of 300 °C. This prevents steam bubbles forming, which would complicate the heat transfer process.

As Figure 2.7 shows, a PWR has two coolant circuits: the primary and secondary circuits (water-steam circuit). In the primary circuit, the coolant water flows round the fuel elements directly: the water which is heated in the reactor core of the reactor pressure vessel is then fed to the boiler and back to the reactor core via circulating pumps. The steam generator then transmits the heat to the secondary circuit, producing steam which drives the turbine and consequently the generator, so the steam passing to the turbine is not radioactive. At the end of the secondary circuit, the steam which was depressurised and condensed in the condenser (coolant water circuit) is pumped back to the steam generator via heat exchangers (preheater unit).

Unlike a pressurised water reactor, in a boiling water reactor (Figure 2.8), the water in the reactor core of the reactor pressure vessel is heated to boiling point: so compara­tively little pressure is required at the proposed operating temperature of 300 °C. A pressure of 70 bar is sufficient. Nor is a boiler required, so only one coolant circuit (direct circuit) is necessary. The live steam is fed directly from the reactor pressure vessel to the turbine, which means that the turbine becomes radioactively contaminated to a limited extent. Unlike with the PWR, in which the reactor is controlled and can be crash shut down by control rods from above, with the BWR, control rods are inserted into the reactor core from below. (Please note: control rods are used to control and shut down nuclear reactors.)

If a loss of coolant accident (LOCA) occurs (Section 2.5) in a BWR the pressure is reduced by condensing the steam released in a condensation chamber, so the safety

1.

Reactor pressure vessel

4. Turbine set

11. Cooling water pump

12. Power station basin

13. Cooling tower

Fig. 2.8 Nuclear power plant with boiling water reactor (BWR) vessel containing the reactor pressure vessel in the reactor building is much smaller than for a PWR of comparable output.

Of the eleven PWRs and six BWRs operating in Germany, the three PWRs of the Convoy model (Siemens KWU) with an output of approx. 1400 MW are the most advanced. One of these Convoy plants, which may be classified as Generation III, operates at the Isar site (near Landshut) together with a BWR unit (Figure 2.9).

Fig. 2.10 Overall view European pressurised water reactor EPR (AREVA, 3D visualisation)

In the course of the further development of the Convoy nuclear power plant design, German nuclear power plant operating companies decided to join forces with the French state company EDF to develop the EPR (European Pressurised Water Reactor) in 1991. This EPR, a Generation III+ model generating 1600 MW, is currently being built in Finland and France (Figure 2.10). It is being supplied by French plant supplier AREVA, which acquired the former Siemens KWU some years ago.

As well as EPR, AREVA with German involvement (E. ON Kernkraft) is also developing the boiling water reactor KERENA (formerly designated SWR 1000) with an output of 1250 MW (Figure 2.11).

Fig. 2.11 Overall view boiling water reactor KERENA (AREVA, 3D visualisation)

Fig. 2.12 AP 1000 pressurised water reactor (Westinghouse)

Further new developments in Generation III+, which are now being offered and preferred as large-scale power plants with outputs of well over 1000 MW each, are the boiling and pressurised water reactors as listed below (Figures 2.12 and 2.13):

Fig. 2.13 ABWR boiling water reactor (Westinghouse/Toshiba) [3]

— ABWR: BWR — 1350 MW;

Supplied by: Westinghouse (USA)/Toshiba (Japan),

— AP1000: PWR — 1000 MW;

Supplied by: Westinghouse (USA),

— AES 92: PWR — 1000 MW;

Supplied by: ASE (Russia),

— APR1400: PWR — 1400 MW;

— Supplied by: KOPEC (South Korea).