The reactor operates as a recirculation boiler with boiling occurring within the core. The RPV of a BWR6 is shown in Fig. 10.11. The BWR RPV is taller than

that of a PWR to accommodate the steam separators and driers. The pressure (nominally 7.2 MPa) is lower than for a PWR and the vessel is constructed from both forgings (in the lower regions nearer to the core) and welded plate. The reactor internals are supported by the lower head and/or vessel walls.

The overall fluid flow is illustrated in the schematic (Fig. 10.2). The coolant water boils in the core producing a two-phase steam/water mixture, which exits the core, is separated and the steam dried. Steam (>99.9% quality) leaves the vessel and enters the turbine. The condensate is pumped back into the vessel entering the interspace between the vessel wall and the core shroud. Flow through the core is controlled by the means of recirculation and jet pumps. There are two recirculation loops each of which supplies ten jet pumps in a semicircle around the lower part of the downcomer. The later ASEA BWRs used directly powered internal jet pumps with motor assemblies mounted on the lower head of the RPV. This design was adopted for the Advanced BWR (ABWR), see Section 10.11. The core design is discussed in 10.8, below.

Water quality is very important in the BWR because the steam produced is directly used to power the turbine generator. A reactor coolant cleanup system takes water from the recirculation system and RPV bottom head as illustrated in Fig. 10.12 . This is passed through filters and demineralisers to remove fission products, corrosion products and other impurities. The purified water is then returned to the feedwater system.

BWR6

Reactor assembly

1. Vent and head spray

2. Steam drier lifting lug

3. Steam drier assembly

4. Steam outlet

5. Core spray inlet

6. Steam separator assembly

7. Feedwater inlet

8. Feedwater sparger

9. Low pressure coolant injection inlet

10. Core spray line

11. Core spray sparger

12. Top guide

13. Jet pump assembly

14. Core shroud

15. Fuel assemblies

16. Control blade

17. Core plate

18. Jet pump/recirculation water inlet

19. Recirculation water outlet

20. Vessel support skirt

21. Shield wall

22. Control rod drives

23. Control rod drive

24. In-core flux monitor

GENERAL ф ELECTRIC