Boiling Water Reactors

Boiling water reactors (BWRs) were first developed in the US by the General Electric Company. The first commercial BWR, Dresden, sold to the Commonwealth Edison Company, was a 200 MW plant commissioned in 1960. This was followed by subsequent orders in the US, Europe and Japan. Ratings were increased up to the 1300 MW plants in operation today. Other vendors developed designs, independently of the US, notably Asea — Atom, later ABB Atom, in Sweden, Figure 1.2.

The characteristic feature of BWRs compared with PWRs is that boiling occurs within the core. Due to the axially changing void fraction, the axial flux becomes asymmetric. After drying in moisture separators (as in a PWR), the steam is passed directly to the turbine. The loop is completed by condensing the steam; the condensate is then returned to the reactor vessel. The Forsmark 3 BWR loop is shown in Figure 1.2.

BWRs burn uranium oxide fuel at a typical enrichment of around 2%. Fuel rods are grouped in a square lattice of 6 X 6 up to 8 X rods, the full assembly being smaller than in the PWR. The enrichment within the rods depends on their position in the fuel assembly, the reason for this being to correct for the effects of water spaces between the fuel assemblies. Reactor control is achieved with control rods inserted from the bottom of the core. The absorber material in the rods is boron carbide.

There have been changes in the main recirculation systems employed in BWRs during their evolution. For example, in some of the older BWR designs, the water is circulated by external pumps, one pump on each loop external to the vessel. In the more recent designs, the tendency is to utilise internal pumps, to avoid the risk of loss of coolant in the event of an external line break. General Electric employed an intermediate system with both external and internal pumps. Reactor power can be controlled by altering the flowrate

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since this affects the core water temperature and steam bubble level, thereby affecting the neutron moderation.

BWRs operate at a lower pressure than PWRs, typically 7-8 MPa. BWR vessels are generally larger than PWRs, which is a disadvantage, despite having the advantage of a single cycle system. The turbine area in a BWR has to be monitored to ensure that health physics regulations are satisfied. Radioactive products can be transported in the steam, from a failed fuel rod for example.

BWRs are constructed with a leak tight containment, which is designed to withstand the load from a large break in the coolant or steam system. Safety systems have the provision to inject water directly into the reactor vessel to cool the fuel. Containment pressure increase is relieved via condensation in water filled areas. There is an additional cooling system to spray the chamber surrounding the reactor vessel.

The design of containment has evolved through the years, mainly in relation to the designs of the dry well that surrounds the reactor and the wet well that contains the water for pressure suppression in the event of a reactor vessel penetration failure. For example, General Electric developed the Mark I, II and III design containments, the principal driver being to simplify design and increase capacity. Six different models, BWR 1 -6, have been developed, incorporating different pump configurations, increased fuel assembly arrays and power density.

BWRs exhibit many of the advantages and disadvantages associated with PWRs. They have also been operated very successfully over a long period of time and much experience in operation has been accumulated. They are fuelled off line, utilise similar fuel coolant and moderator and have relatively complex technology, albeit that the single cycle system of the BWR is clearly a simplification of the two loop cycle of the PWR (and hence capital costs tend to be somewhat lower). Comparative data for the PWR and BWR and also for the other reactor designs are given in Table 1.3.

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