The core of a BWR is similar to that of a PWR being on a square pitch but the individual fuel assemblies are surrounded by a Zircaloy channel box. Thus the core, hydraulically, consists of a series of parallel channels rather than the open structure of a PWR core. The control assemblies are made up of absorber rods held in a cruciform stainless steel sheath. Each absorber rod consists of a stainless steel tube containing boron carbide absorber pellets. These assemblies (or control blades) are positioned in the spaces between the fuel assemblies as shown in Fig. 10.13. The water gaps between the assemblies also increase the moderation.

The fuel assemblies are more complex than PWR ones and have evolved over time. They are based on a square lattice with pin geometries ranging from 6 x 6 to 10 x 10. Modern 10 x 10 fuel assemblies contain mostly full length fuel rods but also have a small number of part length fuel rods distributed through the bundle. Because the core operates with a two-phase mixture in the upper part of the core, the removal of the upper part of some of the rods increases the moderator to fuel ratio in this region, partly offsetting the reduction due to boiling. It also reduces

1. Top fuel guide

2. Channel fastener

3. Upper tie plate

4. Expansion spring

5. Locking tab

6. Channel

7. Control rod

8. Fuel rod

9. Spacer

10. Core plate assembly

11. Lower tie plate

12. Fuel support piece

13. Fuel pellets

14. End plug

15. Channel spacer

16. Plenum spring

the two-phase pressure drop in the upper bundle, which improves core and channel stability. In addition some of the central fuel rods are replaced with large ‘water rods’ (Zircaloy tubes containing water), which increases the moderation.

Reactivity is controlled in a BWR by means of both control rods and by varying the core flow rate. Because it is a boiling system the use of dissolved absorber is not practicable and so reactivity compensation for burnup effects must be undertaken using either control rods or integral burnable poisons. Since the
control rods enter the core from below they are generally inserted hydraulically but later designs use electro-hydraulic fine motion control rods, which give better control in normal operation as well as increased protection against inadvertent control rod withdrawal or insertion. A series of local power range detectors are distributed throughout the core in positions between the fuel assembly boxes to provide inputs to the power control scheme. The hydraulic systems are such that there is a balance between the hydraulic forces that would insert the rods and those holding them out. Rapid insertion is achieved by venting the pressure holding the rods out.

The recirculation system provides increased flow through the core to allow higher power levels to be achieved but it also provides a means of controlling the power. Increasing the flow reduces the average voidage by sweeping the two — phase mixture more quickly through the core, which increases the moderation and power output. Variations in power of about 25% can be achieved using flow control alone; larger changes will require control rod movement as well.

10.3 Safety features and issues

The fundamental safety functions (IAEA, 2000) required for any reactor are:

• control of reactivity

• core heat removal

• confinement of radioactive material and control of operational discharges as well as limitation of accidental releases

The safety features provided are based on these safety functions. Under accident or incident conditions the reactor must be safely shutdown, decay heat removed from the core and radiation confined. This is traditionally achieved using the principles of defence in depth (IAEA, 1996). This provides multiple administrative and physical barriers to ensure the fundamental objective (IAEA, 2006) of the protection of the public against the effects of ionising radiation is met.

The reactors must be protected against all faults which may be expected to occur. In defining what should be designed against the nuclear industry has always been very conservative in its definition of what should be considered in the design of a reactor and over time this has become even more stringent so that new plants are designed to cope with severe accidents involving multiple failures of systems. Thus the safety features use the principles of redundancy (to increase reliability) and diversity (to provide protection against common mode or common cause failure).

Faults which affect the core arise as a result of a mismatch between core power generation and heat removal. This can be caused by either changes in core power (e. g. a reactivity insertion due to control rod withdrawal) or a change in the heat removal capacity (e. g. failure of a coolant circulation pump). Because LWRs operate at pressure, failures in the pressure boundary will lead to depressurisation and loss of coolant. Emergency core cooling systems (ECCS) are provided to both replace any fluid lost and to provide heat removal from the core under these circumstances.

Confinement of radioactivity is based on the provision of multiple barriers and the defence of these barriers. For LWRs there are four main barriers to radioactive release to the environment. These are:

• the fuel matrix

• the fuel cladding

• the primary circuit

• the reactor containment buildings

The first two barriers are sometimes combined into a single barrier, the fuel rod, but should really be treated separately. In normal operation the vast majority of the radioactive fission products are held within the fuel itself. The fuel operates at relatively low temperatures and so only a small proportion of the volatile fission products are released to the fuel clad gap or the fission gas plenum. For these the fuel clad provides containment but for the majority of the fission products the fuel itself will confine them, provided that it is kept cool. In some faults the cladding may fail (e. g. due to rapid depressurisation following a hypothetical rupture of a major coolant pipe) as a result of the initiating event. Confinement of reactivity by containment building systems will be discussed in 10.9.3.