The main distinguishing feature of these types of plants is the use of water as both a moderator and a coolant. Although water is an effective moderator it also has a high neutron capture cross section. It can, however, be used as a coolant, but for electricity production it needs to be pressurised to produce the steam necessary to operate turbines. To overcome neutron absorption, slightly enriched fuel is used. LWRs are designed to be compact and slightly under-moderated.

The obvious difference between the two designs of LWR is the fact that in a PWR the pressure is sufficiently high that boiling in the core is suppressed. The pressure in a BWR is lower and so boiling occurs in the core itself. This has led to the use of a direct cycle in which the steam produced in the core is used to directly drive the turbine generator. Figures 10.1 and 10.2 show overall schematics of a ‘generic’ PWR and BWR. It should be noted that the core cooling water will become radioactive as a result of its passage through the core. Nitrogen-16 is produced by the activation of oxygen. This has a half-life of 7.13 seconds and so decays rapidly once the reactor is shutdown. In addition corrosion products may become activated and if there are fuel failures or tramp fuel is present then fission products will be present in the coolant water. The control of activity in the coolant will be discussed in Section 10.4.

The PWR consists of a primary circuit in which water at high pressure (typically —15.5 MPa) is circulated through the core to provide both cooling and moderation. The water enters the core at about 293 °C and leaves at —324 °C and then passes through a number of steam generators, which are water/water heat exchangers. Each of the subsystems consisting of a steam generator, its associated pipework and reactor coolant pump(s) is known as a reactor coolant loop. The secondary side of the steam generator is at a lower pressure (typically —6.9 MPa) so the water boils generating steam. The steam is saturated and is relatively wet so the upper section of the steam generator contains separators and driers. The steam then passes through the turbine and the condensate is returned to the steam generators by the main feed pumps.

As will be discussed below, although the early BWRs also had steam generators direct cycle plants were introduced in the early 1960s and modern plants were developed from these. The primary circuit is maintained at a lower pressure than is the case for a PWR (typically —7 MPa) and boiling occurs in the core region. The reactor pressure vessel is taller than that of a PWR because it contains steam separators and driers in the upper part of the vessel. The steam, which will contain some radioactive material, then passes through the turbine and the condensate is

Cooling tower

10.2 Schematic of a BWR (Source: USNRC).

returned to the vessel by the main feed pumps. In addition the current BWRs also have recirculation pumps. These take water from the pressure vessel downcomers, which are fed by both condensate and water draining from the separators. This is then pumped into the vessel lower plenum to increase the flow through the core.

Although the two reactor types have characteristics in common there are sufficient differences to make it more convenient to discuss them separately. However, before doing so it is worth saying a little about the overall operational performance requirements demanded of modern nuclear power plants.

In the early days of reactor development for electricity production the plants were designed on the assumption that they would operate as baseload stations (i. e. they would run constantly between maintenance and refuelling outages) with load following and frequency control being undertaken by conventional fossil fuelled plants. This is still the case in countries where the proportion of nuclear generation is still relatively low. However, in a number of European countries nuclear generation exceeds baseload. In addition as electricity markets have been deregulated and access to grid systems has been opened up, the grid system operators now require almost all plants wishing to connect to the grid to be able to contribute to its stabilisation by offering load following and frequency control.

In this respect LWRs are able to do this since despite a widespread belief that all nuclear plants are inflexible, they are both capable of doing so and are already providing the required services (Pouret et al., 2009). The ways in which each reactor type achieves this are slightly different, which will be discussed below.