1.2.1 Pressurised Water Reactors

The pressurised water reactor (PWR) owes its origin to nuclear submarine reactor technology. The first civil PWR was built at Shippingport in the US and it entered commercial operation in 1957. This was a 60 MW (Net) reactor utilising high enrichment uranium fuel. This was soon followed by the Yankee Rowe plant, which included uranium oxide fuel and then other plants commenced operation both in the US and in Europe. Subsequent plants were progressively increased in capacity, in respect of the size of

the components, the number of coolant loops (increasing from 1 to 4) and overall improvements in design. Large modern PWRs now generate typically up to 1300 MW (Net).

The basic components common to all PWRs are a reactor pressure vessel containing the core and the core barrel, primary circuit loops to convey the heat to steam generators, secondary loops to take steam to the turbine, together with a variety of other systems, e. g. control and safety systems. The primary side pressure is controlled by a pressurizer on one of the primary loops. The primary circuit is enclosed in a containment. There have

been various differences in the design of these major components across the various vendors but the fundamental principles are common. Figure 1.1 shows a schematic of the modern Sizewell B PWR.

Principal PWR vendors included Westinghouse, Babcock and Willcox, Combustion Engineering in the US; in Europe, Framatome in France and Kraftwerk Union (KWU) in Germany.

Modern PWR cores comprise assemblies containing fuel rods and absorber rods in a vertical bundle. The rods are arranged in a lattice of 17 X 17 positions. Of these, about 264 positions are occupied by Zircaloy-4 clad fuel rods of about 3% enriched U-235, the remainder of positions are occupied by absorber rods.

The vessel contains light water at a sufficiently high pressure to prevent boiling. The discharge temperature and pressure are about 320°C and 15.7 MPa, respectively. Reactivity is controlled by positioning of the control rods and by managing an appropriate concentration of boron in the coolant. Water is pumped to the steam generators, from which heat is transferred to the secondary side operating at a pressure in the region of 6-8 MPa. Steam produced is passed through moisture separators and dryers before entering the turbine generator. It is subsequently condensed, reheated and returned to the steam generators and the cycle is repeated. There are some differences in detail between different designs.

Typical features of some of the principal designs are as follows. In the Westinghouse PWR for example, the steam generators consist of inverted U tubes immersed in water within the secondary side loop. Other designs, e. g. Babcock and Willcox incorporate once through steam generators, which enable the steam to be slightly superheated.

Other designs exhibit different distinctive features, e. g. in the KWU reactor there are no penetrations in the lower head of the reactor vessel. The KWU design also incorporates a

spherical (as opposed to a cylindrical) containment principle. It includes a steel containment structure encompassing the primary system, which is itself enclosed in a reinforced concrete building.

Framatome have introduced boron carbide control rods in contrast to the silver-indium- cadmium rods of other designs to enable greater flexibility of control. The company has also pioneered further improvements in respect of extended fuel cycles and the use of MOX fuel.

PWRs have operated very successfully over many years. A wealth of experience has therefore built up that has resulted in improved operational, cost effectiveness and safety. PWRs are the most widely used plants in operation in the world today, both in terms of the number of units, the quantity of electricity produced and in their distribution worldwide. Table 1.2 indicates that by the end of the 1990s, PWRs dominated the generating capacity of nuclear reactors worldwide; there are about 204 units producing a gross capacity of 203,228 MWe in 15 different countries. This trend continues today.

PWRs are refuelled off-load. During refuelling, a third of the spent fuel is removed, the remaining two-thirds is relocated to different parts of the core and new fuel is loaded. The core is arranged to provide optimal performance. A disadvantage of the PWR is that it can only be fuelled off-load, which means that the reactor has to be down for 4-6 weeks. During the outage, maintenance operations can be carried out. Typically, once every 3 years, the pressure vessel and internals are inspected, which means that all the fuel has to be removed and this outage might take up to 3 months.

In terms of running costs, these reactors along with most other current plants require some degree of uranium enrichment, and therefore fuel costs are relatively high. Against this they utilise abundantly available water both as moderator and coolant — the cost of these being low. Overall PWRs can compete economically with fossil fuel plants over many years.

PWRs have a relatively complex technology requiring diverse safety systems to guard against major loss of coolant accidents. Modern PWR designers have recognised this weakness and have attempted to simplify the complexity (and hence reduce capital costs) in new proposed designs. These are discussed in later chapters in the book.