The majority of PWR designs use unshrouded fuel assemblies. In these the core effectively consists of an open array of fuel rods. The fuel assemblies consist of square arrays of pins. The number of pins and the pin diameters vary but a typical 17 x 17 fuel assembly is illustrated in Fig. 10.9. The fuel rods are held on 17 x 17 grids. This provides 289 positions. The central position is reserved for in-core instrumentation and another 24 positions are occupied by thimble tubes. These provide part of the skeleton of the fuel assembly and are structural elements, which are joined to the top and bottom nozzles as well as the spacer grids. The reactor control rod cluster assemblies (see Fig. 10.9) are inserted into these tubes in some of the assemblies. The control rod positions are fixed by the design of the plant but as all the fuel assemblies can accommodate RCCAs this allows flexibility in the placement of the assemblies in the core loading pattern.

The fuel rods themselves consist of zirconium alloy tubes into which are inserted pellets of enriched UO2. The rods incorporate a gas plenum to accommodate fission gases produced during irradiation. In general two lengths of fuel rod are commonly used, corresponding to active core heights of 12 foot (‘standard’) or 14 foot (‘XL’), or their metric equivalent. In the Westinghouse design these cores can be accommodated in the same vessel. The XL core needs a longer core barrel, which extends into the lower head region. PWRs can also use mixed oxide (MOX) fuel where plutonium oxide provides the initial fissile loading rather than 235U. Because plutonium has a lower delayed neutron fraction than uranium the shutdown margin is affected and if more than about 30% of the core is loaded with MOX then additional RCCAs are required.

The enrichment used depends on the target burnup and the fuel cycle length. Originally PWRs were designed to run on a 12 month cycle and achieve a burnup of about 30 GWd/t. Each fuel assembly stayed in the core for three cycles and a third of the core would be changed at each refuelling outage. The factor determining the burnup and hence dwell time was largely the performance of the fuel cladding. For all plants which refuel periodically rather than on-load, the reactor must be designed to cope with a number of failed fuel rods, since it would be uneconomic to shutdown, to replace fuel failures, on an individual basis. At one time the design basis was specified in terms of being able to operate with a very small percentage (~0.25%) of fuel failures. In practice it is now controlled by limiting the maximum allowable activity levels in the primary coolant and operators demand very high fuel reliabilities. Operating with failed fuel makes maintenance more complex and increases operator radiation exposure. Cladding materials have been developed to give greater resistance to radiation effects, which has allowed burnups to be extended to more than 60 GWd/t. Modern cladding materials are based on zirconium alloys. Stainless steel has been used in the past, in some fuel, but the performance of zirconium seems to be superior.

Within the constraints provided by the fuel design, the actual fuel cycle used is largely a matter of economics. Long fuel cycles mean that the ratio of generation time to refuelling time is increased. However, increasing the cycle length increases the enrichment required and also increases the number of fuel assemblies which have to be replaced at each refuelling. Small batch sizes tend to allow more efficient fuel utilisation. The replacement of individual assemblies when they reach optimum burnup is ideal, but can only be practically achieved by on-load refuelling, which is not possible for PWRs. Thus the fuel costs tend to be higher for long fuel cycles, but the average outage costs are lower. In addition there is a practical limit on the maximum enrichment currently used. Most fuel fabrication plants and transport containers are designed for enrichments of up to 5%.

Cycle lengths of between 6 months and 2 years are currently used. The short cycles are used by some German Konvoi plants, which were designed to allow rapid refuelling. The commonest cycle lengths are 12 and 18 months. Eighteen months is widely used, particularly in countries which have peaks of demand in both summer and winter, since the outages can be alternated between spring and autumn when demand is lower.