4.4.1 Fuel designs in the smaller cores

From a nuclear design perspective, the first iPWR design specific that needs to be considered is the compactness of the core compared with large PWRs, both in terms of the number of fuel assemblies, and in some cases, the height of the fuel assemblies. Table 4.1 provides a summary of the key nuclear design parameters for the four US iPWR designs submitted in 2011 to the US Department of Energy (DOE) first solicitation, along with a modern large PWR, the Westinghouse AP1000.2 It is important to note that although the fuel heights, and overall loadings are different in each of the iPWRs, the 17 X 17 array of fuel pins is consistently the same throughout (Figure 4.6). This is primarily driven by the excellent operational performance of these fuel types in large PWRs operating today, and in particular it builds on the extensive development work that has evolved fuel designs towards a larger number of thinner fuel pins in order to improve thermal margins and allow higher rod powers. These are equally important drivers in the deployment of iPWRs, hence the choice of 17 X 17 fuels. The lower linear heat ratings in the case of the NuScale and mPower iPWRs in particular will reduce fuel and cladding temperatures during operations and accidents, as well as allow a higher relative power for the lead fuel rods during normal operations and therefore the ability to tolerate more power peaking in the core.

As shown in Figure 4.4, a large PWR has anywhere from 157 to 193 fuel assemblies compared with iPWRs which typically have between 37 (SMR-160)3 and 89 (Westinghouse SMR).4 It may appear that having fewer fuel assemblies make the design of the core and the loading pattern more straightforward. But in fact, fewer assemblies means fewer degrees of freedom to optimize the loading pattern and smooth out the reactivity and resulting power distribution across the core. The degrees of freedom are not just in terms of the fuel design and BP loadings of the fresh fuel, but once beyond the first cycle of operation, the batches of previously irradiated fuel will have a variety of burnups and hence resulting reactivities that can be distributed throughout the core to smooth the power distribution to achieve the power peaking limits.

The smaller core size also results in more neutron leakage radially (and axially, as described later in this section), and results in a more significant variation in assembly power across the core from the center where there is more fuel and hence more power, to the outside of the core where lower power is generated. The radial

Table 4.1 Summary of key nuclear design parameters for a modern, large PWR (AP1000) and a range of iPWRs iPWR name (vendor) AP10002 (Westinghouse) Westinghouse-SMR4,5 (Westinghouse) mPower6 (Babcock and Wilcox) SMR-16037 (Holtec) NuScale15 (NuScale Power) Power Thermal (MWth) 3400 800 530 500 160 Electrical (MWe) 1150 225 180 160 45 Reactivity control Control rods Control rods Control rods Control rods Soluble boron Soluble boron Control rods Soluble boron Fuel in core No. of assemblies 157 89 69 37 37 Array 17 X 17 17 X 17 17 X 17 17 X 17 17 X 17 Active fuel height 4.3 m/14 ft 2.4 m/8 ft 2.4 m/8 ft 3.7 m/12 ft 2.0 m/6.5 ft Mass in core (MTHM) 85 27 20 15 9 Fissile enrichment <5 wt% U235 <5 wt% U235 <5 wt% U235 <5 wt% U235 <5 wt% U235 Cycle length Month 18 24 48 48 24 Fuel demand MTHM per reload 36.6 20 14.7 MTHM per GW year 21.2 Unknown 29.3 33.8 Unknown Linear rating kW/m 19 14 12 14 8 kW/ft 5.8 4.2 3.6 4.2 2.4 *All values are approximate.

leakage can be reduced by ensuring only the fuel with lower reactivities (generally higher burnups) are loaded on the periphery of the core, or by developing and using radial reflectors in the outer core such as using stainless steel, rather than water, as used in the large PWRs. Although non-standard in large PWRs, there have been designs (for example, the AREVA EPR), where stainless-steel reflectors have been developed and demonstrated to be effective. Additional development work on radial reflectors for iPWRs is likely to yield notable benefits, not only in the economics of the fuel, but also by raising the power in the outer assemblies and hence improve the radial power variations across the core.

Neutron leakage is further exacerbated by the use of short fuel in some iPWRs. Typical large PWR fuel assemblies have between 12 and 14 feet (3.66 and 4.27 m) of fuel in 264 fuel pins, whereas several of the iPWRs have moved to a partial height version of the standard 17 X 17 fuel designs, principally because full fuel heights are not required for the lower-powered reactor designs. In each case, the fuel designs have been developed and optimized for the specific requirements and demands of each iPWR, e. g., rating and natural convection. For example, the Westinghouse SMR and mPower iPWRs has an active height of approximately 8 feet (2.4 m), and the NuScale design is approximately 6.5 feet (2.0 m); see Table 4.1 and Figure 4.6 for a relative comparison of fuel heights.

However, there is extensive experience in large PWRs to reduce the axial leakage by using what are known as ‘axial reflectors’. These are simply sections of the active fuel height (typically a few inches at the top and bottom of the fuel stack) where the usual fissile enrichment is reduced or in some cases, natural (0.71 wt% U-235) or even tails (~0.3 wt% U-235, a by-product of the enrichment process) uranium is used. This not only improves the fuel economy by reducing the neutron leakage from the core, but also reduces the costs of the fuel in terms of enrichment needs in an area of the fuel and core where there is relatively little power generated, and therefore little need to enrich the fuel.

However, axial variation in the fuel stack tends to increase the cost of the fuel, not in terms of enrichment or ore costs, but simply because of the complexity of manufacturing a fuel type with more enrichments, zones and diversity; the manufacture and in particular the quality assurance (QA) and quality control (QC) cost components are increased.

By way of illustration, based on current market prices for uranium fuels, if an increase of 0.25 wt% U-235 was required in a reload of fuel to offset either neutron leakage, or inefficient use of the fuel (such as lower burnups achieved), the increase in fuel price would be of the order of a few million dollars per reload. Cost increases because of additional fuel complexity would be the same order of magntiude.