Another variation in some iPWRs that also affects the BP loadings and the complexity of the fuel design is core operation in a once through/single batch reload, i. e., all of the core is replaced each cycle of operation. mPower and SMR-160 are examples where this design option has been examined. Since the fuel is all fresh for each reload, it simplifies the nuclear design because the same fuels, loaded in the same locations with the same enrichment, BP loadings, etc., can be repeated for every cycle of operation, i. e., once a truly optimized nuclear design has been produced, it is simply duplicated for every cycle and for every reactor deployed.

Fresh fuel also has a distinctive axial power profile, with a distinctive chopped cosine power distribution, peaking at the center. Therefore, if all fresh fuel is loaded into the core, in the absence of either control rods inserted or axially varying burnable poison distribution, the power peaking limits will be violated. Irradiated fuel has a much flatter axial power distribution and in those cores with a mix of fresh and depleted fuel, the flattening influence helps to reduce axial power peaking.

All fresh fuel also means that the designer cannot rely on irradiated fuel to help in smoothing the power distribution across the core, and so only BPs and control rods can be used. This tends to result in higher control rod and BP use in the designs rather than utilizing the lower excess reactivity that arises because some of the fuel has been irradiated. As with fresh cores in large PWRs, the use of asymmetric BPs in certain core locations may be required to achieve the power peaking limits, which also adds to the cost of the fuel.

Special attention also has to be paid to the use of a single batch core in terms of uranium ore and enrichment utilization, by ensuring that the fuels achieve their full potential burnup. Fuel loaded on the periphery of the core will tend to have lower powers and resulting burnups compared with the leading power assemblies towards the center of the core, and since the fuel is loaded for only one cycle of operation, there is no opportunity to reload the fuel to achieve higher burnups and ensure that all of the fuel within a given batch achieve similar discharge burnups. This is an indication of how single-batch cores are not as efficient as multi-batch (see Section 4.3.1).

Some of the single-batch core concepts include a cartridge type fuel unit. This suggests (although the designs are yet to be finalized) that the fuel is loaded into the reactor as one single unit, including control rods already inserted. Although this will speed up the core loading and inspection routine, and assist in the criticality safety case for fresh and spent fuel, the cartridge approach will also make it more difficult to inspect and replace any damaged fuel pins. For example, fuel inspection once at the reactor site prior to startup, and rod replacement in the event of a leaker during operations.

Due to the power output required, and the fuel management of the iPWRs under development today, there is a large variation in the fuel demands per reload (see Table 4.1). These limited examples indicate that the iPWRs tend to require more fuel on a per GW year-electrical basis compared with a large PWR such as AP1000. What has to also be take into account in the overall economics, however, is the outage time — for an iPWR with a three-year cycle length compared with an AP1000 with a one-and-half-year cycle length, the iPWR will have half as many outages for refueling and maintenance, which represents something of the order of 750 more days availability to produce power.

It is likely that each of the iPWR designs will have the potential to operate with a range of fuel management schemes, depending on customer and market needs, and overall economics, and further developments of the nuclear design options can continue during the development and demonstrations phases.