Once an initial estimate has been determined of the enrichment, the number of fuel assemblies to be loaded in the cycle, and the types of BPs that are most suited for the iPWR being developed, the next phase is to determine how to load the core, and which of the assemblies loaded in the previous cycle of operation are available and suitable to be reloaded. A major part of the nuclear design effort is concerned

GTGuide tube [t] Instrument tube ^Gadolinium pin

Figure 4.3 Examples of burnable poison distribution designs.

with this phase, typically referred to as the in-core fuel management, and it brings together many of the key interactions and issues discussed above.

The objective is to load the core to provide a flat/smooth distribution of reactivity across the core, i. e., high and low ^-infinities balancing each other. This distribution of the assemblies in the core, is known as the ‘loading pattern’. If reactivity is too high, then assembly powers will be high, and this in turn will result in power peaking in the fuel rods being too high (see Section 4.2.3). The core is loaded in quarter core symmetry to avoid power and performance tilts in the core, e. g., one quadrant running at higher power than the others. This means that the fuel assemblies are

loaded in groups of four, one in each quadrant. In most iPWRs (and most large PWRs) the fresh fuel is loaded towards the center, and the previously irradiated batches of fuel are loaded towards the edge of the core. This improves the neutron economy by reducing the radial neutron leakage out of the core and is known as a ‘low leakage loading pattern’ (or L3P). This is particularly important for iPWRs because the smaller cores, with a high surface to volume ratio result in a much higher neutron leakage than large PWRs. This does tend to make the iPWRs more prone to power peaking (with all of the higher reactivity at the center of the core), and as such, higher BP loadings are required.

Typically the fresh fuel is loaded in a checkerboard configuration (towards the center as explained above), and the highest ranking in reactivity of the previously irradiated fuels are then loaded into the core design according to ^-infinity ranking. Three-dimensional reactor analysis tools (such as CASMO-SIMULATE) are then used to calculate the assembly, and pin power distribution and to determine if the constraints are met for the fuel. If the peaking is not met, then the fuel can be ‘shuffled’ to improve the power peaking. This can either be done by use of the designer’s skill and experience, or by using loading pattern optimization tools. At the same time as assessing the core loading for power peaking, the energy requirements (cycle length) are also checked to ensure that there is sufficient enrichment loaded in the fresh fuel assemblies. The number and content of the BPs can be changed if sufficiently low peaking cannot be achieved, but the designer also has to be aware that once the BPs deplete in the core the power distribution will change, and can lead to higher pin power peaking during the cycle than at the start.

For the designer, the other key criterion during this stage of the design process is the economics of the required fuel. The main variables that affect the fuel costs for a given cycle of operation will be (a) the number of fresh fuel assemblies required, and (b) their enrichment. Since the fuel is loaded in quarter core symmetry, an additional new fuel assembly required in a quarter core means the purchase of four assemblies, an increase of the order of several million dollars in fuel price. Similarly, if the core is designed such that there is too much radial or axial leakage (particularly relevant to the small cores of the iPWRs), then additional enrichment in the fuel will be required to achieve the required cycle length and fuel discharge burnup. This results in an increase not only in the enrichment costs, but also the uranium ore required to be purchased.

Reducing the number of fuel and BP rod types is also a means by which the designer can improve the fuel economics. Good practice involves keeping the number of fissile enrichments to a minimum across the assemblies, rather than having multiple enrichments, either within an assembly, or within a reload batch of fuel. For a reload of fuel, two or three enrichments would not be unusual. Similarly for the BP types and enrichments of fissile material in the BP rods and the BP loading itself, minimizing the variations is key to the fuel costs. In this case, good practice may be to use only one BP type (e. g. gadolinium) and, for example, only three different weight percentages across all fuel designs, e. g., four, six and eight. Also, the designer must ensure that the BP burns out fully, i. e., there is no residual absorption of the BP in the fuel, which is particularly important for highly poisoned cores where the
impact would be greater. For those iPWRs that require a high BP loadings (e. g., for long cycles, or single-batch cores), minimizing the residual absorption by using boron rather than gadolinium, or a combination of the two would be warranted.

The compactness of the iPWR combined with fewer assemblies available (in terms of absolute number, variation in burnup, BP types, etc.,) makes the loading pattern design process more challenging and achievement of an optimized core design more difficult. For example, iPWRs have of the order of one-fifth of the fuel assemblies of a large PWR as shown in Figure 4.4.12

Once the cycle length has been achieved with pin power peaking, fuel rod and assembly burnup within the allowed constraints, a brief safety analysis is often completed prior to the full set of detailed analyses and reports being completed. This overall saves time and money in the design process. The usual checks include:

• power peaking through the cycle;

• moderator temperature coefficient at hot full power and hot zero power;

• shutdown margin.

However, certain iPWR designs will have their own nuances that the designers will learn are key to a successful design prior to committing the nuclear design for a full safety analysis, and so ad hoc checks will be identified.