Once the enrichment and number of assemblies required has been determined as outlined above, the designer needs to evaluate the need for BPs in the fuel assemblies. Since sufficient enrichment (and reactivity) has to be present for the lifetime of the fuel, the excess reactivity has to be controlled, particularly during the first cycle of irradiation. In general for large PWRs and for iPWRs, the core-wide (global) reactivity during the cycle is controlled by means of either boron in the coolant, or by use of control rods. The BP content (type of material, number of rods, content in the rods) is not a design requirement, other than it assists in controlling power peaking within (fuel rod powers) and between the assemblies (assembly powers), and it assists in reducing core wide excess reactivity, thereby reducing the soluble boron concentration and resulting in a negative moderator temperature coefficient (MTC).

Materials are chosen for BPs that initially absorbs neutrons (have a high neutron capture cross-section), but upon capture, they become an isotope with a low-absorption cross-section, i. e., during irradiation, they are ‘burnable’. Examples include boron, gadolinium, erbium and dysprosium; the first two are used today on a routine basis in commercial, large PWRs and these are the most likely candidates for iPWRs also.

The absorbing material can either be mixed in with the fuel itself during the manufacture of the fuel pellets, (known as ‘integral BPs’) or can be loaded as separate components into the fuel assemblies (into the guide tubes) and so can be removed at the end of a cycle of operation (known as ‘discrete BPs’).

Boron (specifically B-10) burns out quickly because of its high absorption cross­section, whereas gadolinium, because of self-shielding effects, tends to burn out more slowly. Generally, this makes boron-based BPs more suitable for short cycles of operation, and gadolinium more suitable for longer cycles, but varying the weight percent of the poison material is used to tailor the rate of burnout required.

A comparison of the burnout rates, and extent of the reactivity hold down can be seen in Figure 4.2 for examples containing gadolinium. It can be seen that the overall reactivity doesn’t quite return to the level of the no burnable poison cases. This is due to residual absorption (albeit relatively small) from the remaining gadolinium in the fuel. For fuels that contain boron as the BP, such as IFBA (which is an ‘integral fuel burnable absorber’ technology where a boron coating is sprayed onto the outside of the fuel pellets), there is no residual absorption in the fuel.

Other performance considerations for the designer may include helium build-up (a result of neutron capture in B-10), or thermal conductivity, both of which can limit the fuel performance. For example, gadolinium has a lower thermal conductivity that uranium, and so the fissile enrichment of the carrier material for the gadolinium poison is deliberately lowered to avoid power peaking concerns.

For those iPWRs that are looking at either very long cycle lengths (of the order of a few years), or those that need a much longer lasting reactivity hold down (for example, those that do not use boron in the coolant), erbia is another contender. Erbia has a relatively low absorption cross-section compared with boron-10 or gadolinium, which means it depletes slowly. In addition, all of the isotopes of erbia have reasonable absorption cross-sections, which means the isotopes produced by capture also have a reactivity hold down effect.

The designer has to consider the magnitude of the reactivity hold down, the duration, and the rate of depletion. Combining the number of BP rods, weight percent of the BP in those rods, as well as the BP material type gives the designer sufficient degrees of freedom to achieve the desire outcome. Examples of indicative gadolinium pin locations are provided in Figure 4.3 by way of illustration. Note that the BPs are loaded near the water holes (instrument and guide thimbles) as additional thermalization of neutrons improves their effectiveness.