The engineering design process starts with the utility defining its preferred refueling interval (usu­ally between 12 and 24 months depending on the grid demand profile) and the average load factor expected. This in turn determines the excess reactivity at the beginning of the cycle and the difference between this and the reactivity hold-down capability of the control rods and soluble poison (if applicable) defines the reactivity hold-down requirement for burnable poisons. The nuclear designer must then set the num­ber and location of burnable poisons to meet this requirement. The next step is to adjust the burnable poison loading in each location such that the poison material is nearly completely depleted by the end of the cycle. For IFBA poisons, this may involve choosing to use enriched 10B, while for gadolinia, it will involve choosing the optimum initial concentrations (usually in the range 4-8 wt% gadolinia). This is usually an iterative process carried out as part of the core design; it involves finding the fuel assembly loading pattern that best meets the design and safety constraints. Clearly, a compromise is required between the nuclear designer’s ideal, which might demand different num­bers of poison rods in each assembly and different initial poison concentrations, and the manufacturing constraints, which demand as simple a solution as possible.

The initial choice of burnable poison is usually dictated by the manufacturing capabilities of the fuel supplier. The IFBA design requires a major capital investment in the fuel manufacturing plant, while gadolinia rods usually have to be manufactured in a dedicated facility to avoid inadvertent contamination of the main fuel production lines (where even a few ppm of gadolinia contamination can cause fuel to go out of specification). These capital costs and the associated operational costs are passed on to the utility as part of the fuel fabrication price and appear as an indirect cost to the utility. Depending on the burnable poison type, there may also be a residual absorption penalty. For a given cycle length, this will increase the initial enrichment requirement of the fuel, which the utility sees as a direct cost.

Though large in absolute terms, relative to the overall generating costs of a commercial reactor, they are not so important. The benefit to the utility comes through being able to extend the fuel cycle length as much as possible. For a fixed refueling and mainte­nance outage time, the overall electrical output can be increased if the time between refueling outages is extended. Since fuel costs typically only represent around 20% of the overall generating cost (with capital and finance accounting for about 60%), the penalty on the fuel cost is easily outweighed by even a modest increase in output. Therein lies the main benefit and justification of burnable poisons. Absorp­tion of neutrons is generally regarded as a thing to be avoided in reactor design and operation, but burnable poisons are usually important in allowing reactor operations to be optimized. Even relatively small incre­mental improvements in reactor operations are very often sufficient to offset the negative aspects of burnable poisons, such as residual neutron absorption.