There are three burnable poison materials in common use in current fuel assembly designs, and they are boron, gadolinium, and erbium. There is also an advanced fuel design for CANDU reactors (called the CANFLEX bundle) that uses dysprosium burnable poison, but it has yet to be used on a commercial scale. They all have one or more isotopes with a large neutron capture cross-section that, following a neutron capture, are transformed into isotopes with small cross-sections.

2.16.3.1 Boron

Natural boron consists of the two isotopes 10B and 11B at 19.9% and 80.1% natural abundance,

respectively.1,2 10B has a thermal neutron capture cross-section (meaning the neutron absorption cross­section for a neutron moving with 0.0256 eV kinetic energy) of 3837 barns2 (where a barn is a convenient unit of area 10—28m2). Following a neutron capture, it is transmuted to 7Li and 4He, both ofwhich have very low neutron absorption cross-sections. For some appli­cations, for example, where it is difficult to incorporate

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sufficient quantity of 10B as natural boron, isotopically enriched 10B is available commercially. The neutron capture cross-section of 10B decreases smoothly with increasing neutron energy (Figure 2), so that slow moving (thermal) neutrons are the most likely to be absorbed. (To be strictly accurate, the cross-section shown in Figure 2 is the cross-section for neutron capture, followed by transmutation to 7Li and 4He; the true neutron capture cross-section of 10B is actually very small and is insignificant relative to the 10B! 7Li + 4He reaction.)

Early applications of boron burnable poisons in PWRs used natural boron in borosilicate glass rods that fitted into the control rod guide tubes of assem­blies not positioned under a control rod location, as illustrated in Figure 3. Such a design is called a dis­crete burnable poison rod. It has the advantage that the burnable poison material does not need to be incorporated in the fuel, and can therefore be manu­factured separately. Also, the precise number ofpoison rods can be fine-tuned before the fuel assembly is loaded in the core. Such discrete poison rods are now seldom used because the stainless steel outer clad and other structural components continue to absorb neu­trons after all the 10B is depleted; this represents a parasitic absorption penalty that costs the utility because a higher initial enrichment of 235U is needed to compensate. This effect is made worse because the
poison rods also displace water, reducing moderation in the fuel assembly and further decreasing the mul­tiplication factor. Another disadvantage is that dis­crete poison rods constitute an additional source of intermediate level waste that adds to waste manage­ment costs.

An alternative to discrete boron burnable poison rods is to incorporate a boron compound with the fuel rods themselves. The so-called integral fuel burnable absorber (IFBA) is used in many commer­cial PWRs in the United States. It consists of a thin coating of zirconium diboride (ZrB2), which is applied to the surface of some of the UO2 fuel pellets in the fuel assembly. Since zirconium has a very low capture cross-section and there is no structural mate­rial, IFBA has practically zero residual absorption penalty. A disadvantage is that the application of the coating adds an extra process in fuel manufacture and since it is hygroscopic, manufacturing needs to take place under a dry atmosphere in glove boxes. Figure 4 illustrates a typical distribution of IFBA fuel rods in a PWR assembly.