Natural dysprosium contains a mix of seven stable isotopes 156, 158, 160, 161, 162, 163, and 164 with natural abundances of 0.06%, 0.10%, 2.34%, 18.9%, 25.5%, 24.9%, and 28.2%, respectively. The low abundance of 156Dy and 158Dy are such that only the latter five make a significant contribution to neu­tron capture in a dysprosium poison rod. 164Dy has the highest thermal capture cross-section (Figure 10) and combined with 28.2% initial abundance, it dominates the overall neutron capture rate. On capturing a neu­tron, 164Dy changes to 165Dy, with a small cross­section. The presence of the 160, 161, 162, and 163 isotopes, each of which has a small but nevertheless significant neutron capture cross-section, causes dys­prosium to have a high residual absorption penalty. Moreover, the residual absorption does not burn out because for all the isotopes, a neutron capture event generates the next higher dysprosium isotope and a significant residual cross-section remains.

The first application of dysprosium burnable poi­sons is likely to be in advanced CANDU heavy water moderated reactors. A new fuel design consisting of a bundle of 42 fuel rods arranged in three annular rings around a central uranium oxide/dysprosium

oxide (UO2/Dy2O3) poison rod. The dysprosium plays an important role in helping to ensure a more negative void reactivity coefficient and more negative total power reactivity coefficient, as well as con­trolling the neutron flux shape across the core, while maximizing the power density of the reactor (which in turn has a direct impact on the economics of reactor operation). The void coefficient is the reactivity change in response to steam void formation in the core. The power coefficient is the change in reactivity following a perturbation to reactor power. Both of these coefficients are important in ensuring that the inherent response of the reactor in transient conditions is safe. The presence of a persistent neu­tron absorber, such as dysprosium, helps to remove thermal neutrons in the presence of steam void and thereby helps to maintain a negative void coefficient.