For a long cycle length, burnable poison rods are used to maintain moder­ator temperature coefficient negative as mentioned in the list [1](1) of Sect. 3.3.4. However, a large amount of burnable poisons causes a reactivity penalty at EOC that cannot be ignored (even if burnable poison were depleted, the effect of a structure such as the cladding remains) and it gives rise to a problem in spent burnable poison as a solid waste. Gadolinia-added fuel for PWRs was developed and employed to solve these problems.

In the design of the gadolinia-added fuel assembly, gadolinia content and the number and location of gadolinia-added fuel rods are investigated for required performance. Having a large number of gadolinia-added fuel rods gives a high reactivity suppression effect (reactivity reduction at BOC). The location of the gadolinia-added fuel rods in a fuel assembly can be determined to reduce power peaking through the fuel burnup. Figure 3.58 depicts an arrangement of 24 gadolinia-added fuel rods in the 17 x 17 type fuel assembly and Fig. 3.59 shows the variation in infinite multiplication

factor with burnup. is suppressed by the gadolinia-added fuel rods at the beginning of burnup, but increases with burning of the gadolinia and reaches a peak when the gadolinia is almost depleted, and then decreases as fuel burnup continues.

The larger content of gadolinia (wt%Gd2O3) leads to its slower depletion and a longer reactivity suppression effect. This is because the self-shielding effect of gadolinia becomes stronger. Figure 3.60 compares the nuclear enthalpy rise hot channel factor (F^H), which usually occurs at a gadolinia — added fuel assembly or a neighboring fuel assembly. In considering the behavior of reactivity variation, the power peaking factor in the case of low-content gadolinia tends to increase with gadolinia depletion and it becomes a maximum in the late of the cycle. High-content gadolinia oppositely leads to a mild variation and no peak in the nuclear enthalpy rise hot channel factor. Thus, higher content of gadolinia is suitable for a longer cycle length.

The addition of gadolinia to uranium fuel causes deterioration of thermal conductivity and lowering of melting point, and reduces the margin of fuel centerline temperature against melting. To secure the same mechanical

integrity as that of conventional uranium fuel, the gadolinia-added fuel rod is designed to have low uranium enrichment to suppress an increase in linear power density.

In 10 wt% Gd2O3 gadolinia-added fuel, for example, the gadolinia-added fuel with 3.2 w% enriched uranium is used instead of the conventional 4.8 w% enriched uranium.