A thorough investigation was performed to understand the physics behind the calculated trends in the reactivity coefficients of hydride-fueled pressurized water reactor cores. A two-step procedure was developed for this purpose: ranking the contribution to a reactivity coefficient of each of the system's constituents; investigating, for each of the more important constituents, the spectral reasons for their specific response to the temperature perturbation. This procedure was applied to understand (a) the difference in the beginning-of-life and end-of-life fuel temperature coefficient of reactivity (FTC) behavior of hydride- as compared to oxide-fueled cores; (b) the different effect integral fuel burnable absorber (IFBA) and erbium burnable poisons have on the FTC and coolant temperature coefficient of reactivity (CTC); (c) the effects on the FTC and CTC of inclusion of thorium hydride in the fuel; (d) the effects of plutonium on the FTC and CTC in hydride- as compared to oxide-fueled cores.

It is found that the physics characteristics of hydride-fueled cores are fundamentally different from those of oxide-fueled cores as particularly manifested by the behavior of the FTC. In oxide-fueled cores the main phenomenon affecting the FTC is the well-known Doppler broadening of the fuel resonances. Hydride cores feature an additional unique phenomenon of spectral shift in the thermal energy range; it is the result of upscattering of the thermal neutrons due to the increase in the fuel hydrogen temperature. The interplay between the spectral shift and the shape of the low-energy cross sections of the fuel isotopes is responsible for the sometimes very different values of the calculated FTC for hydride- versus oxide-fueled cores and even for the same fuel type at different burnups. It is also concluded that fissile plutonium can have different effects on the FTC and, although the physics phenomena are quite different, on the CTC in hydride-fueled cores. If the plutonium is present in sufficiently large quantities, its effect can be negative, while if it is present in relatively small quantities, it is more likely to give a positive contribution. An additional finding is that the buildup of 135Xe makes the FTC less positive in hydride-fueled cores, while it has little effect on the FTC of oxide-fueled cores. Also concluded is that thorium-containing hydride fuel cores feature a smaller FTC than that of oxide-fueled cores. This is due to a harder neutron spectrum in the Th-containing hydride-fueled cores leading to a smaller spectral shift, combined with the buildup of 233U the contribution to the FTC of which in hydride-fueled cores is mostly negative.

The insight gained through the analyses reported in this work facilitated the identification of an optimal, safe uranium-based hydride-fueled core design; it consists of U-ZrH1.6 fuel in which 25% (volumetric) of the ZrH1.6 is replaced by ThH2. Burnable poisons have to be used to compensate part of the excess reactivity; IFBA is the preferred choice.