The utilization of freeze-cast scaffolds for burning transuranic elements in SMRS

This work aimed at investigating the viability of burning transuranic elements in SMRs, using the IRIS model as a basis, including freeze-cast sc olds in the fuel pins. The IRIS reactor is an integral design reactor, having all of its primary components inside the pressure vessel, assuring unique characteristics regarding economy, safety and non-proliferation. Freeze-cast scaffolds are strong solid structures, built in a process of freezing powder material. When applied to the fuel pins, they allow a precise placement of different materials. Am-241 is a representative of the transuranic elements with high cross section values in the thermal spectrum, presenting itself as a great candidate for this application. Two models were compared: the Am model, having two types of fuel - the americium alloy and UO₂ -, and a reference model, with the freeze-cast structures but only uranium as a fuel. Through the utilization of the package MCNP5, it was possible to obtain and analyze: (I) k_eff values, (II) axial and radial flux proles, (III) maximum fuel temperature and (IV) power peaking factors. It was possible to observe that the k_eff values, even though higher than 1, are considered small in both models when compared to PWRs in beginning of life, compromising the fuel life, and showing a tendency of neutron absorption by the ceramic scaffolds. The values were even lower for the Am model, confirming the suspicion that americium would be a great neutron absorber due to the resonances observed in the cross section. However, shutdown margin was achieved in both models, both in hot and cold conditions. The flux profiles showed consistency, showing a visible flattening in the radial flux profile for the Am model. The power peaking factors are close to those of a typical PWR, and fuel maximum temperature also showed satisfying numbers. (author)