Abstract
UO2–Gd2O3 fuel is mostly used as a burnable absorber fuel in the form of a homogenous mixture of Gd2O3 and UO2. More effective reactivity control can be achieved by lumping Gd2O3 within the UO2 because this enhances the spatial self-shielding factor of the burnable absorber fuel. The fabrication of lumped burnable absorber fuel containing lumped Gd2O3 spherical particles or compacts has been experimentally demonstrated using yttrium-stabilized zirconia (YSZ) as a UO2 fuel surrogate. Interfacial cracks or gaps forming under the interfacial stress that develops during the fabrication of the fuel can be eliminated by controlling the initial density of the lumped Gd2O3. In this study, this interfacial stress during the fabrication process was simulated using finite element methods. The effect of the size, shape, and initial density of the lumped Gd2O3 on the distribution and magnitude of the interfacial stress was investigated. The addition of Gd2O3 spherical particles resulted in a lower and more uniform interfacial stress distribution than the addition of cylindrical Gd2O3 compacts. The interfacial stress was increased with increasing Gd2O3 size and initial density. The calculated interfacial stress was compared with experimental results to estimate the threshold stress for crack development in a lumped burnable absorber fuel.
Highlights
The potential use of Gd as a burnable absorber was recognized many years ago, considering its extremely high thermal neutron absorption cross-section
As the pre-sintering temperature increased from 1400 ◦C to 1600 ◦C, the maximum sintering stress increased, which could be attributed to the increase in the shrinkage rate mismatch
The maximum sintering stress in the yttrium-stabilized zirconia (YSZ) oxide pellet containing a lumped Gd2O3 mini-pellet pre-sintered at 1400 ◦C during the fabrication process was calculated by finite-element analysis (FEA) as approximately 120 MPa; that with the mini-pellets presintered at 1500 ◦C was approximately 150 MPa
Summary
The potential use of Gd as a burnable absorber was recognized many years ago, considering its extremely high thermal neutron absorption cross-section. The main limitations of Gd2O3 are the significant end-of-cycle penalty due to the existence of Gd isotopes with higher thermal neutron absorption cross-sections (i.e., 155Gd and 157Gd) (Grossbeck et al, 2001) and the degradation of the fuel’s thermophysical properties (IAEA, 1995; Durazzo et al, 2013; Choe et al, 2016a). One possible way to overcome these limitations is changing the design of the UO2–Gd2O3 mixed fuel. Several burnable absorber fuel designs have been developed for small modular pressurized water reactors (SMPWRs) in order to eliminate the use of soluble boron and improve the reactor performance in terms of increasing the fuel cycle length while maintainig a flat power distribution
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