Positron annihilation lifetime spectroscopy (PALS) has the potential to determine open volume defect identities and concentrations only if the spectrum can be accurately decomposed into its constituent parts. The intrinsic difficulty of decomposing PALS spectra into their constituent lifetimes and intensities is demonstrated, and it is shown that the global minimum of the objective function does not represent the true solution for a range of typical experimental scenarios. We show that the function currently employed in standard fitting methods cannot be improved upon with alternate weighting schemes. Resolution function width minimally impacts fit decomposition quality but errors are reduced with higher counts. A regression model is developed based on the experimental count, intensity of the defect component, and difference between the defect and bulk lifetime which predicts the anticipated intrinsic error of the objective function global minimum in estimating the fraction of positrons which annihilate in the bulk. This can be employed to determine whether a given PALS spectrum can be successfully decomposed into defect types and lifetimes.

Accident tolerance of LWR fuels and structures is of paramount importance, as highlighted by the accident at the Fukushima Daiichi nuclear power station. The ability of fuel cladding and core internals to resist runaway oxidation during a beyond design basis accident (BDBA), as well as to minimize corrosion during steady state operation and design basis accidents (DBAs), determines its degree of accident tolerance. A gap was identified in the accident tolerant fuel (ATF) concepts under consideration, as most consisted either of coatings which may not hold their integrity under prolonged operation, or different materials altogether which incur significant neutronic penalties. A compromise was therefore sought – by minimizing the amount of coating material while ensuring a strong microstructural bond, it was thought that a Zircaloy-steel layered composite would achieve the best of both approaches.The goal of this NEUP project was to develop a multi-metallic layered composite (MMLC) tailored to addressing accident tolerance of LWR fuel cladding and core internal structures using an innovative fabrication technology. The MMLC developed in this program is expected to enhance the accident tolerance of LWRs, thereby reducing the cost by recovering lost operating margins and/or increasing operating windows of peak cladding temperature, peak linear power, reducing steady-state corrosion, and enhancing severe accident tolerance.