Microstructure, thermophysical properties and neutron shielding properties of Gd/316 L composites for spent nuclear fuel transportation and storage

Abstract

In this paper, (0.5–5.0 wt%)Gd/316 L composites were prepared by vacuum arc melting, and the microstructure, thermophysical properties, and neutron shielding properties were analyzed deeply. The results showed that the Gd/316 L composites started solidification in primary ferrite mode and terminated solidification by a peritectic-type reaction forming (Fe, Ni, Cr)3Gd that is rich in Ni and Gd. In the peritectic-type reaction, local convection in the Ni-rich and Gd-rich peritectic liquid phase leads to the formation of multiple variants of (Fe, Ni, Cr)3Gd. The higher the Gd content, the greater the amount of (Fe, Ni, Cr)3Gd and ferrite, and the harder it is to keep the austenitic matrix of 316 L stainless steel. The solidification temperature range of Gd/316 L composites is 1074–1460 °C, which severely limits the ability to further process it by high-temperature deformation techniques such as hot rolling or hot forging. (Fe, Ni, Cr)3Gd preferentially nucleates and grows in the triangular region of austenite grain boundaries at 1074 °C. At 50–500 °C, with increasing Gd content, thermal expansion and thermal diffusion coefficients become decrease, and thermal conductivity and specific heat capacity increase. The trend of thermophysical property changes is favorable for SNF transportation and storage applications. The thermal neutron shielding properties of 1.0 wt%Gd/316 L composite is comparable to that of 4.45 wt% B content boron steel and 15 wt%B4C/Al composite. The Gd content in Gd/316 L composites material should not exceed 2.5 wt%. If the Gd content continues to increase, it is of little significance to improving shielding properties and material thinning. A shielding thickness of 3 mm is more appropriate for Gd/316 L composites. Too thin would not provide sufficient mechanical properties and increase processing and manufacturing costs.