Hydride Formation in Cladding Materials Studied via In Situ Environmental Heating Transmission Electron Microscopy

Rajasekhara, Shreyas, K. Hattar, V. Tikare, R. Dingreville and B. G. Clark, 2012
Zr-based nuclear cladding materials observed in situ TEM
Image courtesy of Sandia National Laboratories

Abstract

The mechanical properties of Zr-based nuclear cladding alloys are inherently affected by the presence of Zr hydrides, which are known to embrittle the cladding if oriented along radial directions. Radially oriented hydrides can be precipitated by a reorientation process when the cladding tubes cool under stress from temperatures at which pre-existing hydrides dissolve into the host matrix. This is of particular concern for fuel assemblies in long term dry storage, where the cladding must maintain its strength and ductility over potentially hundreds of years. The aforementioned precipitation mechanism is highly dependent on the underlying host microstructure, comprised of grains, precipitates and line defects. In order to develop mesoscale models capable of predicting the structural integrity of nuclear cladding materials during dry storage, a detailed knowledge of the hydride precipitation mechanisms as a function of alloy composition, microstructure, cladding texture, and cooling rates is needed. To date, past studies have employed ex-situ characterization techniques such as optical and transmission electron microscopy (TEM), electron backscatter diffraction, and synchrotron X-ray diffraction to examine hydride precipitation in cladding materials. These ex-situ studies have suggested hydrogen uptake and hydride formation depend on hydrogen content, modes of cladding material fabrication, heat treatment procedure, and the presence or absence of stress in the cladding material. However, there remain questions regarding the details of the hydride precipitation mechanisms as a function of alloy composition, microstructure, cladding texture, and cooling rates is needed.

Impact Statement

The study of hydride formation in Zirlo™ cladding material using a Protochips environmental in-situ heating TEM stage. Two Si-based chips with 5 nm-thick SiNx viewing windows (one of which was patterned with a resistive heater) comprised the gas cell held in the stage tip. This contrivance allowed the researchers to observe in-situ the material’s microstructural transformation as it was exposed to temperatures up to 1200 °C and pressures up to 1 atm. The results show the disappearance of microstructural features and the formation of a new grain, demonstrating the possibilities of using in situ observation to verify and validate predictive material models.