Hydrothermal crystallization was conducted to improve the diffusion barrier performance of Zr thin films to prevent a fuel clad chemical interaction (FCCI) at the interface between a clad material and lanthanide elements (Mischmetal: 75Ce-25La; intermetallic compound) as fission product. The crystalline phase and size of Zr thin films deposited on a HT9 disk by RF magnetron sputtering were varied by hydrothermal crystallization in an autoclave at 393 K, 423 K and 453 K. Diffusion couple tests of the clad with and without a Zr diffusion barrier were performed at 933 K for 25 h with mischmetal, which have diffusion properties similar to uranium metal composite fuel. While substantial FCCI occurred at the interface between the mischmetal and clad in the specimen without hydrothermal crystallization, the Zr barrier with hydrothermal crystallization showed excellent resistance to FCCI. The performance of the Zr FCCI barrier was improved due to a decrease in interdiffusion by the grain boundary, which can increase the FCCI in the Zr barrier. (C) 2013 Elsevier B. V. All rights reserved.