Polycrystalline diamond and magnetron sputtered chromium as a double coating for accident-tolerant nuclear fuel tubes

Abstract

Controlling the hot steam corrosion of nuclear fuel tubes is crucial for power plant efficiency and safety. We demonstrate an effective method of protecting ZIRLO ® (ZIRLO) fuel tubes against corrosion at accident temperatures in a water-cooled nuclear reactor environment. The tubes surfaces were coated by double layers consisting of the polycrystalline diamond (PCD) and magnetron-sputtered Cr. Coated and bare ZIRLO tubes were subjected to hot steam tests for 30 min at 900 °C followed by 40 min at 1000 °C. Surprisingly, Cr and PCD double layers ability to protect ZIRLO fuel tubes against hot steam corrosion strongly depends on the order of the layers in coatings. ZIRLO coated with Cr as the bottom layer and PCD as the top layer exhibited the best ability to slow down the oxidation process. This double layer coating reduced ZIRLO tube corrosion by 47% compared to unprotected ZIRLO. On the contrary, the double layer coating with PCD as bottom and Cr as the top layer reduced the ZIRLO tube corrosion by 20–25% compared to unprotected ZIRLO. To explain the specific roles of Cr and PCD coatings experimental and theoretical procedures were performed. Our results pointed to two basic parameters, significantly affecting the corrosion of coated ZIRLO tubes: 1. the amount of Cr carbides in the surface part of the coated ZIRLO fuel tubes and 2. adhesion of the coatings to the ZIRLO substrate.