Corrosion resistance of microfuel elements in air during contact with austenitic steel fuel-assembly components

Abstract

A problem arising in the development of light-water reactors with a microfuel-based core is guaranteeing safety in accident regimes where the temperature of the outer surfaces of the microfuel elements reaches 1200-1400°C [1-3]. It has been determined experimentally [4] that unirradiated microfuel elements with a silicon carbide outer protective layer possess high corrosion resistance and integrity in a vapor-gas medium at temperatures up to 1600°C and have successfully passed heat-resistance tests by preheating in air up to 800-1600°C followed by rapid cooling in water. However, these tests were performed with no contact with the structural materials of fuel assemblies, while during a serious accident the chemical interaction between the metal and microfuel elements can destroy the protective outer layer of silicon carbide. Investigations of the interaction of sintered silicon carbide with zircalloy, austenitic steel, and oxides of various elements which are constituents of steel (FeO, Fe 3 O 4 , Cr 2 O 3 , MnO, and others) in air at high temperature 1450-1800°C established that silicon carbide is unstable under conditions where it comes into contact with type 316 austenitic steel at its melting temperature of 1450°C [5]. However, calculations have shown [1] that the most likely maximum temperature in a reactor core during a loss-of-coolant accident is ∼1200°C (for nominal VVER-1000 parameters). At the present time, there were no data on the behavior of the outer silicon carbide coating of microfuel elements in oxidative media in the case of contact with austenitic corrosion-resistant steel at the indicated temperature. It should also be noted that the structure and physical properties of sintered silicon carbide differ substantially from those of the carbide obtained by chemical gas phase deposition in a fluidized bed [5]. In this connection, investigations were performed on the corrosion resistance and integrity of silicon carbide coatings on microfuel elements coming into contact with austenitic stainless steels 08Kh18N10T and EI-847 (05Kh15N16M3B) in air at 1100-1200°C. In addition, the corrosion resistance of this steel under the same conditions was evaluated. Microfuel elements from two batches with different uranium oxide kernel diameters (300 and 500 μm) and multilayer cladding consisting of pyrolytic carbon and silicon carbide were chosen for the investigations. The outer protective silicon carbide coating on the microfuel elements was -50 pm thick in the first batch and -140 μm thick in the second batch. The nominal characteristics of the microfuel elements are presented in [4].