Alloying Effect of Niobium and Tin on the Zirconium Alloy Fuel Cladding Behavior at High Temperature Oxidation in Steam

Abstract

As the materials for fuel claddings for water-cooled reactors, binary and multicomponent zirconium alloys zirconium-niobium (Zr-Nb) (E110, M5), zirconium-tin-iron (Zr-Sn-Fe) (Zircaloy-2,-4), and zirconium-niobium-tin-iron (Zr-Nb-Sn-Fe) (E635, ZIRLO®) most commonly are used. Improvement extends, in particular, by varying their composition by niobium, tin, and iron. At the same time, importance is given to ensuring the safe operation of the fuel rods not only in normal conditions but also in emergency situations such as a loss of coolant accident. In this paper, we present the research results of studies of the influence of alloying elements niobium, tin, and iron on corrosion resistance and embrittlement at high-temperature steam oxidation of fuel claddings based on the alloy types Zr-xNb (x = 1.0 ÷ 2.5) and Zr-xNb-ySn-zFe (x = 0.6 ÷ 2.4; y = 0.24 ÷ 1.1; z = 0.18 ÷ 0.34) made by using zirconium sponge. We conducted high-temperature steam oxidation tests at temperatures of 1,000°C, 1,100°C, and 1,200°C with continuous measurement of the weight gain during the experiment and subsequent cooling in steam with rate ∼20°C/s. We conducted studies of kinetics of high-temperature oxidation in steam, structural-phase state changes, alloying elements distribution, absorbed hydrogen content, and residual ductility after rapid cooling of the oxidized specimens. We revealed the difference in the oxidation kinetics of the materials studied, which decreased as the oxidation temperature increased. Likewise, an increase of the tin content in the alloy influenced a specimen's breakaway oxidation intensification at 1,000°C. This shows that the mechanical properties of fuel claddings oxidized under the same conditions depend on their alloying composition, and the hydrogen fraction is absorbed by the specimen and formed by the oxidation of ZrO2, α-Zr(O), and ex-β layer structures, which in turn varies depending on the alloy composition. The increase of niobium, tin, and iron content in the zirconium alloy leads to a decrease in the residual ductility of fuel claddings after high-temperature steam oxidation. According to obtained results, among the materials studied, Zr-1.0Nb alloy is the most resistant to embrittlement during high-temperature steam oxidation at temperatures of 1,000–1,200°C.