Causes and nature of leakage of VVER-1000 FA fuel rods examined by JSC "SSC RIAR" in 2016 – 2018

Post-irradiation examinations of leaky VVER-1000 fuel assemblies are conducted as part of the 'Zero Failure of Nuclear Fuel' industry-based project. The main goal of such research is to determine the cause and nature of fuel rods leakage, to assess their condition after operation in a leaky state. In 2016 – 2018, JSC "SSC RIAR" examined five leaky VVER-1000 fuel assemblies spent at the Kalinin, Rostov and Balakovo NPPs. The main characteristics of the examined fuel assemblies are presented. The PIE results obtained for three FAs TVSA from the Kalinin NPP 3rd Unit and two FAs TVS-2M from Balakovo and Rostov NPPs showed the leakage cause to be a debris-damage of claddings with foreign items. Primary defects were located mainly in the lower part of leaky fuel rods under the AVG and SG1. Only an FA TVS-2M Gd-containing fuel rod from Balakovo NPP had a primary defect in the upper part under the SG12. The state of examined leaky fuel rods in most cases is typical for the leaky VVER-1000 fuel rods. The ingress of water into fuel rods leads to an increase in fuel temperature and gas swelling, and gas and volatile fission products release from fuel rods into coolant. Since the cladding absorbs hydrogen resulted from oxidation, its hydrogenation and embrittlement are observed in the upper part of the fuel rod. The paper contains the description of leaky fuel rods appearance, results of fuel and claddings metallography, measurements of water content in various parts of the cladding. The influence of size and location of primary defect on the degradation properties of fuel and cladding is analyzed. Keywords: VVER-1000, fuel rods, leakage, fuel, hydrogenation.