Quenching of high temperature VVER fuel after long term oxidation in hydrogen rich steam

Abstract A cleaning tank incident at Paks NPP resulted in severe fuel damage of thirty VVER-440 type assemblies. The fuel rods heated up due to insufficient cooling and the zirconium components suffered heavy oxidation. Opening of the tank and quenching of the assemblies by cold water led to fragmentation of brittle zirconium components. In order to improve the understanding of the phenomena that took place during the Paks-2 incident, integral tests with electrically heated fuel rods have been carried out. The experiments covered the whole scenario of the incident, including the long term oxidation in hydrogen rich steam atmosphere and final quenching by cold water. The final state of the bundles was very brittle, the fuel rods were fragmented and they showed many similarities with the results of the incident at the NPP. For this reason, it is very probable that the thermal conditions and chemical reactions were also similar in the tests and in the incident. The maximum temperature in the cleaning tank was probably in the range of 1200–1300 °C. The hydrogen in the Zr components could reach 20,000 ppm, while the hydrogen concentration in the atmosphere of the cleaning tank could be above 80 vol.%. The post-test examination of test bundles indicated that the high degree of embrittlement was a combined result of oxidation and hydrogen uptake by the Zr components.