Hungarian Academy of Sciences KFKI Atomic Energy Research Institute
Abstract
The cleaning tank incident at the unit 2 of Paks NPP in 2003 resulted in severe fuel damage of 30
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. Due to the poor instrumentation there were many
open questions concerning the course of the incident and the behaviour of fuel assemblies. In order to
improve the understanding of the phenomena that took place during the Paks-2 incident integral tests
have been carried out in the CODEX (Core Degradation Experiment) facility. The tests simulated the
whole scenario of the incident using electrically heated fuel rods. The final state of the fuel rods
showed many similarities with the conditions observed after the incident at the NPP and 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 post-test examination of CODEX-CT-1 bundle indicated that the high
degree of embrittlement was a common result of oxidation and hydrogen uptake by the Zr
components
