CODEX-CT-2 experiment: Long term treatment in high temperature hydrogen and water quenching of a fuel bundle KFKI-2008-02/G

The simulation of the Paks-2 incident was carried out in the frame of an experimental programme in the CODEX facility with electrically heated fuel rod bundles. The main boundary conditions for the CODEX-CT-2 were similar to the previous CODEX-CT-1 test. The most significant difference between the two tests was the operation of the air let down valve that was open in the first test and closed in the second one. In the second test the hydrogen produced in the Zr-steam reaction could not escape from the test section and it prevented the access of steam to the Zr surfaces and caused much less oxidation than was observed in the first tests. The final quench by water led to temperature excursion in the bundle and in the shroud. The final state of the bundle was very brittle, the fuel rods and the shroud were cracked and fragmented

CODEX-CT-1 experiment: Quenching of fuel bundle after long term oxidation in hydrogen rich steam KFKI-2008-01/G

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

Impact of thermal and chemical treatment on the mechanical properties of E110 and E110G cladding tubes

The mechanical and corrosion behavior of the Russian zirconium fuel cladding alloy E110, predominantly used in VVERs, has been investigated for many decades. The recent commercialization of a new, optimized E110 alloy, produced on a sponge zirconium basis, gave the opportunity to compare the mechanical properties of the old and the new E110 fuel claddings.Axial and tangential tensile test experiments were performed with samples from both claddings in the MTA EK. Due to the anisotropy of the cladding tubes, the axial tensile strength was 10–15% higher than the tangential (measured by ring tensile tests). The tensile strength of the new E110G alloy was 11% higher than that of the E110 cladding at room temperature.Some samples underwent chemical treatment – slight oxidation in steam or hydrogenation – or heat treatment – in argon atmosphere at temperatures between 600 and 1000 °C. The heat treatment during the oxidation had more significant effect on the tensile strength of the claddings than the oxidation itself, which lowered the tensile strength as the thickness of the metal decreased. The hydrogenation of the cladding samples slightly lowered the tensile strength and the samples but they remained ductile even at room temperature. Keywords: E110, E110G, Tensile test, Tensile strength, Heat treatment, Oxidation, Hydrogenatio

Applying the TRANSURANUS Code to VVER Fuel under Accident Conditions

The TRANSURANUS code development started in 1973 and initially focused on fuel pins for fast breeder reactors. There was a shift in 1982 towards LWR applications. The VVER version of the code has been under development since the mid 90s. Specific thermal and mechanical properties for Nb-containing cladding and specific correlations for annular UO2 fuel pellets were implemented to simulate fuel rod performance under normal operating conditions. Simulation of accident conditions came to the front in the EXTRA project, which was completed in 2003. The project focused on the development of a data-base for Zr1%Nb cladding and on the elaboration of new models and correlations for plastic deformation, high-temperature oxidation and cladding failure. The new models were incorporated into the TRANSURANUS code and validated against burst tests for as-received, oxidised, and irradiated cladding specimens. Furthermore, the upgraded version of the code was applied for assessing the fulfilment of safety acceptance criteria in design basis accidents. An example for the modelling of fuel rod performance in large break LOCA is presented. In the follow-up project of EXTRA it is intended to account for the hydrogen uptake by the cladding material under accident conditions, which have detrimental effects on the mechanical properties of the cladding. New experiments concerning the oxidation kinetics, the mechanical strength and the embrittlement of hydrogen charged cladding are carried out at the AEKI (KFKI Atomic Energy Research Institute) in Hungary. The experimental program and its recent results are demonstrated. Finally, an outlook is given about the planned activities for code development. The experimental data of the new tests are to provide background for the development of new correlations and the improvement of the present models.JRC.E.3-Materials researc

Behavior of Zr1%Nb Fuel Cladding under Accident Conditions

The behavior of the VVER fuel (E110) cladding under accident conditions has been investigated at the AEKI in order to study the role of oxidation and hydrogen uptake on the cladding embrittlement and to understand the phenomena that took place during the Paks-2 cleaning tank incident (2003). The test programme covered small scale tests and large scale tests with electrically heated 7-rod bundles in the CODEX (Core Degradation Experiment) facility. Since a hydrogen rich atmosphere could have been formed in the closed tank, the experiments were carried out in hydrogen-steam mixture.JRC.E.4-Nuclear fuel