Ruthenium Oxidation in High Temperature Air and Release of Gaseous Ruthenium KFKI-3/2008

The RUSET experimental programme was launched in order to study Ru oxidation and release from fuel in high temperature air. More than forty small scale tests have been performed with mixed powder components of inactive materials and with short fuel rods. The influence of temperature, air flow rate and the presence of other fission products on the gaseous Ru release and the retention role of fuel pellets and cladding have been investigated. The test series indicated that if an air ingress type severe accident occurs most of the initial Ru mass can be released from the reactor core to the containment or environment. Some part of the released gaseous Ru undergoes precipitation and deposits on the cold surfaces, another part is released in gaseous form. The deposited Ru oxides can serve as a secondary source for further gaseous Ru releas

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

Transport of ruthenium in primary circuit conditions during a severe NPP accident

International audienceRuthenium species, volatilized from damaged fuel during a severe accident in a nuclear power plant, are radiotoxic and can be transported to the containment atmosphere in gaseous form. To limit the possible source term to the environment, it is of interest to understand the behaviour of Ru after it has been released from fuel and the phenomena taking place within the decreasing temperature section of the reactor coolant system. This was investigated in the framework of EC SARNET and SARNET2 projects, as a part of the Source Term work package, with several separate-effect tests on the transport and speciation of Ru in primary circuit conditions considering the influence of other fission products as well. The source of Ru was metallic Ru, RuO2 powder or gaseous RuO4. The large-scale integral tests of the Phébus FP program were conducted with real irradiated fuel, and more realistic analysis on the release and transport of Ru could be performed. Experimental studies proved that the transport of ruthenium to the containment atmosphere took mainly place as RuO2 particles when Ru source was oxidized above 1250 °C. The fraction of Ru transported in gaseous form was at its highest when ruthenium was oxidized at approx. 1000-1100 °C. A major part of the released Ru was deposited at the decreasing temperature area of the circuit as RuO2. Revaporisation of the deposited Ru at low temperature was a significant source of gaseous ruthenium. In order to understand the behaviour of ruthenium in these tests, the analysis work was extensive and several simulations were carried out. As an outcome, the observed transport and deposition of ruthenium was explained. The simulation studies gave also an insight into the performance of the ASTEC code and some model improvements for Ru transport through the reactor coolant system have been identified. © 2014 Elsevier Masson SAS

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

Progress on ruthenium release and transport under air ingress conditions

A particular concern in the event of a hypothetical severe accident is the potential release of highly radiotoxic fission product (FP) isotopes of ruthenium. The highest risk for a large quantity of these isotopes to reach the containment arises from air ingress following vessel melt-through. One work package (WP) of the source term topic of the EU 6th Framework Network of Excellence project SARNET is producing and synthesizing information on ruthenium release and transport with the aim of validating or improving the corresponding modelling in the European ASTEC severe accident analysis code. The WP includes reactor scenario studies that can be used to define conditions for new experiments. The experimental database currently being reviewed includes the following programmes:•AECL experiments conducted on fission product release in air; results are relevant to CANDU loss of end-fitting accidents;•VERCORS tests on FP release and transport conducted by CEA in collaboration with IRSN and EDF; additional tests may potentially be conducted in more oxidizing conditions in the VERDON facility;•RUSET tests by AEKI investigating ruthenium transport with and without other FP simulants;•Experiments by VTT on ruthenium transport and speciation in highly oxidizing conditions. In addition to the above, at IRSN and at ENEA modelling of fission product release and of fuel oxidation is being pursued, the latter being an essential boundary condition influencing ruthenium release. Reactor scenario studies have been carried out at INR, EDF and IRSN: calculations of air ingress scenarios with respectively ICARE/CATHARE V2; SATURNE-MAAP; and ASTEC codes provided first insights of thermal-hydraulic conditions that the fuel may experience after lower head vessel failure. This paper summarizes the status of this work and plans for the future. © 2008 Elsevier B.V. All rights reserved

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

Recent advances in understanding ruthenium behaviour under air-ingress conditions during a PWR severe accident

In a hypothetical severe accident in a Pressurised Water Reactor (PWR), Fission Products (FPs) can be released from the overheated nuclear fuel and partially transported by gases, composed of a mixture of superheated steam and hydrogen, to the reactor containment. Subsequent air ingress into a damaged reactor core may lead to enhanced fuel oxidation, affecting some FP release, especially that of ruthenium. Ruthenium is of particular interest because of its high radiotoxicity and its ability to form very volatile oxides. In the reactor containment, such volatile forms are very hazardous as they are much less efficiently trapped than particulate forms by emergency filtered venting. In the four and a half years of SARNET, collaborative research dedicated to the "ruthenium story" has been performed by several partners. This paper presents the main achievements over the whole project period. Starting from experimental observations showing that fuel could be extensively oxidised by air to, and that a significant fraction of ruthenium inventory can be released, rather satisfactory models have been developed. In addition, the effect of the air interaction with Zircaloy cladding, as well as with UO2 itself, has been studied. Experiments on the complex transformations of ruthenium oxides upon cooling through the reactor circuit have been performed. An unexpectedly large effect of temperature on the decomposition rate of gaseous ruthenium compounds has been found, as well as effects of the nature of circuit internal surfaces and other FP deposits. So it has been highlighted that various forms of ruthenium can reach the containment, but the most probable gaseous species under these conditions is ruthenium tetroxide. Preliminary analysis of ruthenium transport supports these conclusions. Experiments and analysis have also been launched on the radio-chemical reactions undergone by these ruthenium oxides in the reactor containment. Competing effects of gaseous decomposition to solid particles and re-volatilization from these ruthenium deposits have been demonstrated and modelled. The paper concludes by identifying the remaining work needed to achieve full resolution of the ruthenium source term issue. Recommendations are made for future research activities in the follow-up programme SARNET2. © 2009 Elsevier Ltd. All rights reserved