Nuclear Energy for New Europe 2009

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No: 412

Conference: Nuclear Energy for New Europe 2009

Title: Modelling of cladding oxidation by air under severe accident conditions

Theme: Severe Accidents

Author(s): Emilie Beuzet, Jean Lamy, Eric Simoni

Contact : Emilie Beuzet

E-mail: emilie.beuzet@edf.fr

Address: EDF, Electricite de France, Research and Development Division
92141 Clamart Cedex

Country: France

In a nuclear power plant, a potential risk in some low probable situations of severe accident is an air ingress in the vessel. Air is a highly oxidizing atmosphere that can lead to an enhanced core oxidation and degradation affecting the release of FP, especially increasing that of ruthenium which is of particular importance because of its high radio-toxicity and of its ability to form highly volatile oxides.Oxygen affinity is decreasing between cladding in Zircaloy, fuel and ruthenium inclusions. It is consequently of great need to understand the phenomena governing cladding oxidation by air as a prerequisite for the source term issues.
A review of existing data in the field of Zircaloy oxidation in air containing atmosphere shows that this phenomenon is relatively well understood. The cladding oxidation process can be divided in two kinetic regimes separated by a breakaway transition. Before transition, a protective dense zirconia scale grows following a solid state diffusion- limited regime for which experimental data are well fitted by a parabolic time dependence. For a given thickness which depends mainly on temperature and pre-oxidation in steam, the dense scale can potentially breakdown. In case of breakaway combined with oxygen starvation, cladding oxidation can then be much faster because of the combined action of oxygen and nitrogen through a complex self sustaining nitriding-oxidation process.
A review of the pre-existing correlations used to simulate the zirconia scale growth under air atmosphere shows a high degree of variation from parabolic to accelerated time dependence. Variations also exist in the choice of the breakaway parameter. Multiple correlations and breakaway parameters found in the literature were implemented in the MAAP4 SA code. They were assessed thanks to the simulation of the QUENCH10 test which is a semi-integral test built to study fuel assembly exposure to steam and air. This paper deals with the main achievements obtained with MAAP when simulating the QUENCH10 test and with plans for future code improvement and validation

No:	412
Conference:	Nuclear Energy for New Europe 2009
Title:	Modelling of cladding oxidation by air under severe accident conditions
Theme:	Severe Accidents
Author(s):	Emilie Beuzet, Jean Lamy, Eric Simoni
Contact :	Emilie Beuzet
E-mail:	emilie.beuzet@edf.fr
Address:	EDF, Electricite de France, Research and Development Division 92141 Clamart Cedex
Country:	France

In a nuclear power plant, a potential risk in some low probable situations of severe accident is an air ingress in the vessel. Air is a highly oxidizing atmosphere that can lead to an enhanced core oxidation and degradation affecting the release of FP, especially increasing that of ruthenium which is of particular importance because of its high radio-toxicity and of its ability to form highly volatile oxides.Oxygen affinity is decreasing between cladding in Zircaloy, fuel and ruthenium inclusions. It is consequently of great need to understand the phenomena governing cladding oxidation by air as a prerequisite for the source term issues. A review of existing data in the field of Zircaloy oxidation in air containing atmosphere shows that this phenomenon is relatively well understood. The cladding oxidation process can be divided in two kinetic regimes separated by a breakaway transition. Before transition, a protective dense zirconia scale grows following a solid state diffusion- limited regime for which experimental data are well fitted by a parabolic time dependence. For a given thickness which depends mainly on temperature and pre-oxidation in steam, the dense scale can potentially breakdown. In case of breakaway combined with oxygen starvation, cladding oxidation can then be much faster because of the combined action of oxygen and nitrogen through a complex self sustaining nitriding-oxidation process. A review of the pre-existing correlations used to simulate the zirconia scale growth under air atmosphere shows a high degree of variation from parabolic to accelerated time dependence. Variations also exist in the choice of the breakaway parameter. Multiple correlations and breakaway parameters found in the literature were implemented in the MAAP4 SA code. They were assessed thanks to the simulation of the QUENCH10 test which is a semi-integral test built to study fuel assembly exposure to steam and air. This paper deals with the main achievements obtained with MAAP when simulating the QUENCH10 test and with plans for future code improvement and validation