Characterization of Oxygen Distribution in LOCA Situations

Abstract

Oxygen distribution in the rod cladding tubes during a Loss of Coolant Accident (LOCA) situation is known to play a key role regarding the post-quench mechanical properties of fuel rods. Analytical solutions of the diffusion equations in the complex multilayer and multiphase systems describing the cladding material tube during high temperature oxidation are limited to stationary isothermal conditions. To address the problem of non-stationary transient LOCA situations, a numerical tool (named DIFFOX) has been developed. In the first part, the main assumptions and main features of the code are described. In parallel to the code development, an experimental program has been launched to provide a validation database as well as to input data for improvement of the code models. Zircaloy-4 cladding samples have been pre-oxidized at low temperature then vacuum annealed or steam oxidized at high temperature. Oxygen distribution profiles are investigated by Electron Probe Microanalysis. The ability of the code to describe the chemical reduction of the pre-oxide layer and oxygen diffusion into the metal during high temperature annealing is demonstrated. Regarding high temperature steam oxidation, the pre-oxide layers are observed to be either protective or not on a same given sample. While a good code to experimental data agreement is obtained in the first case, the growing of a high temperature oxide underneath the pre-oxide is not well reproduced by the code. The effect of hydrogen has also been investigated by performing small scale oxidation tests on 600 wppm hydrogen charged Zy-4 cladding. Samples have been long-term annealed for homogenization after oxidation. The main purpose of these experiments is to provide quantitative data on the modification of the α/α+β and α+β/β transus of the Zy4-O pseudo binary phase diagram induced by the presence of hydrogen. A variation of the transus locations deduced from these experiments is proposed.

Oxygen segregation in pre-hydrided Zircaloy-4 cladding during a simulated LOCA transient

“…Oxidation is interrupted by dropping the sample into a water bath at room temperature as illustrated in Figure 2. More details about the oxidation protocol and its qualification are provided by Duriez et al [10] and Guilbert et al [13,14]. Radial cross sections are prepared for hydrogen measurements and microstructure analyses.…”

“…It has been developed by IRSN to address LOCA induced oxidation [10]. The oxygen concentration profile is calculated by solving the Fick's 2nd law diffusion equation in cylindrical coordinates according to equation (1).…”

“…In the open literature, analytical oxygen diffusion models are restricted to samples having simple geometry that is usually not satisfactory for analyzing tubular geometry [9]. A finite-difference software solving the oxygen diffusion equations named DIFFOX (for DIFFusion of OXygen) has been developed by Institut de Radioprotection et de Sûreté Nucléaire (IRSN) [10]. The comparison between the calculations and the experimental results in the mixed a+b region shows that the oxygen diffusion is not accurately predicted.…”

Development and validation of the multi-physics DRACCAR code

“…CORA 13 (OCDE/NEA-CSNI, 1993), EDGAR (Forgeron et al, 2000), FEBA (Ihle and Rust, 1984), HALDEN IFA 650 (OCDE/NEA-CSNI, 2010), MARGO-R (Duriez et al, 2011), PBF SFD 1.4 (Petti et al, 1989), PER-ICLES, 1 PHEBUS SFD B9+(OCDE/NEA-CSNI, 1992), PHEBUS LOCA 218 (Scott De Martinville et al, 1987), REBEKA-6 (Karwat, 1985), ROSCO (Bascou et al, 2012), SEFLEX (Ihle and Rust, 1986) experiments). In this document, tests are investigated focusing on coolability of an intact and of a ballooned rod bundle (Sections 3 and 4) and on spent fuel pool draining accidents (Section 5).…”