Experimental study and modeling of high-temperature oxidation and phase transformation of cladding-tubes made in zirconium alloy
Description
One of the hypothetical accident studied in the field of the safety studies of Pressurized light Water Reactor (PWR) is the Loss-Of-Coolant-Accident (LOCA). In this scenario, zirconium alloy fuel claddings could undergo an important oxidation at high temperature (T≅ 1200 C) in a steam environment. Cladding tubes constitute the first confinement barrier of radioelements and then it is essential that they keep a certain level of ductility after quenching to ensure their integrity. These properties are directly related to the growth kinetics of both the oxide and the αZr(O) phase and also to the oxygen diffusion profile in the cladding tube after the transient. In this context, this work was dedicated to the understanding and the modeling of the both oxidation phenomenon and oxygen diffusion in zirconium based alloys at high temperature. The numerical tool (EKINOX-Zr) used in this thesis is based on a numerical resolution of a diffusion/reaction problem with equilibrium-conditions on three moving boundaries: gas/oxide, oxide/αZr(O), αZr(O)/βZr. EKINOX-Zr kinetics model is coupled with ThermoCalc software and the Zircobase database to take into account the influence of the alloying elements (Sn, Fe, Cr, Nb) but also the influence of hydrogen on the solubility of oxygen. This study focused on two parts of the LOCA scenario: the influence of a pre-oxide layer (formed in-service) and the effects of hydrogen. Thanks to the link between EKINOX-Zr and the thermodynamic database Zircobase, the hydrogen effects on oxygen solubility limit could be considered in the numerical simulations. Thus, simulations could reproduce the oxygen diffusion profiles measured in pre-hydrided samples. The existence of a thick pre-oxide layer on cladding tubes can induce a reduction of this pre-oxide layer before the growth of a high-temperature one during the high temperature dwell under steam. The first simulations performed using the numerical tool EKINOX-Zr showed that this particular kinetics pathway was qualitatively well reproduced by the modeling. It was also showed that the pre-oxide layer (formed at low-temperature) has particular diffusion properties. Consequently, some 18O tracer experiments were performed on pre-oxidized cladding tubes (pre-oxidizing done in autoclave) in order to study the diffusion properties of this oxide layers formed at low-temperature. Furthermore, EKINOX-Zr was modified to take into account the diffusion of tracer species. Both the diffusion properties of the low-temperature oxide layer and the evolution of the diffusion properties of the αZr(O) phase (formed by reduction of the oxide layer) were studied by comparing experiments and numerical simulations. (author)
Abstract (French)
Parmi les scenarios accidentels hypothetiques etudies dans le cadre des etudes de surete des Reacteurs a Eau Pressurisee (REP), figure l'Accident par Perte de Refrigerant Primaire (APRP). Dans ce scenario, les gaines en alliage de zirconium qui contiennent le combustible nucleaire sont soumises a une oxydation importante a haute temperature (T≅ 1200 C) dans de la vapeur d'eau. Les gaines etant la premiere barriere de confinement des radioelements, il est primordial qu'elles conservent une certaine ductilite residuelle apres la trempe pour conserver son integrite. Cette propriete est directement liee aux cinetiques de croissance de la zircone et de la phase αZr(O), ainsi qu'au profil de diffusion de l'oxygene dans le metal au cours du regime transitoire. Dans ce cadre, la comprehension et la modelisation du phenomene d'oxydation et de diffusion de l'oxygene dans les alliages de zirconium a haute temperature ont fait l'objet de cette these. Le modele cinetique (EKINOX-Zr), developpe au cours de cette these, est base sur la resolution numerique d'un probleme de diffusion/reaction avec des conditions aux limites sur trois interfaces mobiles: gaz/oxyde, oxyde/αZr(O) et αZr(O)/βZr. Le couplage du code cinetique avec le logiciel ThermoCalc et la base de donnees thermodynamiques Zircobase permet de prendre en compte l'influence des elements d'alliages (Sn, Fe, Cr, Nb) et de l'hydrogene. Cette etude s'est plus particulierement interessee a deux aspects de l'APRP: l'influence d'une couche de pre-oxyde (formee aux temperatures des conditions de service du reacteur) et les effets de l'hydrogene. Grace au couplage avec la base thermodynamique Zircobase, l'effet de l'hydrogene sur les limites de solubilite de l'oxygene dans les differentes phases a pu etre pris en compte dans le modele cinetique. Les simulations ont ainsi permis de reproduire les profils de concentration en oxygene mesures sur differents echantillons pre-hydrures. Par ailleurs, l'existence de couche de pre-oxyde de forte epaisseur peut conduire a une reduction transitoire de la couche de pre-oxyde dans les premiers instants du palier a haute temperature sous vapeur d'eau, avant formation de l'oxyde haute temperature. Une premiere serie de simulations a l'aide du modele cinetique EKINOX-Zr a permis de reproduire qualitativement ce chemin cinetique et a montre que cette couche formee a basse temperature possede des proprietes de diffusion particulieres. Des experiences de traceurs sous 16O2/18O2 ont ete realisees sur des gaines pre-oxydees en autoclave pour etudier la diffusion de l'oxygene a haute temperature dans ces couches formees a basse temperature. D'autre part, le modele cinetique EKINOX-Zr a ete modifie pour prendre en compte la diffusion des traceurs. La confrontation des experiences avec les calculs a permis d'etudier les proprietes de diffusion particulieres de la couche d'oxyde basse-temperature ainsi qu'une evolution des proprietes de diffusion de la phase αZr(O) formee par dissolution de l'oxyde a haute temperature. (auteur)
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Additional details
Additional titles
- Original title (French)
- Etude experimentale et modelisation de l'oxydation a haute temperature et des transformations de phases associees dans les gaines en alliage de zirconium
Identifiers
Publishing Information
- Imprint Pagination
- 241 p.
- Report number
- FRCEA-TH--6126
INIS
- Country of Publication
- France
- Country of Input or Organization
- France
- INIS RN
- 46031699
- Subject category
- S36: MATERIALS SCIENCE; S21: SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS;
- Resource subtype / Literary indicator
- Thesis
- Descriptors DEI
- ACCIDENTS; COMPUTERIZED SIMULATION; DIFFUSION; DUCTILITY; KINETICS; LOSS OF COOLANT; OXIDATION; OXYGEN; TEMPERATURE RANGE 0400-1000 K; ZIRCONIUM ALLOYS
- Descriptors DEC
- ACCIDENTS; ALLOYS; CHEMICAL REACTIONS; ELEMENTS; MECHANICAL PROPERTIES; NONMETALS; REACTOR ACCIDENTS; SIMULATION; TEMPERATURE RANGE; TENSILE PROPERTIES; TRANSITION ELEMENT ALLOYS
Optional Information
- Notes
- 78 refs.; Available from the INIS Liaison Officer for France, see the 'INIS contacts' section of the INIS-NKM website for current contact and E-mail addresses: http://www.iaea.org/inis/Contacts/; Also available from Bibliotheque Universitaire de Sciences Universite Paul Sabatier 118 route de Narbonne 31062 Toulouse cedex 09, (France)