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Thesis proposals

Secondary hydriding and embrittlement of a M5 clad after a LOCA Test

References: RES21-7

Themes: Engineering sciences: solid mechanics, materials, civil engineering

Thesis location: Mechanics and Materials Experimental Research Laboratory (LE2M)​ - Cadarache (13) 

Start: October 2021

Skills required

Master​'s Degree in materials or mechanics
Knowledges in cardiology​
Notions in oncology and radiotherapy

Age limit: 26 years old unless otherwise stated.

Thesis subject

This PhD thesis proposal concerns study of secondary hydriding and embrittlement of a M5 clad after a LOCA Test. This PhD thesis offer aims at studying the Loss of Coolant Accidents (LOCA) in Pressurized Water nuclear Reactors (PWR. These accidents result from a leak in the primary loop of the reactors. LOCA studies contribute to determination of the volume of the safety injection tank. More specifically the thermal-mechanical behavior of fuel cladding samples with inserted pellets will be studied. As-received material will be studied to better understand the behavior of irradiated cladding. During a LOCA the cladding temperature increases as well as internal pressurization stresses intensified by both temperature increase and primary loop depressurization. This loading result in a creep induced ballooning and potentially cladding burst affecting the core coolability. The bursted cladding is then exposed to an oxidizing steam environment at elevated temperature. The outer surface of the cladding is exposed to almost uniform oxidation conditions but the situation is rather different at internal surface of the cladding. Actually, the heated steam penetrates the internal volume of the cladding through the burst opening and locally oxidizes the cladding. This oxidation induces a hydrogen release modifying progressively the composition of the oxidizing environment at increasing distance from the burst opening. There is a progressive increase of gaseous hydrogen content and concomitantly a decrease of steam content in the internal volume. When the gaseous environment contains a sufficient hydrogen partial pressure, hydrogen uptake by the cladding is observed. This phenomenon is called secondary hydriding. For low permeability environment within the cladding, secondary hydriding bands are observed at a almost constant distance of the burst opening. A long term collaboration between INSA de Lyon and IRSN was launched since 2005. INSA de Lyon developed a high technical level in on-line recording of optical fields over complex surfaces such as temperatures, displacements (and strains) during thermal mechanical transients. During this thesis both experimental and modeling results are expected. The main challenges to overcome are the following:

• Adaptation of the experimental high temperature testing bench available at INSA de Lyon to include the capacity of performing tests under combined steam environment, internal pressurization up to sample burst and finally axial load to test the resistance of the embrittled, oxidized and hydrided cladding to axial loads,
• Neutron tomography will be used to characterize the secondary hydriding bands,
• The room temperature mechanical resistance to axial loads will be studied with special care to the competition between the influence of ballooning and secondary hydriding,
• Test interpretation will be performed using already developed multi-physics softwares such as the SHOWBIZ code developed at IRSN, It is expected that, at the end of this thesis, a better understanding of the key mechanisms and parameters affecting the M5 cladding embrittlement will be obtained. This new understanding will be used to better describe the competition between secondary hydriding and ballooning. This thesis will mainly take place at INSA de Lyon (Villeurbanne, France) with periods at Cadarache. 

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phone: 04 42 19 94 90 
adress: CEN de Cadarache. Bat 327. 13115 St Paul lez Durance cedex.​


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