Fuel Cladding post-quench LOCA Embrittlement: Mechanical test Relevance
Résumé
During a loss-of-coolant-accident (LOCA), the fuel cladding is submitted to a high temperature
increase in a steam environment, along with an internal pressure increase which would eventually lead to ballooning
and burst of the cladding. Steam could then penetrate through the burst opening and lead to inner surface oxidation
and secondary-hydriding of the cladding. Post-quench cladding rupture can then occur either in two-sided oxidized
area with wall thinning due to ballooning, or in single-sided oxidized area with large hydrogen content.
Common tests involved in the evaluation of the post-quench ductility are Ring Compression Tests (RCT) [1-
4], Expansion Due to Compression Test (EDC) [5] or Axial Tensile Tests (AT) [6].
Questions arise about these mechanical tests, as for instance, the influence on sample failure of the oxide layer
thickness partitioning between inner and outer diameter.
A systematic study was carried out to compare several mechanical tests on both single-sided and two-sided
Zircaloy-4 oxidized samples. Five different mechanical tests were compared, including RCT: “C”-shaped ring
Compression Test (CCT), Axial Tensile test (AT), Ring Tensile Test with well-defined machined gage section (RTT)
and plain Ring Tensile Test (pRTT). All samples were taken from cladding tubes which had been oxidized in steam
at 1200°C and then water-quenched. Mechanical tests were performed at room temperature and two to three samples
were tested for each oxidation conditions and for every sample geometry to assess the reproducibility.
Metallographic examinations were carried out to measure the ZrO2 and αZr(O) layer thicknesses and the ex-β/α(O)
phase fraction. Mean hydrogen content measurements were also performed systematically on each sample.
Results indicate that the different mechanical tests have not the same sensitivity to the oxide layer partitioning: RTT
and pRTT load at failure seems to depend mainly on the cumulated oxide layer thickness rather than on its
partitioning, whereas this parameter has an impact on CCT and AT test results. For all tensile tests, stresses at failure
were derived from load-displacement curves. The failure of brittle samples was assumed to be governed by linear
elastic fracture mechanics. Using conventional stress intensity factor solution, a value of the material fracture
toughness controlling the sample failure was determined from experimental results and discussed.
Domaines
Sciences de l'ingénieur [physics]Origine | Fichiers produits par l'(les) auteur(s) |
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