High Temperature Expansion Due to Compression Test for the Determination of a Cladding Material Failure Criterion under RIA Loading Conditions

Centre des Materiaux, Mines Paris, CNRS UMR 7633, BP 87, 91003 Evry, France * Tel: +33 1 69 08 18 18, Fax: +33 1 69 08 93 24, Email: matthieu.lesaux@cea.fr Abstract – This paper is mainly dedicated to the development of an out-of-pile test reproducing the thermomechanical loading conditions encountered during the first stage of a Reactivity Initiated Accidents (RIA) transient, dominated by Pellet Clad Mechanical Interaction (PCMI). In particular, the strain-controlled clad loading under high strain rate associated with temperatures up to 600°C expected during the PCMI phase is simulated by an Expansion Due to Compression (EDC) test achievable at high temperature. The use of appropriate materials for the inner pellet made it possible to achieve the tests from 20°C up to 900°C. The interpretation of the test data is supported by Finite Element Analysis (FEA) including parameters tuned using an inverse method coupling FEA and tests results. A deformation model, identified upon the PROMETRA (TRAnsient MEchanical PROperties) experimental database and describing the anisotropic viscoplastic behavior of Cold-Worked Stress Relieved Zircaloy-4 cladding alloys under typical RIA loading conditions, is exploited. The combined analysis of experimental results and finite element simulations provides a deeper understanding of the deformation mode (near pure hoop tension) that arises during the tests. The failure mode appears to be representative of that obtained on tubes during the PCMI stage of RIA experiments. An appropriate device is currently developed in order to reach a biaxiality of the loading path closer to that expected during the PCMI stage (between plane-strain and equal-biaxial tension). I. INTRODUCTION The increase of fuel burnup limits and the extension of operating cycles, leading to optimize the management of Pressurized Water Reactors (PWR), have motivated numerous studies related to the behavior of high burnup fuels during design basis accidents. Reactivity Initiated Accidents (RIAs), caused by the inadvertent ejection of a control rod assembly, are amongst the most serious postulated scenarios. The resulting very fast local power increase in the surrounding fuel assemblies (~50 ms) results in a nearly adiabatic heating of the fuel pellets which expand due to thermal and fission gases expansion. This produces a fast multiaxial straining (typically 1 s