The behavior of PWR fuel rod cladding under LOCA conditions strongly depends on the interactions between the metallurgical evolution (phase transformation) and the thermomechanical behavior at high temperature. FRAMATOME needs for the qualification of advanced zirconium alloys have motivated an extensive study of the phase transformation kinetics and the development of a new test facility, EDGAR-2, to perform thermomechanical tests. The first part of the paper deals with an experimental study and modeling of the α↔β phase transformation kinetics. High-temperature/high-sensitivity calorimeter and fast dilatometer facilities have been used to measure the on-heating α⃗β and on cooling β⃗α phase transformations from near equilibrium conditions up to LOCA conditions (heating-cooling rates up to 100°C/s). The equilibrium fraction of α/β phase as a function of temperature is derived from calorimetric measurements using the Zhu and Devletian model and described using a modified Johnson-Mehl-Avrami equation. Modeling of the α↔β kinetics is given by the differential Holt equation. The present results show that the Holt model gives a good description of the α⃗β kinetics upon heating but is not able to describe accurately the β⃗α phase transformation upon cooling, probably because it does not take into account the occurrence of the partial martensitic transformation during cooling, especially for the higher cooling rates. An important feature of the current study is that, despite the α/β equilibrium temperatures for M4 (ZrSnFeV) and M5 (ZrNbO) alloys being lower than that of Zy-4, for kinetic reasons the α to β phase transformation occurs in the same temperature range for the three alloys in the case of fast thermal transients (i.e., 10°C/s). This observation could be related to the slower thermal diffusion of Nb and V atoms compared to that of Fe and Cr. This slower diffusion explains why the thermalmechanical behavior of new M4 and M5 alloys is quite equivalent to Zy-4 in LOCA conditions. The second part of this paper deals with the thermomechanical tests. The EDGAR-2 test facility performs single rod tests under any internal pressure and clad temperature transients in steam environment. The advanced zirconium alloys M4 and M5 have been tested under steady-state conditions of pressure and temperature, continuous-heating and constant-pressure conditions for various heating rates and pressure, and some LOCA representative pressure temperature transients. The experimental program covers the LOCA conditions, and the rupture occurs from a few seconds up to 2000 seconds. The results are compared with those of the SRA optimized Zy-4 clad using the Monkman-Grant correlation, rupture ductility versus burst temperature, and burst criteria. EDGAR models for the prediction of the evolution of the transformed β-phase volume fraction and of the diametral deformation, time to rupture, and uniform diametral rupture elongation in typical LOCA conditions are derived for advanced alloys from the two parts of this study. These models may easily be implemented in most accident simulation computerized codes for safety analysis.

An assessment of the mechanical properties of the highly irradiated fuel claddings under high strain rate has been carried out in the framework of the PROME-TRA program undertaken by the French Institut de Ra-dioprotection et de Sûreté Nucléaire in collaboration with Electricité de France and Commissariat à l'Energie Atom-ique (CEA). Three types of tests, including burst tests, hoop and axial tensile tests, have been performed at CEA-Saclay hot laboratories to determine the cladding tensile properties to use in the SCANAIR code. The prototypicality of each test with regard to the reactivity-initiated accident loading conditions can be addressed and ana-lyzed in terms of strain or stress ratio. The high-strain-rate ductile mechanical properties of irradiated ZIRLO and M5 alloys derived from the PROMETRA program and their comparison to the stress-relieved irradiated Zircaloy-4 are reported. Then, the clad brittle behavior, in particular for highly corroded or spalled Zircaloy-4 cladding, is investigated.

The PROMETRA material testing program is a support program related to the study of high burnup fuel rod behavior under Reactivity Initiated Accidents (RIA) and to the interpretation of the CABRI REP-Na RIA test results. Hoop and axial tensile tests have been performed on fresh and irradiated Zircaloy-4 cladding alloy first at CEA Grenoble hot labs and now at CEA Saclay in order to assess the cladding mechanical behavior during RIA transients. Efforts have been continuously carried out in order to improve the prototipicallity of the tests for RIA studies involving new specimens and new testing techniques. The corrosion level of irradiated specimens reached up to 130 μm of oxide layer thickness. The influence of in-pile oxide layer spallation has also been addressed. High strain-rate material properties of irradiated Zircaloy-4 and the consequences of hydride embrittlement can be derived from the PROMETRA program.

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