A fuel cladding mechanical test, performed under conditions of plane strain deformation in the transverse direction of tube axis, was originally developed at Pennsylvania State University. It was decided to implement this original test within the PROMETRA program using the same experimental procedure and its optimization for a ring mechanical testing on plane strain conditions (PST tests) in hot cells laboratory.This paper presents a detailed description and an interpretation of the Plane Strain Tensile (PST) tests performed in the framework of the PROMETRA program on fresh and irradiated claddings. At first, the context of the PST tests is situated and the specificities of these tests implemented at CEA are justified. Indeed, a significant adjustment of the original experimental procedure is carried out in order to test the irradiated fuel cladding in the best possible conditions.Then, the tests results on fresh Zircaloy-4 and on irradiated Zircaloy-4, M5 (TM) and ZIRLO (R) specimens are gathered. The main analyses in support of these tests, such as metallographies, fractographic examinations and finite element simulations are detailed. Finally, a synthesis of the interpretation of the tests is proposed.The PST test seems only representative of plane strain fracture conditions when the test material is very ductile (fresh or high temperature or low hydride material like M5TM). However, it provides a relevant representation of the RIA rupture initiation which is observed in irradiated cladding resulting from hydride rim damage due to the strong irradiation of a fuel rod. (C) 2016 Elsevier B.V. All rights reserved.

The mechanical test procedures that address fuel cladding failure during a RIA are reviewed with an emphasis on the development of test procedures that determine the deformation and fracture behavior of cladding under conditions similar to those reached in a RIA. An analysis of cladding strain data from experimental research reactor test programs that have simulated the RIA is presented. These data show that the cladding undergoes deformation characterized by hoop extension subject to a range of multiaxial stress states and strain paths comprised between plane-strain (no axial extension of the cladding tube) and equal-biaxial tension (equal strain in both the hoop and the axial orientations). Current mechanical test procedures of cladding material are then reviewed with a focus on their ability to generate the appropriate deformation response and to induce the prototypical multiaxial stress states and failure modes activated during a RIA. Two main groups of tests currently exist. In the first group, the deformation behavior of the cladding is examined by several variations of hoop tensile tests in which an axial contraction of the specimen gage section occurs such that a near-uniaxial tension stress state results; finite element analyses are then usually employed to deduce the deformation response, often under conditions of an assumed coefficient of friction between the specimen and test fixtures. The second group includes test procedures which attempt to reproduce the deformation and failure conditions close to those seen during a RIA such that any stress-state corrections of the failure conditions are comparatively small. The advantages and disadvantages of all of these deformation/fracture tests are discussed with special reference to testing high burnup fuel cladding.

The CABRI REP-Na program was performed in the sodium loop of the CABRI reactor by the French Institut de Radioprotection et de Sûreté Nucléaire. The objective was to study the behavior of high-burnup UO2 and mixed-oxide (MOX) fuel during a reactivity-initiated accident (RIA) and involved eight tests with UO2 and four with MOX fuel. Failures of some UO2 and MOX fuel rods at enthalpy levels ranging from 125 to 472 J/g (30 to 113 cal/g) demonstrated the need for further development of the present safety criteria pertaining to fuel behavior. Detailed interpretation of the test data led to identifying the deleterious influence of a high clad corrosion level with hydride concentrations on clad failure, the contribution of grain boundary gases on fission gas release, and potential clad loading, mainly in MOX fuel during the first phase of the transient without significant clad temperature increase. Questions still remain concerning the transient fission gas behavior and its impact on clad loading during the entire transient, the rod behavior with high clad temperature and internal pressure, and the postfailure phenomena (fuel ejection, fuel/coolant interaction with finely fragmented solid fuel). These issues will be addressed by the CABRI International Program tests under typical pressurized water reactor conditions.

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 PROMETRA program undertaken by the French Institut de Radioprotection et de Surete Nucleaire in collaboration with Electricite de France and Commissariat a l' Energie Atomique (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 analyzed 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.

In the frame of reactivity-initiated accidents (RIA) studies, the CABRI REP-Na programme is currently performed, focused on high burn-up UO2 and MOX fuel behaviour. From 1993 to 1998, seven tests were performed with UO2 fuel and three with MOX fuel. In all these tests, particular attention has been devoted to the role of fission gases in transient fuel behaviour and in clad loading mechanisms. From the analysis of experimental results, some basic phenomena were identified and a better understanding of the transient fission gas behaviour was obtained in relation to the fuel and clad thermo-mechanical evolution in RIA, but also to the initial state of the fuel before the transient. A high burn-up effect linked to the increasing part of grain boundary gases is clearly evidenced in the final gas release, which would also significantly contribute to the clad loading mechanisms.