Charpy-size tensile specimens have been reconstituted starting from broken Charpy specimens. Precautions have been taken to prevent fracture in the weld regions of the reconstituted specimens. Careful qualification of the tensile properties of the reconstituted tensile bars has been established and is in agreement with test results on ASTM/E8-size tensile specimens. The tensile properties of the unirradiated weld material of the Belgian PWR Doel 2 and of the unirradiated base metal of the German BWR Philippsburg 1 have been determined using this methodology. Both the resulting yield and maximum loads are in agreement with the actual expectations. Qualification to apply the technique on irradiated specimens is in preparation.
In the nuclear field, the importance of direct fracture toughness measurements on reactor pressure vessel (RPV) materials has been nowadays widely recognized, as opposed to Charpy-based estimations. However, sample dimensions have to be kept small in order to optimize the use of available material (often in the form of previously broken Charpy specimens) or, in the case of new irradiations, make effective use of the limited space available inside irradiation facilities. One of the most appealing geometries for fracture toughness measurements is the miniature Compact Tension specimen, MC(T), which has the following dimensions: B = 4.15 mm, W = 8.3 mm, cross section 10 x 10 mm2. Four MC(T) specimens can be machined out of a broken half Charpy, and in the case of irradiation ten MC(T) samples occupy approximately the same volume as a full-size Charpy specimen. A comprehensive investigation is presented in this paper, aimed at assessing the applicability of MC(T) specimens to measure fracture toughness in fully ductile (upper shelf) conditions. In this study, 18 1TC(T) and 20 MC(T) specimens have been tested at different temperatures from three RPV steels and one low-alloy C-Mn steel. The results obtained clearly show that MC(T) samples exhibit lower fracture toughness properties, both in terms of initiation of ductile tearing (according to various test standards) and resistance to ductile crack propagation (J-R curve). The reduction of tearing resistance might be attributed to work hardening prevailing over loss of constraint in the uncracked ligament for a side-grooved specimen, or to the inadequacy of J-integral to represent ductile crack extension in very small specimens. Both arguments need to be verified with further investigations.
L'entreposage a sec de crayons combustibles uses des reacteurs a eau sous pression pour une duree comprise entre plusieurs decennies et un siecle est actuellement a l'etude. Un tel entreposage temporaire garantissant l'integrite du materiau de gainage permettra d'exploiter ulterieurement la mise en œuvre de solutions optimales pour la gestion des dechets radioactifs de haute activite et a vie longue que contiennent ces colis. Au cours de cet entreposage, la rupture du crayon par corrosion sous contrainte par l'iode ne peut etre exclue. Une analyse critique des donnees actuelles montre que l'etude des parametres chimiques a l'origine du phenomene est une approche pertinente. Sur cette base, on discute le risque de rupture du materiau dans les conditions d'entreposage. L'etude experimentale prealable etablit qu'un mecanisme chimique susceptible d'assister la fissuration du metal requiert la presence d'une quantite optimale d'iode mais aussi d'oxygene. La disponibilite de l'iode dans le crayon est ensuite evaluee. La methode repose sur un modele thermochimique permettant de modeliser le comportement de la pastille combustible. Les equilibres impliquant l'iode ont ete identifies et on montre que la pression d'iode est controlee par la formation soit d'uranate de cesium soit de molybdate de cesium. Les interactions chimiques entre l'iode et le zirconium sont ensuite estimees en modelisant l'etape de chimisorption de l'iode sur le metal. A partir des resultats obtenus, on montre que le risque de rupture par corrosion sous contrainte existe a condition que : (i) il y ait formation de molybdate de cesium pour que la pression d'iode soit suffisante et (ii) la temperature et les pressions d'iode et d'oxygene depassent certaines valeurs seuil. Sur la base de ces resultats, le risque de corrosion sous contrainte en conditions d'entreposage peut etre exclu dans un domaine de temperatures donne.
A comprehensive review of the status of the reconstitution technique, as applied to reactor pressure vessel steel, is given. The reconstruction techniques in use at this moment on different small-size specimen geometries are highlighted, and the reconstitution methodology is evaluated. The advantages and limitations of reconstituted specimens are discussed, in relation to preparation and material conditions and in view of the test techniques to be used. For plant life management issues, the importance of finding an indexing parameter independent of the reconstitution process will be indicated.
