SCK-CEN Contribution to the IAEA Round Robin Exercise on WWER-440 RPV Weld Material: Irradiation, Annealing, and Re-Embrittlement

Mechanical properties of WWER-440 RPV weld joints have been studied with account of different states of the material: baseline (unirradiated), irradiated up to the average fast neutron fluence of 6.5*10^19 n/cm^2, irradiated and eventually annealed, re-irradiated with the accumulated fast neutron fleunce 1.3*10^20 n/cm^2. For each state of the material, tensile, impact fracture, and fracture toughness tests were performed with the use of Charpy (standard, reconstituted, and pre-cracked) specimens.Исследованы механические свойства материалов сварных соединений корпусов реакторов ВВЭР-440 с учетом различных состояний: исходного (неохрупченного), после радиационного охрупчивания со средней плотностью потока нейтронов до 6,5*10^19 нейтр/см^2, охрупченного с последующим отжигом, повторно охрупченного с флюэнсом частиц 1,3*10^20 нейтр/см^2. Для каждого состояния материала проведены испытания на растяжение, ударное разрушение и трещиностойкость с использованием образцов Шарпи (стандартные, восстановленные, с предварительно нанесенной трещиной).Досліджуються механічні характеристики зварних з ’єднань корпусів реакторів BBEP-440 з урахуванням різного стану: початкового (неокрихчувано- го), після радіаційного окрихчування з середньою густиною потоку нейтронів до 6,5*10^19 нейтр/см^2, окрихчуваного з наступним відпалом, повторно окрихчуваного з флюенсом частинок 1,3*10^20 нейтр/см^2. Для кожного стану матеріалу проведено дослідження на розтяг, ударне руйнування та тріщиностійкість із використанням зразків Шарпі (стандартні, відновлені та з попередньо нанесеною тріщиною)

Copper precipitate hardening of irradiated RPV materials and implications on the superposition law and re-irradiation kinetics

Copper is known to play an important role in irradiation hardening and embrittlement of RPV materials. This is particularly true for old vessels. Indeed, while the Cu-content is low (< 0.1%) in modern RPV materials, it often exceeded 0.15% in older vessels. Within the RADAMO irradiation program aiming to provide a reliable and extensive (chemistry, heat treatments, fluence, irradiation temperature) databank to investigate irradiation-induced hardening of RPV materials, we irradiated steels and welds with copper contents ranging from 0.06 to 0.31% at 300 and 265 degrees C. Experiments on re-irradiation after annealing were also performed to investigate the re-irradiation kinetics. It is found that copper plays a role in the very early stage of irradiation but saturates quite rapidly. The peak hardening is in agreement with the ageing data. Considering a two-component model, the linear superposition law provides the most appropriate one to rationalize the experimental data including re-irradiation path. (c) 2005 Elsevier B.V. All rights reserved

Analysis of the geometry dependence of fracture toughness at cracking initiation by comparison of circumferentially cracked round bars and SENB tests on Copper

In the first part of the paper, the use of circumferentially cracked round bars (CRB geometry) for characterizing fracture toughness of a ductile material, namely copper, is assessed experimentally through a comparison with the single edge notched bend (SENB) geometry. The J(R) curve method with multiple-specimens was applied, but, as unstable cracking appeared very early in the CRB specimen, an engineering definition of fracture toughness was not pertinent. Unloaded specimens were analyzed metallographically to determine the CTOD at physical cracking initiation. The fracture toughness measured using the CRB geometry was 50% larger than using the SENB geometry. The second part of the paper aims at justifying this difference of fracture toughness at cracking initiation. Finite element simulations revealed a slightly higher constraint in the SENB specimens. The main difference between the two specimen geometries lies in a 50% larger extension of the finite strain zone with respect to the CTOD in the case of the SENB specimens. Based on the observation that, in the studied material, the critical CTOD is one order of magnitude larger than the void spacing, we conclude that the geometry dependence of the fracture toughness is caused by the difference in the finite strain zone extension rather than by a stress triaxiality effect

Micromechanical Modeling of Ductile Fracture Initiation to Predict Fracture Toughness of Reactor Pressure Vessel Steels

Two micromechanical models of ductile fracture are investigated and have been applied to two reactor pressure vessel steels, 18MND5 and A508 Cl.3 : the Beremin model, based on the Rice et Tracey void growth model, and the damage work model that combines the plastic strain work to the work spent in void growth. Due to the local nature of these models, finite element analysis needs to be performed to derive stress and strain history in order to obtain the damage kinetics of the material and geometry under consideration. Tensile tests were performed on various geometries (notched and precracked tensile) and sizes. All specimens are in large scale yielding condition. It is found that while the critical void growth ratio decreases with the triaxiality ratio, the critical damage work is not affected. Geometry, size and orientation effects are also investigated. These effects are well described by these micromechanical models. Similar concepts are applied to sharp notch and crack tip situations. An additional parameter, the so-called characteristic distance, characterizing the process zone size, is introduced. The fracture toughness, derived from the notched bars, is within experimental uncertainties in reasonable agreement with the results obtained for cracked geometries

A Radiation Hardening Model of 9%Cr-Martensitic Steels Including dpa and Helium

This paper provides a physically-based engineering model to estimate radiation hardening of 9%Cr-steels under both displacement damage (dpa) and helium. The model is essentially based on the dispersed barrier hardening theory and the dynamic re-solution of helium under displacement cascades but incorporating a number of assumptions and simplifications [Trinkaus, J. Nucl. Mater. 318 (2003) 234-340]. As a result, the kinetics of the damage accumulation kept fixed, its amplitude is fitted on one experimental condition. The model was rationalized on an experimental database that mainly consists of similar to 9%Cr-steels irradiated in the range of 50-600 degrees C up to 50 dpa and with a He-content up to 5000 appm. The test temperature effect is taken into account through a normalization procedure based on the change of the Young's modulus and the anelastic deformation that occurs at high temperature. Despite the large experimental scatter, inherent to the variety of the material and irradiation as well as testing conditions, the obtained results are very promising. (C) 2009 Elsevier B.V. All rights reserved