Hieronymus Hein

Flux dependence of cluster formation in neutron-irradiated weld material

The effect of neutron flux on the formation of irradiation-induced clusters in reactor pressure vessel (RPV) steels is an unresolved issue. Small-angle neutron scattering was measured for a neutron-irradiated RPV weld material containing 0.22 wt% impurity Cu. The experiment was focused on the influence of neutron flux on the formation of irradiation-induced clusters at fixed fluence. The aim was to separate and tentatively interpret the effect of flux on the characteristics of the cluster size distribution. We have observed a pronounced effect of neutron flux on cluster size, whereas the total volume fraction of irradiation-induced clusters is insensitive to the level of flux. The result is compatible with a rate theory model according to which the range of applied fluxes covers the transition from a flux-independent regime at lower fluxes to a regime of decelerating cluster growth. The results are confronted with measured irradiation-induced changes of mechanical properties. Despite the observed flux effect on cluster size, both yield stress increase and transition temperature shift turned out to be independent of flux. This is in agreement with the volume fraction of irradiation-induced clusters being insensitive to the level of flux.

Final Results from the Crack Initiation and Arrest of Irradiated Steel Materials Project on Fracture Mechanical Assessments of Pre-Irradiated RPV Steels Used in German PWR

Pre-irradiated original reactor pressure vessel (RPV) materials covering all four German pressurized water reactors (PWR) construction lines were tested in the Crack Initiation and Arrest of Irradiated Steel Materials program to create a database of fracture toughness and arrest values for neutron fluences beyond the end of life range. The new database comprises data from both unirradiated and irradiated RPV base and weld materials generated by tensile, Charpy-V impact, fracture toughness KJc, and crack arrest KIa tests. The test matrix consists of materials with optimized chemical composition and with high Copper or high Nickel content, respectively. Based on the generated and already existing data the RTNDT and the RTT0 (Master Curve) concepts are applied with specific view on reference temperatures, transition temperature shifts, and on possible correlations between the criteria used in both concepts. In this context the consequences of some influencing factors like type and chemical composition of the RPV steel, its manufacturing conditions, and the specimen type and size on the reference temperatures are discussed. Moreover, the test results are assessed with respect to the American Society of Mechanical Engineers (ASME) code and German Nuclear Safety Standards Commission safety standards. The crack arrest characteristics for these typical RPV materials are also determined in a twofold way by testing Compact Crack Arrest specimens and by evaluation of instrumented Charpy-V impact test data. The available results made a good point that crack arrest is a reliable phenomenon that doubtless exists. It is also shown that the obtained KIa data can be enveloped by applying the ASME KIc lower bound curve indexed by different reference temperatures. Finally, the results show that the used RPV materials are well designed in terms of material behavior under irradiated conditions and that optimized manufacture specifications are of great benefit particularly after long operation times.

Irradiation Damage and Embrittlement in RPV Steels Under the Aspect of Long Term Operation: Overview of the FP7 Project LONGLIFE

The increasing age of the European NPPs and envisaged lifetime extensions up to 80 years require an improved understanding of RPV irradiation embrittlement effects connected with long term operation (LTO). Phenomena which might become important at high neutron fluences (such as late blooming effects and flux effects) must be considered adequately in the safety assessments. Therefore the project LONGLIFE was initiated within the 7th Framework Programme of the European Commission. The project aims at: i) improved knowledge on LTO phenomena relevant for European reactors; ii) assessment of prediction tools, codes, standards and surveillance guidelines. In the paper, we give an overview of the project structure and the related tasks. Furthermore we present two examples for the experimental evidence of LTO relevant phenomena: the first example is related to the flux dependence of defect cluster formation in a neutron irradiated weld material. We have found that the size of the irradiation induced defects exhibits a flux effect whereas the mechanical properties are almost independent of the flux. The second example refers to the acceleration of irradiation hardening after exceeding a threshold fluence. This effect was observed by means of both small angle neutron scattering (SANS) and tensile testing for low Cu RPV steels irradiated at a temperature of 255 °C. These examples demonstrate that LTO irradiation effects have to be investigated in more detail to guarantee the applicability of the embrittlement surveillance guidelines beyond 40 years of operation.Copyright

ENPOWER: Repair Welds and Residual Stresses in Clad Plates

One of the tasks of the European Commission sponsored project ENPOWER was to manufacture repair welds on clad plate specimens simulating the inner wall of a Reactor Pressure Vessel (RPV) and to establish their structural integrity. The paper summarizes the main results from the repair welds carried out on clad plates with an anticipated sub-clad defect including the results from various residual stress measurements and from numerical welding simulations as well as from some fracture mechanical calculations. The results are discussed with respect to support the repair weld optimization in particular by minimizing the residual stresses. Moreover, the application ranges and capabilities of numerical simulations for this kind of weld processes are discussed.Copyright

Management of Nuclear Plant Operation by Optimising Weld Repairs - Enpower Project Overview

ENPOWER is a EURATOM collaborative research project sponsored by the European Union. The aim is to produce advanced weld repair techniques and residual stress mitigation procedures for nuclear components in order to eliminate the need for expensive global Post-Weld Heat Treatment (PWHT). These procedures are based on weld repair optimisation combined with novel Alternative Post Weld Treatments (APWT). Repair procedures have been developed by carrying out parametric numerical studies for various repair and component configurations. APWTs are based on local heating at moderate temperatures (300–600°C) to produce local plasticity that redistributes the internal stresses and results in compression in the areas of interest. Two techniques have been identified as prime candidates: namely “hot compression” and “thermal shock” techniques. In combination with internal pressure the hot compression technique reveals itself to be very efficient and of most general use. The thermal shock method is effective for repairs in sections up to 25mm thick. The APWTs were established using numerical models applied to mock-up nuclear components, and validated by measurements. Numerical modelling methods have also been used to study the interactions between residual stresses, post weld treatments, operational loads, crack growth and fracture. Integrity assessment procedures using a modified J-integral definition are applied to confirm that optimised repair and APWT procedures can be developed to reduce welding residual stresses without any secondary detrimental effect, even with the presence of pre-existing cracks.Copyright

Investigation of Thermal Ageing in Long Term Operated RPV Materials

The phenomenon of thermal ageing of low alloy steels comes more into focus in terms of long term operation of nuclear power plants (NPP). Safety-relevant components such as the RPV or the pressurizer have to bear the respective loads at elevated temperatures for longer times. However the mechanical properties of the applied materials might experience certain degradations such as a decrease of the impact energy levels and a shift in the ductile to brittle transition temperature (e.g. T41) leading to higher ductile-brittle reference temperatures and a reduction of material toughness. In terms of a safe long term operation it is important to understand in how far thermal ageing alone, meaning for the RPV without the cumulative damaging effects through neutron irradiation, has detrimental influences on the respective materials of interest.First of all an overview is provided of the current state of the art with respect to thermal ageing by describing influencing mechanisms, its implementation into different nuclear codes, standards and selected experimental investigations in this field. Following this, the test results of the thermal surveillance sets from three German PWRs are presented and discussed. The tested Charpy-V specimens, taken from representative RPV base and weld metals (22NiMoCr3-7 / NiCrMo1UP) as well as their heat affected zones, were exposed to ∼290°C for ∼30 years on the cold leg of the according plants’ main coolant loops. The obtained results are compared with the existing thermal aging data base (baseline and ∼7 years data) of the materials concerned. Finally, the role of thermal ageing particularly with respect to RPV irradiation surveillance will be assessed.Copyright

Extended Mechanical Testing of RPV Surveillance Materials Using Reconstitution Technique for Small Sized Specimen to Assist Long Term Operation

For the Ringhals 3 and 4 PWR RPV, results from the irradiation surveillance program are available also for neutron fluences which cover long-term operation (LTO). These standard surveillance result ...

A Fracture-Toughness Based Transition Reference Temperature for Use in the ASME Code With the Crack Arrest (KIA) Curve

In the early 2000s, ASME adopted Code Cases N-629 and N-631 [1–2], both of which permit the use of the Master Curve reference temperature (To) to define an reference temperature RTTo, as follows (in SI units, as are used throughout the paper):Display FormulaRTTo=To+19.4℃ The Code Cases state that “this reference temperature … may be used as an alternative to [the] indexing reference temperature RTNDTfor the KIcand KIatoughness curves, as applicable, in Appendix A and Appendix G [of Section XI of the ASME Code].” KIa is now only used in Appendix A. The functional form of the ASME KIc and KIa curves dictate that the temperature separation between them remains constant irrespective of the degree of neutron radiation embrittlement, as quantified by ΔRTNDT or ΔRTTo. However, data collected from the literature and new data reported by Hein et al. show that radiation embrittlement brings the KIc and KIa curves closer together as embrittlement increases. As a result, current Code guidance will not produce a bounding KIa curve in all situations when RTTo is used as an reference temperature. To reconcile this issue, this paper summarizes available data and, on that basis, concludes that use of the following reference temperature will ensure that the ASME KIa curve bounds currently available KIa data:Display FormulaRTKIa=RTTo-19.4+44.97×exp⁡−0.00613×RTTo-19.4Copyright

Crack Arrest Test Results of Unirradiated and Irradiated German RPV Steels

In the CARISMA[1] and CARINA[2] projects comprehensive tensile, Charpy-impact and fracture toughness tests were performed for unirradiated and irradiated original reactor pressure vessel (RPV) steel specimens from German pressurized water reactors (PWR) up to neutron fluences in the range of 60 operational years and beyond.In addition, crack arrest fracture toughness tests were performed to demonstrate the crack arrest behavior of the materials. To determine the crack arrest properties of ferritic steels, the designated test method according to ASTM E1221 [3] was used. However, in particular for irradiated reactor pressure vessel materials with higher irradiation embrittlement, the prescribed standard test specimen does not always provide adequate test results. During starter notch preparation annealing effects occurred in the heat affected zone (HAZ) of the brittle weld of the starter notch causing crack arrest in the HAZ after unstable crack initiation. Therefore a modified test method to perform crack arrest tests with so called duplex specimens was investigated.In this paper this modified method and the test results of five base and four weld metals with a fluence up to 4,69E+19 cm−2 (E >1 MeV) are discussed.The available test results show that the duplex specimen is an appropriate alternative to the standard compact crack arrest (CCA) specimen. The measured KIa fracture toughness data are enveloped by the “lower bound” of the ASME KIa-curve indexed with RTNDTj or TKIa but not all data are enveloped by indexing the “lower bound” curve with RTT0 like described in the ASME Code Case N-629 [4]. Furthermore correlations of the crack arrest test results with Charpy-impact and fracture toughness test results will be shown.Copyright

CARINA: A New Project to Extend the Data Base for Fracture Mechanical Characteristics of Irradiated German RPV Materials at High Neutron Fluences

Within the recently finished project CARISMA “Determination of Fracture Mechanics Values on Irradiated Specimens of German PWR Plants” a data base was created for pre-irradiated original RPV steels of the four construction lines of German PWRs, which allowed to examine the consequences if the Master Curve (T0 ) approach instead of the RTNDT concept is applied for the RPV safety assessment. In the new research project CARINA the experimental data base for both the safety concepts RTNDT and Master Curve for the proof against RPV brittle fracture will be extended by additional representative materials irradiated under different conditions and with respect to the accumulated neutron fluences and specific impact parameters (neutron flux, chemical composition, manufacturing effects). The investigation of materials irradiated to higher neutron fluences and different irradiation conditions complements the representativeness of the conclusions for the applicability of safety concepts to RPVs with longer operation times and beyond EoL respectively in terms of “Upper Bound” coverage. The obtained results will be evaluated on the basis of the already elaborated approaches and will be represented by the aid of the Master Curve concept in an appropriate way. An overview on the objectives, the experimental program, and the current status of the project is given. Finally first test results are presented and evaluated.Copyright