Reactor Pressure Vessel Welds Research Articles

Study of mechanical properties of RPV base and weld material using SPT technique

Small Punch Test (SPT) is one of the miniature test techniques used for evaluating tensile properties when limited dimension of material is available for standard mechanical test or handling of large specimen is difficult, especially while dealing with irradiated materials. In order to use SPT for determination of mechanical properties, it is crucial to establish correlation coefficients by comparing results obtained by SPT with that from standard tensile tests. Present work involves developing correlation coefficients and evaluation of tensile properties of reactor pressure vessel (RPV) steel using SPT technique. Specimen dimensions used for SPT was 10x10x0.5 mm3. These specimens were prepared from Charpy V-Notch specimens, which were earlier subjected to standard impact testing. Correlation coefficient for RPV material was established by comparison of SPT data with the tensile properties obtained using standard tensile test. Subsequently, SPT data was acquired on RPV thermal material and tensile properties were obtained using the correlation coefficient for validation. This paper describes the SPT set-up used during the current studies and methodology used for establishing the correlation coefficient on RPV base material.

Elemental distribution in a decommissioned high Ni and Mn reactor pressure vessel weld metal from a boiling water reactor

Elemental distribution in a decommissioned high Ni and Mn reactor pressure vessel weld metal from a boiling water reactor

Small-angle neutron scattering study of neutron-irradiated and post-irradiation annealed VVER-1000 reactor pressure vessel weld material

Post-irradiation annealing of neutron-irradiated reactor pressure vessel steels is a matter of both technical and scientific interest. Small-angle neutron scattering (SANS), while being sensitive to nm-sized irradiation-induced solute-atom clusters, provides macroscopically representative and statistically reliable measures of cluster volume fraction, number density and size. In the present study, SANS was applied to uncover the size distribution of clusters in as-irradiated samples of a VVER-1000 weld and their gradual dissolution as function of the post-irradiation annealing temperature. The same samples were used to measure Vickers hardness. The results are consistent with Mn-Ni-Si-rich clusters of less than 2 nm radius to be the dominant source of both scattering and hardening. Annealing gave rise to small but significant partial recovery at 350°C and almost complete recovery at 475°C. The dispersed-barrier hardening model was applied to bridge the gap between the characteristics of nano-features and macro-hardness.

Mechanical behavior of high-Ni/high-Mn Barsebäck 2 reactor pressure vessel welds after 28 years of operation

To assess long-term operation of the reactor pressure vessel (RPV), surveillance programs are applied for periodic monitoring and prediction of the aging of the mechanical properties due to irradiation and thermal embrittlement. In literature, there are limited data sets to compare the results from the surveillance program to the aging of the RPV. In this work, the tensile and impact toughness properties of the high-Ni, high-Mn welds from decommissioned Barsebäck 2 RPV are characterized. The results indicate that the surveillance program describes sufficiently the aging of the RPV welds. Differences in mechanical properties of the welds from various regions are explained by variations in post-weld heat treatment. The synergetic effect of Ni and Mn on embrittlement appears not to result at low fluences in a significant difference in the embrittlement rate when compared to ASTM E900 embrittlement trend curve prediction.

Post-irradiation annealing of high flux irradiated and surveillance material reactor pressure vessel weld metal

In this study, high flux irradiated and surveillance high Ni and Mn and low Cu welds identical to those of the belt-line region of Ringhals R4 were subjected to annealing at temperatures between 390 and 455 °C for 24–30 h, in order to study the dissolution of irradiation induced clusters and possible matrix defects using hardness testing and atom probe tomography. It was found that the cluster characteristics did not change during annealing at 390 °C, meaning that the size, number density and composition of the clusters, which mainly consist of Ni and Mn, did not change. Thus, the observed decrease in hardness during annealing of the high flux irradiated material is believed to be due to dissolution of matrix defects that were stable at the operating temperature. Cluster dissolution was observed after annealing at 410 °C in the high flux irradiated material, leaving around 10% of the original clusters. These clusters contained more Cu and less Ni and Mn than before annealing. The cluster dissolution at temperatures above 400 °C correlated with the decrease in hardness. The larger clusters of the surveillance material required a higher temperature or longer time to be dissolved compared to the clusters of the high flux material.

Contribution of Hardening Mechanism to VVER-1000 RPV Welds Flux Effect

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On the Analysis of Clustering in an Irradiated Low Alloy Reactor Pressure Vessel Steel Weld.

Radiation induced clustering affects the mechanical properties, that is the ductile to brittle transition temperature (DBTT), of reactor pressure vessel (RPV) steel of nuclear power plants. The combination of low Cu and high Ni used in some RPV welds is known to further enhance the DBTT shift during long time operation. In this study, RPV weld samples containing 0.04 at% Cu and 1.6 at% Ni were irradiated to 2.0 and 6.4×1023 n/m2 in the Halden test reactor. Atom probe tomography (APT) was applied to study clustering of Ni, Mn, Si, and Cu. As the clusters are in the nanometer-range, APT is a very suitable technique for this type of study. From APT analyses information about size distribution, number density, and composition of the clusters can be obtained. However, the quantification of these attributes is not trivial. The maximum separation method (MSM) has been used to characterize the clusters and a detailed study about the influence of the choice of MSM cluster parameters, primarily on the cluster number density, has been undertaken.

Study of the flux effect nature for VVER-1000 RPV welds with high nickel content

This work extends the research of the basic regularities of segregation processes in the grain boundaries (GB) of VVER-1000 reactor pressure vessel (RPV) steels. The paper considers the influence of irradiation with different fast neutron fluxes on the structure, yield strength and ductile-to-brittle transition temperature (TK) changes as well as on changes of the share of brittle intergranular fracture and development of segregation processes in the VVER-1000 RPV weld metal (WM). The obtained experimental results allow to separate the contribution of the hardening and non-hardening mechanisms to mechanical properties degradation of material irradiated at the operating temperature. It is shown that the difference in TK shift in WM irradiated to the same fluence with different fast neutron fluxes is mainly due to the difference in the GB accumulation kinetics of impurities and only to a small extent due to the material hardening. Phosphorus bulk diffusion coefficients were evaluated for the temperature exposure, accelerated irradiation and irradiation within surveillance specimens (SS) using a kinetic model of phosphorus GB accumulation in low-alloyed low-carbon steels under the influence of operational factors. The correlation between the GB segregation level of phosphorus and nickel, and the TK shift - in WM SS was obtained experimentally and indicates the non-hardening mechanism contribution to the total radiation embrittlement of VVER-1000 RPV steels throughout its extended lifetime.

DEVELOPMENT OF THE ALTERNATE PRESSURIZED THERMAL SHOCK RULE (10 CFR 50.61a) IN THE UNITED STATES

In the early 1980s, attention focused on the possibility that pressurized thermal shock (PTS) events could challenge the integrity of a nuclear reactor pressure vessel (RPV) because operational experience suggested that overcooling events, while not common, did occur, and because the results of in-reactor materials surveillance programs showed that RPV steels and welds, particularly those having high copper content, experience a loss of toughness with time due to neutron irradiation embrittlement. These recognitions motivated analysis of PTS and the development of toughness limits for safe operation. It is now widely recognized that state of knowledge and data limitations from this time necessitated conservative treatment of several key parameters and models used in the probabilistic calculations that provided the technical of the PTS Rule, 10 CFR 50.61. To remove the unnecessary burden imposed by these conservatisms, and to improve the NRC's efficiency in processing exemption and license exemption requests, the NRC undertook the PTS re-evaluation project. This paper provides a synopsis of the results of that project, and the resulting Alternate PTS rule, 10 CFR 50.61a.

A comparison of the structure of solute clusters formed during thermal ageing and irradiation

Nanometre scale clusters form in Cu-containing reactor pressure vessel (RPV) steels during neutron irradiation. These clusters have a deleterious effect on mechanical properties, which can result in embrittlement and limit the reactor operating life. Thermal ageing of RPV steels can also induce the formation of solute clusters but it is not clear how similar these are to those formed during irradiation. In this work atom probe tomography, combined with detailed structural assessments of the structure of solute clusters, is used to address this issue. A series of thermal ageing heat treatments has been performed on several high- and low-Ni RPV welds to produce 1-4 nm diameter solute clusters. The same materials have also been neutron irradiated. The results show that CuMnNiSi enriched clusters formed during thermal ageing have, on average, higher Cu contents and lower Mn, Ni and Si contents than those found in irradiation-induced clusters. The effect of increasing bulk Ni is to encourage the formation of clusters with significantly higher Ni content, slightly higher Mn and Si contents and significantly lower Cu contents. At very high doses and dose rates MnNiSi enriched clusters can form even in high-Cu welds. Despite differences in the compositions of individual clusters formed during irradiation and during thermal ageing, clusters in both exhibit similar structure. In particular, well developed clusters in both materials have Cu-enriched cores whose peripheries are enriched in Ni, Mn and, in most cases, Si.

Small-angle neutron scattering investigation of as-irradiated, annealed and reirradiated reactor pressure vessel weld material of decommissioned reactor

Small-angle neutron scattering (SANS) was applied to characterize the microstructure of weld material taken from the reactor pressure vessel (RPV) of the decommissioned VVER440 (230)-type nuclear power plant (NPP) Greifswald, Units 1, 2 and 4. The welding seam of highest neutron exposure of Unit 1 was subject to a large-scale annealing treatment in 1988 after about 11.5 effective years of operation. The same type of annealing was applied to Unit 2 in 1990 after about 11 effective years of operation. After final decommissioning of NPP Greifswald in 1990, RPV material was left in the reirradiated condition (Unit 1), in the as-annealed condition (Unit 2) and in the as-irradiated condition (Unit 4). Trepans of material from the highly irradiated RPV welds of these Units have recently become available for examination. The results of the SANS investigation are reported and compared with published results obtained for as-irradiated, post-irradiation annealed and reirradiated surveillance material of the same type. A general agreement was found indicating in particular the formation of irradiation-induced Cu-enriched clusters and efficient recovery as a result of the large-scale annealing treatments. The only essential difference was observed for the ratio of magnetic and nuclear scattering indicating differences of the cluster composition for the RPV wall and surveillance material.

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

Abstract 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.

Stress corrosion cracking countermeasure observed on Ni-based alloy welds of BWR core support structure

The effect of hydrostatic test on the residual stress re-distribution was simulated by experiment to confirm the residual stress behavior of the cone-shaped shroud support to reactor pressure vessel (RPV) weld, where a number of cracks due to stress corrosion cracking (SCC) were observed on the inner side only. Test specimen with tensile residual stress was loaded and unloaded with axial plus bending load, which simulates the hydrostatic test load, and the strain change was measured during the test to observe the residual stress behavior. The results verify that the residual stresses of the shroud support to the RPV weld were reduced and the stresses on inner and outer sides were reversed by the hydrostatic test. As the SCC countermeasure, the shot peening (SP) technology was applied. Residual stress reduction by SP on the complicated configuration, and improvement of SCC resistance and endurance of the compressive residual stress were experimentally confirmed. Then, SP treatment procedures on the actual structure were confirmed and a field application technique was established.

Flux dependence of cluster formation in neutron-irradiated weld material

The effect of neutron flux on the formation of irradiation-induced clusters in reactorpressure vessel (RPV) steels is an unresolved issue. Small-angle neutron scattering wasmeasured 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 ofirradiation-induced clusters at fixed fluence. The aim was to separate and tentativelyinterpret the effect of flux on the characteristics of the cluster size distribution. Wehave observed a pronounced effect of neutron flux on cluster size, whereas thetotal volume fraction of irradiation-induced clusters is insensitive to the level offlux. The result is compatible with a rate theory model according to which therange of applied fluxes covers the transition from a flux-independent regime atlower fluxes to a regime of decelerating cluster growth. The results are confrontedwith measured irradiation-induced changes of mechanical properties. Despitethe observed flux effect on cluster size, both yield stress increase and transitiontemperature shift turned out to be independent of flux. This is in agreement with thevolume fraction of irradiation-induced clusters being insensitive to the level of flux.

SANS response of VVER440-type weld material after neutron irradiation, post-irradiation annealing and reirradiation

It is well accepted that the reirradiation behaviour of reactor pressure vessel (RPV) steel after annealing can be different from the original irradiation behaviour. We present the first small-angle neutron scattering (SANS) study of neutron irradiated, annealed and reirradiated VVER440-type RPV weld material. The SANS results are analyzed both in terms of the size distribution of irradiation-induced defect/solute atom clusters and in terms of the ratio of total and nuclear scattering intensity in a saturation magnetic field (A ratio). The measured A ratio is compared with calculations performed on the basis of the cluster composition reported for a similar weld material investigated by means of three-dimensional atom probe field ion microscopy. The observed deviation between both estimates and possible reasons for the discrepancy are discussed. Special emphasis is placed on the differences between the materials response to the original irradiation and to reirradiation after annealing. The results indicate that reirradiation-induced clusters are slightly different in their average composition and their formation saturates at a lower volume fraction than in the case of the original irradiation.

Analysis of WWER-440 and PWR RPV welds surveillance data to compare irradiation damage evolution

It is known that for Russian-type and Western water reactor pressure vessel steels there is a similar degradation in mechanical properties during equivalent neutron irradiation. Available surveillance results from WWER and PWR vessels are used in this article to compare irradiation damage evolution for the different reactor pressure vessel welds. The analysis is done through the semi-mechanistic model for radiation embrittlement developed by JRC-IE. Consistency analysis with BWR vessel materials and model alloys has also been performed within this study. Globally the two families of studied materials follow similar trends regarding the evolution of irradiation damage. Moreover in the high fluence range typical of operation of WWER the radiation stability of these vessels is greater than the foreseen one for PWR.

The effect of non-metallic inclusions on the fracture toughness master curve in high copper reactor pressure vessel welds

The fracture toughness of two high copper reactor pressure vessel welds having low upper shelf energy was evaluated in accordance with the master curve method of ASTM E1921. The resultant data were correlated to the metallurgical factors involved in the brittle fracture initiation to provide a metallurgical-based understanding of the master curve. The tests were performed using pre-cracked Charpy V-notched specimens and the master curve was made with an average of T 0 values determined at different temperatures. In all specimens, the cleavage fracture initiated at non-metallic inclusion ranging from 0.7 to 3.5 μm in diameter showing a scatter with the specimens and testing temperatures. Temperature dependency of the triggering particle size was not found. The fracture toughness ( K J C ) was inversely proportional to the square root of the triggering inclusion diameter ( d i ) at respective temperatures. From this relationship, we determined median K J C values which correspond to the average value of triggering inclusion diameter of all tested specimens and defined them as a modified median K J C ( K ′ J C(med) ). The obtained K ′ J C(med) values showed quite smaller deviation from the master curve at different temperatures than the experimental median K J C values. This suggests that the master curve is on the premise of a constant dimension of key microstructural factor in a material regardless of the testing temperature. But the inclusion size at trigger point played an important role in the absolute position of the master curve with temperature and the consequent T 0 value.

Thermally activated deformation of irradiated reactor pressure vessel steel

Temperature and strain rate change tensile tests were performed on two VVER 1000-type reactor pressure vessel welds with different contents of nickel in unirradiated and irradiated conditions in order to determine the activation parameters of the contribution of the thermally activated deformation. There are no differences of the activation parameters in the unirradiated and the irradiated conditions as well as for the two different materials. This shows that irradiation hardening preferentially results from a friction hardening mechanism by long-range obstacles.

A microstructural examination of irradiated reactor pressure vessel weld samples

An important contribution to the life extension of Magnox nuclear power stations comes from the assessment of microstructural changes in reactor pressure vessel (RPV) weld metal after extended in-service exposure to a neutron flux. A project was undertaken to remove through-thickness weld samples from an RPV at a power station undergoing decommissioning. Sampling was carried out in a region of the reactor where the RPV was exposed to a high fast neutron irradiation dose at a low operating temperature. Each sample was cut into several specimens for mechanical tests to establish fracture toughness, chemical analysis and dosimetry data. This paper describes some of the preliminary results obtained from a microstructural characterisation programme. Samples were cut in various positions along the weld and at different depths through the vessel wall. These samples were examined by a number of techniques to evaluate the macro and microstructure after different amounts of neutron irradiation. In-situ micro-analysis and fractography was carried out on specimens after mechanical testing.

Boat sampling and inservice inspections of the reactor pressure vessel weld No. 4 at Kozloduy NPP, Unit 1

The paper deals with reactor pressure vessel (RPV) boat sampling performed at Kozloduy Nuclear Power Plant, Unit 1, from August to November 1996. Kozloduy NPP, Unit 1 has no reactor vessel material surveillance program. Changes in the material fracture toughness resulting from the fast neutron irradiation which cannot be monitored without removal of the vessel material. Therefore, the main objective of the project was to cut samples from the RPV wall in order to obtain samples of the RPV material for further structural analyses. The most critical area, i.e. weld No. 4 was determined as a location for boat sampling. Replication technique was applied in order to obtain precise determination of the weld geometry necessary for positioning of the cutting tool prior to boat sampling, and determination of divot depth left after boat sampling and grinding of sample sites. Boat sampling was performed by electrical discharge machining (EDM). Grinding of sample sites was implemented to minimize stress concentration effects on sample sites, to eliminate surface irregularities resulting from EDM process, and to eliminate recast layer on the surface of the EDM cut. Ultrasonic, liquid penetrant, magnetic particles, and visual examinations were performed after grinding to establish baseline data in the boat sampling area. The project preparation activities, apart from EDM process, and the site organization lead was entrusted to INETEC. The activities were funded by the PHARE program of the European Commission.