Reactor Pressure Vessel Model Research Articles

Magnetic Indicator for Evaluating Cu Clustering and Hardening Effect in RPV Model Alloy

The reactor pressure vessel (RPV) is a critical barrier in nuclear power plants, but its embrittlement during service poses a significant safety challenge. This study investigated the effects of Cu-enriched clusters on the mechanical and magnetic properties of Fe-0.9 wt.%Cu model alloys through thermal aging. Using Vickers hardness tests, Magnetic Barkhausen Noise (MBN) detection, and Atom Probe Tomography (APT), the study aimed to establish a quantitative correlation between MBN signals, Vickers hardness, and Cu-enriched clusters, facilitating the non-destructive testing of RPV embrittlement. Experimental results showed that the hardness and MBN parameters (RMS and Vpp values) changed significantly with aging time. The hardness increased rapidly in the early stage (under-aged), followed by a plateau and then a decreasing trend (over-aged). In contrast, MBN parameters decreased initially and then increased. APT analysis revealed that Cu-enriched clusters increase in size to 4.60 nm and coalesced during aging, with their number density peaking to 3.76 × 1023 m−3 before declining. An inverse linear correlation was found between MBN signals and the combined factor Nd2Rg (product of the number density squared and the mean radius of Cu-enriched clusters). This correlation was consistent across both under-aged and over-aged states, suggesting that MBN signals can serve as applicable indicators for the non-destructive evaluation of RPV steel embrittlement.

Probabilistic fracture mechanics analyses of a reactor pressure vessel using the irradiation embrittlement evaluation based on the Bayesian nonparametric method

In the structural integrity assessment of an embrittled reactor pressure vessel (RPV) based on probabilistic fracture mechanics (PFM), it is crucial to consider both of the mean value and the uncertainty of the embrittlement prediction. Typically, the embrittlement prediction uncertainty is given by a normal distribution, and its standard deviation is determined from the residuals of the measured and predicted values of all data used to develop the embrittlement prediction method. Therefore, the same uncertainty is assumed regardless of neutron fluence and the available data. To overcome this issue, the Japan Atomic Energy Agency recently developed the embrittlement prediction method based on machine learning and Bayesian statistics, i.e., the Bayesian nonparametric (BNP) method, and introduced it to the PFM analysis code. The BNP method can predict the probability distribution according to data scarcity and provide more reasonable uncertainty because it estimates significant uncertainties where the scatter of actual measured data is large and the number of data is small. In this study, PFM analysis for a Japanese model RPV in a pressurized water reactor is conducted using the PFM analysis of structural components in aging light water reactors, where the embrittlement probability distribution calculated using the BNP method is used. Furthermore, the effects of different embrittlement prediction methods and uncertainties on the failure frequency of RPVs are investigated in the PFM analyses, and the results are presented.

Understanding the effect of phosphorous on the ion-irradiation behaviour of RPV model steels using atom probe tomography and nanoindentation

Understanding the formation of the embrittling Mn-Ni-Si (MNS)-rich clusters in reactor pressure vessel (RPV) steels is of economic, environmental, and safety importance. Hence we investigated the influence of phosphorous (P) on the formation of MNS-rich clusters in RPV model steels employing atom probe tomography and nanoindentation tests. The atom probe tomography results show that the average number density and volume fraction of clusters decrease slightly with an increase in the bulk P content; however, higher bulk P led to a slight increase in the average diameter of the clusters. A higher amount of bulk P led to higher Cu in the clusters; suggesting synergy between Cu and P. An increase in the irradiation hardening values was observed due to higher bulk P content. This is attributed to the stabilisation of the self-interstitial atoms (SIA) clusters by P. A higher recovery for the sample containing higher bulk P indicated that the SIA clusters dissolved after post-irradiation annealing.

Study on the Re-Aging Behavior of Cu-Rich Precipitates in a FeCu Alloy under Electropulsing.

The nanoscale Cu-rich precipitates (CRPs) are one of the most critical microstructural features responsible for degrading the mechanical properties of reactor pressure vessel (RPV) steels. The prospect of the rapid regeneration of the service performance of degraded materials through electropulsing is attractive, and electropulsing has been proven to have the application potential to eliminate the CRPs and recover the mechanical properties of RPV materials. However, few studies have investigated the secondary service issue of electropulsing. This paper provides experimental findings from microstructural investigations and property evaluations of a FeCu RPV model alloy subjected to re-aging following recovery electropulsing and annealing treatments. The evolution behavior of CRPs and the changes in the hardness of the alloy during the re-aging process after electropulsing treatment were consistent with the initial aging process, while the re-aging process of the annealing treatment group was quite different from the initial aging. The difference between the electropulsing and annealing treatment groups was that the annealing treatment failed to eliminate the precipitates completely, leaving behind some large precipitates. This work demonstrates the potential application of EPT in this field.

Experimental study of turbulent flow in lower plenum with flow skirt of reactor pressure vessel of pressurized water reactor

The turbulent flow in lower plenum of reactor pressure vessel (RPV) of pressurized water reactor (PWR) is an important phenomenon for design of flow mixing device. A high-precision matched index of refraction (MIR) technique is requisite for time-resolved particle image velocimetry (TR-PIV) measurements in RPV with complicated internals. In this study, turbulent flow in a scaled-down RPV model was measured by TR-PIV under different Reynolds numbers (Re) and asymmetrical flow rate with aid of the high-precision MIR technique of sodium iodide (NaI) solution and acrylic. The time-averaged flow structures include flow separation at the corner of lower support plate, fluid bifurcation around the lower edge of flow skirt, flow turning upward below the vortex suppression plate, horizontal jets at holes of flow skirt, vertical jets at holes of vortex suppression plate, improving turbulent mixing and Reynolds stresses in the lower plenum. The coherent structures were studied by proper orthogonal decomposition (POD) analysis. The flow skirt and vortex suppression plate break down large vortices through them. With Re increasing, Reynolds stresses decrease fast, and the energy fractions of low-order modes transfer into high-order modes, because turbulent vortices break up. In the case of asymmetrical flow rate, the time-averaged velocity and Reynolds stress decrease slightly, and the cumulative energy fraction decreases at flow skirt and vortex suppression plate.

The investigation of the carbon on irradiation hardening and defect clustering in RPV model alloy using ion irradiation and OKMC simulation

The precipitation of solutes is a major cause of irradiation hardening and embrittlement limiting the service life of reactor pressure vessel (RPV) steels. Impurities play a significant role in the formation of precipitation in RPV materials. In this study, the effects of carbon on cluster formation and irradiation hardening were investigated in an RPV alloy Fe-1.35Mn-0.75Ni using C and Fe ions irradiation at 290 °C. Nanoindentation results showed that C ion irradiation led to less hardening below 1.0 dpa, with hardening continuing to increase gradually at higher doses, while it was saturated under Fe ion irradiation. Atom probe tomography revealed a broad size distribution of Ni–Mn clusters under Fe ion irradiation, contrasting a narrower size distribution of small Ni–Mn clusters under C ion irradiation. Further analysis indicated the influence of carbon on the cluster formation, with solute-precipitated defects dominating under C ion irradiation but interstitial clusters dominating under Fe ion irradiation. Simulations suggested that carbon significantly affected solute nucleation, with defect clusters displaying smaller size and higher density as carbon concentration increased. The higher hardening at doses above 1.0 dpa was attributed to a substantial increase in the number density of defect clusters when carbon was present in the matrix.

Positron annihilation study of the reactor pressure vessel model steels irradiated in the high flux reactor

Twelve reactor pressure vessel (RPV) model steels with parametric variation of alloying elements Ni and Mn were characterized by means of positron annihilation spectroscopy before and after irradiation at the High Flux Reactor (HFR) to ∼0.18 dpa at an average temperature of 286 °C. Two independent positron annihilation lifetime spectrometers (PALS) and one coincidence Doppler broadening spectrometer (CDBS), used in the post-irradiation examination, provided a substantial amount of experimental data used to characterize the vacancy-type defects and their agglomerations in the studied materials. The obtained results show an apparent increase in the positron trapping at vacancy-type defects after irradiation, with the concentration of the radiation-induced vacancy-type defects increasing with the Ni and Mn content. The effect of Si and Cu was found to be less pronounced, and the concentration of vacancy-type defects seems to anti-correlate with their content. In addition to open-volume vacancy-type defects, the PALS technique enabled a quantitative estimation of the dislocation-type defects, where only a modest increase in density was observed after the irradiation at the given temperature.

Analysis of LBLOCA of APR1400 with 3D RPV model using TRACE

It is very difficult to capture the multi-dimensional phenomena such as asymmetric flow and temperature distributions with the one-dimensional (1D) model, obviously, due to its inherent limitation. In order to overcome such a limitation of the 1D representation, many state-of-the-art system codes have equipped a three-dimensional (3D) component for multi-dimensional analysis capability. In this study, a standard multi-dimensional analysis model of APR1400 (Advanced Power Reactor 1400) has been developed using TRACE (TRAC/RELAP Advanced Computational Engine). The entire reactor pressure vessel (RPV) of APR1400 has been modeled using a single 3D component. The fuels in the reactor core have been described with detailed and coarse representations, respectively, to figure out the impact of the fuel description. Using both 3D RPV models, a comparative analysis has been performed postulating a double-ended guillotine break at a cold leg. Based on the results of comparative analysis, it is revealed that both models show no significant difference in general plant behavior and the model with coarse fuel model could be used for faster transient analysis without reactor kinetics coupling. The analysis indicates that the asymmetric temperature and flow distributions are captured during the transient, and such nonuniform distributions contribute to asymmetric quenching behaviors during blowdown and reflood phases. Such asymmetries are directly connected to the figure of merits in the LBLOCA analysis. Therefore, it is recommended to employ a multi-dimensional RPV model with a detailed fuel description for a realistic safety analysis with the consideration of the spatial configuration of the reactor core.

Interplay between carbon and substitutional solute atoms during precipitation of Mn/Ni/Si-rich phases in model reactor pressure vessel steels

Trace C atoms can significantly alter the mechanical properties of steels, however, the effects of C atoms during precipitation of the Mn/Ni/Si-rich phases in reactor pressure vessel (RPV) steels remains unclear. Here, the interaction of C atoms with Mn/Ni/Si atoms during aging is investigated via the internal friction technique. Two Snoek peaks were detected: one is a superposition of the Snoek relaxation in Fe-C-Fe and the Snoek relaxation in Fe-C-Mn, and the other is the Snoek relaxation in Fe-C-Ni(Si). From the height variation of Snoek peaks with aging time, it was found that with the increasing aging time, the C atoms combine with the substitutional solute atoms at first, and then co-precipitate as MnSiC clusters at the GBs after 20 h aging at 600 °C. The variation of Snoek peak with aging time provides a reliable method to non-destructively detect the aggregation and precipitation of the M/Ni/Si-rich phases in RPV steels.

Influence of Ni-Mn contents on the embrittlement of PWR RPV model steels irradiated to high fluences relevant for LTO beyond 60 years

Understanding the irradiation induced degradation of the reactor pressure vessel (RPV) steels at high fluences becomes more important in view of safe long term operation (LTO) of existing nuclear power plants. Specifically, the role of Ni, Mn and Si and associated synergetic effects on embrittlement behaviour need to be further investigated to understand accelerated embrittlement at high neutron fluences (ca. 1 × 10 20 n.cm −2 , E>1 MeV) observed in certain types of RPV steels. In addition, it is required to verify the validity of existing embrittlement trend curves (ETCs) at high fluence values. To address these issues, NRG and JRC performed a unique irradiation experiment called LYRA-10 in the High-Flux Reactor (HFR) Petten. In LYRA-10 a variety of specimens (e.g. tensile, Charpy, mini-Charpy and microscopy) of RPV steels (base metal and welds) with systematic variations in Ni, Mn and Si contents, while keeping the base composition representative of VVER-1000 and PWR RPV steels, have been irradiated for 16 HFR cycles to achieve the nominal fast neutron fluence of 1.11 × 10 20 n.cm −2 at a nominal temperature of 286 °C. A large post-irradiation examination campaign on LYRA-10 specimens has been initiated within the STRUMAT-LTO, which is an international collaboration project co-funded by EURATOM. The research reported in this paper includes results from tensile and mini-Charpy impact testing of four PWR RPV model steel specimens irradiated in LYRA-10, which is performed as part of STRUMAT-LTO in-kind pilot project before the start of the EURATOM co-funded project. The combined influence of Ni and Mn on embrittlement behaviour, characterized by irradiation hardening and shifts in transition temperature (T k ) properties, has been evaluated. The measured shifts in T k values for all four model steels have been compared with predicted values from two selected ETCs in order to check their validity for these steels at the above high neutron fluence values.

Magnetic investigations on irradiation-induced nanoscale precipitation in reactor pressure vessel steels: A first-order reversal curve study

We have investigated magnetic properties for reactor pressure vessel model alloys with variable Cu contents, subjected to neutron irradiation up to a fluence of 9 × 10 19 n cm − 2 . Unlike a monotonic increase of microhardness with neutron fluence, the major-loop coercivity decreases at a higher fluence and the decrease becomes larger for the alloy containing a higher amount of Cu. The measurements of first-order reversal curves (FORCs) for the high-Cu alloy show that the position of the FORC distribution peak shifts toward a lower coercivity just after neutron irradiation, followed by a slight increase, associated with the broadening along both the coercivity and interaction field axes. The results can be explained by the enhancement of magnetic inhomogeneity in a matrix due to Cu precipitation and an increasing magnetostatic interaction between local magnetic regions with different coercivity. The magnetic method using FORCs can be a possible technique which provides in-depth information on microstructural changes due to neutron irradiation, which is not obtained by measurements of a conventional major hysteresis loop.

The ageing behavior and the correlation between hardness and coercivity of Cu-rich reactor pressure vessel model steels

Reactor pressure vessels (RPVs) are the heart of nuclear power plants. RPV steels suffer from neutron irradiation, which would introduce irradiation damages into materials, such as dislocation loops, tetrahedrons, precipitations, etc. Due to shortage of monitoring specimens, nondestructive testing (NDT) has been developed for the degradation evaluation of RPV steels. In this paper, the ageing behavior of hardness and coercivity of Cu-rich RPV model steels are studied at 643 K, 673 K, 743 K and 773 K for magnetic NDT. It is found that the hardness and coercivity both exhibit ageing peaks during thermal ageing and there is a good correlation between them. Furthermore, the apparent activation energy of Cu-rich precipitations (CRPs) is estimated at around 160 kJ∙mol−1.

Effects of cold work on solute atom clustering during thermal aging in RPV model alloy

ABSTRACT The effect of initial material strength induced by cold work on the formation of solute atom clusters was investigated for thermal-aged Fe-Cu alloy specimens with and without cold work to understand the effects of material properties before irradiation on radiation embrittlement in reactor pressure vessel (RPV) steels. The results of solute atom cluster analysis by atom probe tomography supported the idea that cold work enhances the mobility of solute atoms by the vacancies associated with dislocations that were generated by the cold work and influences the formation and growth of solute atom clusters. Formation and growth of solute atom clusters by cold work was greater for more hardening in the RPV model alloy with higher yield strength. Cold work would be expected to worsen irradiation hardening and embrittlement.

Observation of Mn-Ni-Si-rich features in thermally-aged model reactor pressure vessel steels

Atom probe tomography was used to characterise two low-Cu (< 0.04 at. %) model steels after exposure to long-term thermal ageing. Mn-Ni-Si-rich features were observed to form after as little as 20,731 h (∼ 2.4 years) of ageing. The composition of these features were compared to those predicted by thermodynamic models and the similarities and differences are discussed.

Fracture mechanics analyses of a reactor pressure vessel under non-uniform cooling with a combined TRACE-XFEM approach

This paper presents the integrity analyses of a model reactor pressure vessel (RPV) subjected to pressurized thermal shocks (PTS). The analyses are performed with a one-way multi-step strategy that includes the thermo-hydraulics, thermo-mechanical and fracture mechanics analyses to simulate three hypothetical loss of coolant accidents (LOCA). The thermo-hydraulics analyses are performed with the system code TRACE and a three-dimensional (3D) model of the RPV, providing the input for the structural analyses with the finite element code ABAQUS. These employ a sequential use of global model (entire RPV) and submodels (a portion of the RPV containing the crack), where the eXtended Finite Element (XFEM) approach is employed to compute the stress intensity factor (SIF) of a postulated crack in the RPV wall.The results first present a verification of the multi-step strategy with the FAVOR code for uniform temperature distribution in the RPV wall. Under uniform temperature and pressure load, a significant effect of the nozzle geometry on the asymmetric stress distribution is demonstrated. The second part of the results shows that the stresses and the SIFs are also sensitive to the non-uniform temperatures due to the presence of the cooling plumes. It is also confirmed that the analyses with ABAQUS and FAVOR provide very similar results for the medium and large break LOCA transients. For the small break LOCA, the FAVOR code underestimates the SIFs due to the missing nozzle geometry in combination with system pressure. Finally, the paper corroborates that the use of TRACE and XFEM, within the one-way multi-step simulation strategy, reduces the computational costs and the number of assumptions and approximations needed for feasible and relivable 3D fracture mechanics analyses of the RPV with consideration of the cooling plume effect.

The effect of composition variations on the response of steels subjected to high fluence neutron irradiation

A set of low alloy model reactor pressure vessel steels, with systematic variations in their Mn, Ni, and Si contents, were neutron-irradiated to high fluence (1.4 × 1020 n/cm2) in the Advanced Test Reactor at Idaho at 290°C and a flux of 3.6 × 1012 n/cm2s. The alloys were analysed using atom probe tomography and solute clusters were observed in each alloy, including in one alloy that contained low nominal levels of Mn (0.04 at. %) and Si (0.06 at. %). Changes in the mechanical properties of the alloys were correlated with cluster volume fractions. Whilst the effect of nominal composition was observed to influence cluster composition, cluster nucleation site was not observed to affect composition. Several grain boundaries were also analysed and the segregation behaviour of certain elements is discussed.

Ion irradiation-induced the radiation hardening in Fe and Fe-0.3Si alloys

Depth profile of Helium irradiation effects in reactor pressure vessel (RPV) model material is identified using a nano-indentation technique in order to develop a new methodology to clarify the solute-vacancy interaction. 2 MeV He ion was irradiated in pure iron at room temperature with and without an energy degrator. The indentation hardness was measured by nanoindentation changing the load at 9 levels. Hardening in irradiated materials was clearly observed in all indentation loads as well as the well-known indentation size effect. It was interesting that the significant increase of hardness was recognized at 0.8 μm depth, which is corresponding to the obstacle of deformation at 3˜4 μm depth, that would be the cause of helium implantation. Furthermore, dose effects on 3MeV Si irradiated RPV model materials were confirmed.

Redistribution of Mn between α-Fe matrix and θ cementite during long-term thermal aging in a low alloy steel

The addition of certain amounts of Mn in steel has long been known to retard the growth and coarsening of cementite during tempering, which can increase the tempering resistance of carbon steels. It is now well-established that the retarding effect is inherently correlated with the partitioning of Mn between ferrite(α) matrix and cementite(θ). According to the equilibrium thermodynamics, Mn would diffuse from α-Fe matrix to θ cementite after the initial stage of tempering until equilibrium is reached.However, the manner in which Mn diffuses from α-Fe matrix to θ cementite is unclear, which is key in understanding the mechanism in which the partitioning of Mn can retard the growth and coarsening of cementite. Therefore, the measurement of Mn content across the α-Fe/θ interface is of importance to achieve this goal. In this study, the redistribution characteristics of Mn between α-Fe matrix and θcementite after long-term aging at 370 or 400 °C with quenched–tempered or quenched samples of reactor pressure vessel model steel was investigated by atom probe tomography. Results show that Mn diffuses from the α-Fe matrix and enriches in the θ cementite under all heat treatment conditions. The concentration of Mn in cementite is the highest when the specimen is thermally aged directly after quenching. Moreover, Mn is not distributed uniformly within cementite after long-term aging at 400 °C for 35000 h. Instead, a Mnsegregated zone exists within cementite adjacent to theα-Fe/θinterface,with concentration increasing by aging temperature,which acts as a barrier to the coarsening of cementite by hindering the dissolution of small-sized cementite.The redistribution characteristics of Mn between the two phases is correlated with the difference of diffusivities in theα-Fe matrix andθcementite during thermal aging,and the diffusivity of Mn inθcementite is slower than that inα-Fe matrix.

Cu precipitation behaviour in long-term thermally aged, high-copper model reactor pressure vessel steels

Nanoscale Cu-rich precipitates that were formed in Cu-containing reactor pressure vessel (RPV) steels during service have a deleterious effect on mechanical properties, which can result in RPV embrittlement and limit reactor operation life. To understand the nanoscale precipitation mechanism, thermal aging at 370°C for up to 13200 h of high-copper RPV model steels was performed to produce nanosized Cu-rich precipitates. Then the Cu-rich precipitates were systematically investigated by atom probe tomography (APT). The changes in the mechanical properties of the steels were characterized by Vickers hardness test. The results show that the Cu-rich precipitates as determined by APT construction analysis have a spatial core-shell structure. The core is enriched with Cu atoms, and the region near the precipitate/matrix interface is enriched with Mn, Ni, Mo, and Si and C atoms. Cu-rich precipitates lead to precipitation strengthening and hardening/embrittlement effects. Using the Russell-Brown model, we estimated the hardening due to Cu-rich precipitates in the matrix and observed that the measured hardness and the estimated changes in hardness were in good agreement.

A kinetic lattice Monte Carlo study of post-irradiation annealing of model reactor pressure vessel steels

Significant embrittlement in reactor pressure vessel (RPV) steels can be caused by the formation of nanometer-scale Mn–Ni–Si precipitates (MNSPs) and annealing is a promising technique for reducing embrittlement. To achieve better understanding of the evolution of these precipitates at the atomic scale, a kinetic lattice Monte Carlo (KLMC) model, parameterized using CALPHAD and recent atom probe tomography (APT) data, is used to simulate post-irradiation annealing of MNSPs. The model predicts MNSP volume fractions, number densities and sizes that agree well with the experimental observations. The model also predicts that the initial structure of the precipitates may be B2 bcc phases with one sublattice occupied by Ni and the other sublattice occupied by Mn and Si, as well as shows a modest temperature dependence of the MNSP composition. The results show that the simple approach can be used to model MNSP evolution and supports that these precipitates are stable thermodynamic phases.