Warm Prestressing Research Articles

Consideration of special effects for the application of an optimized fracture mechanics approach for the RPV safety assessment (CAMERA)

A comprehensive fracture mechanics test program called CAMERA was carried out for seven RPV base and weld materials in unirradiated and irradiated conditions. The objective was to establish an application-oriented completion of the fracture mechanics approach for the RPV safety assessment over the entire relevant temperature range up to operating temperature. For this purpose, fracture toughness tests in the ductile-brittle transition region with and without warm pre-stress (WPS), and in the ductile region (upper shelf) were performed in order to complete the fracture toughness curves of the material concerned. The test program was completed by various analytical and numerical calculations and microstructural analyses. The benefit of the WPS effect was confirmed for both the material (higher apparent fracture toughness values) and the load path (no initiation after maximum loading). By fracture toughness tests in the ductile regime the fracture toughness curve could be completed up to the operating temperature and the T0 based criterion temperature TUS above which no brittle fracture occurs was determined and successfully verified. Based on the results obtained by the Master Curve, WPS and crack resistance tests it was demonstrated how to integrate the criterion temperature TUS in the fracture toughness-temperature diagram including the loading transient to an application window for the RPV safety assessment that is based on T0 only. For three out of nineteen data sets the homogeneity screening procedure described in ASTM E1921 did indicate a macroscopically inhomogeneous material for that a generally conservative reference temperature T0IN was calculated that is on average 11 K higher than the reference temperature T0. A higher susceptibility of weld materials for macroscopic material inhomogeneity compared to base materials was found. The suitability of the applied SE(B) and C(T) specimens for the specific fracture mechanics tests (WPS and/or crack resistance in ductile region) was proven. Micromechanical Local Approach damage models (Bordet and Gurson) were successfully applied for test design and prediction of fracture toughness. The overall results reveal even so some open gaps remaining for future work that are addressed as well.

Numerical prediction of warm pre-stressing effects for a steam turbine steel

In warm pre-stressing (WPS), the fracture resistance of cracked steel components is raised when subjected to certain temperature-load histories. WPS’s beneficial effects enhance safety margins and potentially prolong fatigue life. However, understanding and predicting the WPS effects is crucial for employing such benefits. This study utilised pre-cracked compact tension specimens made from steam turbine steel for WPS and baseline fracture toughness testing. Two typical WPS cycles were investigated (L-C-F and L-U-C-F), and an increase in fracture resistance was observed for both cycles. The WPS tests were simulated using finite element analysis to understand its effects and predict the increase in fracture resistance. A local approach was followed based on accumulative plastic strain magnitude ahead of the crack tip. Since cleavage fracture is triggered by active plasticity, the WPS fracture is assumed when accumulated plasticity exceeds the residual plastic zone formed at the crack tip due to the initial pre-load.

Influence of preliminary thermomechanical load on the integrity of the vessel of Ukrainian nuclear reactors in case of accidents of "large" and "small" leak

In works of extending lifetime of WWER-type reactors, it is necessary to obtain the brittle fracture resistance (BF) of the reactor pressure vessel (RPV). Implementation calculations for brittle fracture resistance is regulated by the technical PNAE of Ukraine. The objective of these calculations is to prevent catastrophic brittle destruction of the reactor pressure vessel, pipelines and pressure vessels from the existence crack-like defects for all operating regimes, including emergency situations (ES). The paper considers the most dangerous postulated emergency situation in operation is "large" and "small" leak in the RPV NPP. Calculations with Warm Pre-Stressing effect (WPS) of the RPV for the most dangerous scenarios have been presented, and an assessment of the brittle strength of RPV NPP is taking into account with WPS. The results of studies with factor of brittle strength safety are also presented without taking into account the Warm Pre-Stressing, comparing with the existing method for accounting this type of load.

Statistical modelling of fracture using cellular atomata finite element

Fracture is an inherently statistical phenomenon as it is a function of micro-structural heterogeneities such as distributed defects and inclusions. This is evidenced by scatter in the toughness of seemingly identical specimens. Therefore, deterministic approaches do not give full picture of scatter in fracture behaviour. More suitable probabilistic methods have been devised to describe the scatter associated with fracture. While the probabilistic approaches provide a sound scientific basis for capturing the scatter in the fracture data through assuming a probability for the presence of fracture initiators, their microstructurally agnostic assumptions can limit their predictive capability. This is because there is no information on the microstructure such as grain size and morphology, texture, and other important features considered in them. An alternative class of models which take into account the distribution of toughness is cellular automata finite element models (CAFE). CAFE models are stronger in simulating the scatter in the fracture data through their ability to represent the microstructure although so far, they have been limited to fully brittle or quasi-brittle materials. In addition, the CAFE models they are computationally expensive, and their running time can be prohibitive for their application to large scale engineering components thus reducing their appeal. In this study, a CAFE model was developed to take advantage of the microstructural fidelity of CAFE but presented within the context of a probabilistic fracture approach. The CAFE based model calculates the macroscopic strain from the continuum FE model. The strain is then used to load a model which is defined in the cellular automata space. The CAFE model then simulates the initiation and propagation of fracture in the microstructure to fully capture the heterogeneity of the material at the lower length scale. The critical stress acting normal to the cleavage plane of each grain is calculated in the CAFE model and used to decide the onset of cracking in a probabilistic manner; the stress depends on the orientation of the grain in which microcrack initiates as well as depending on the orientation of the surrounding grains. To evaluate its performance, the model was calibrated using a set of experimental fracture toughness data and the results of its prediction were compared with an intendent and separate set of warm prestress experiments of the same material. Good agreement between the prediction and the experiment of the second set was observed giving confidence in the model.

Improvements on Evaluation Functions of a Probabilistic Fracture Mechanics Analysis Code for Reactor Pressure Vessels

Abstract In Japan, a probabilistic fracture mechanics (PFM) analysis code PASCAL was developed for structural integrity assessment of reactor pressure vessels (RPVs) considering neutron irradiation embrittlement and pressurized thermal shock (PTS) events. By reflecting the latest knowledge and findings, the evaluation functions are continuously improved and have been incorporated into PASCAL4 which is the most recent version of the PASCAL code. In this paper, the improvements made in PASCAL4 are explained in detail, such as the evaluation model of warm prestressing (WPS) effect, evaluation function of confidence levels for PFM analysis results by considering the epistemic and aleatory uncertainties in probabilistic variables, the recent stress intensity factor (KI) solutions, and improved methods for KI calculations when considering complicated stress distributions. Moreover, using PASCAL4, PFM analysis examples considering these improvements are presented.

Probabilistic Fracture Mechanics Analysis of Boling Water Reactor Vessel for Cool-Down and Low Temperature Over-Pressurization Transients

Abstract The failure probabilities of the reactor pressure vessel (RPV) for low temperature over-pressurization (LTOP) and cool-down transients are calculated in this study. For the cool-down transient, a pressure–temperature limit curve is generated in accordance with Section XI, Appendix G of the American Society of Mechanical Engineers (ASME) code, from which safety margin factors are deliberately removed for the probabilistic fracture mechanics analysis. Then, sensitivity analyses are conducted to understand the effects of some input parameters. For the LTOP transient, the failure of the RPV mostly occurs during the period of the abrupt pressure rise. For the cool-down transient, the decrease of the fracture toughness with temperature and time plays a main role in RPV failure at the end of the cool-down process. As expected, the failure probability increases with increasing fluence, Cu and Ni contents, and initial reference temperature-nil ductility transition ( RT NDT ). The effect of warm prestressing on the vessel failure probability for LTOP is not significant because most of the failures happen before the stress intensity factor reaches the peak value while its effect reduces the failure probability by more than one order of magnitude for the cool-down transient.

The probabilistic structural integrity assessment of reactor pressure vessels under pressurized thermal shock loading

The pressurized thermal shock (PTS) event poses a potentially significant challenge to the structural integrity of the reactor pressure vessel (RPV) during the long time operation (LTO). In the USA, the “screening criteria” for maximum allowable embrittlement of RPV material, which forms part of the USA regulations, is based on the probabilistic fracture mechanics (PFM). The FAVOR software developed by Oak Ridge National Laboratory (ORNL) is used to establish the regulation. As the technical basis of FAVOR is not the most widely-used and codified methodologies, such as the ASME and RCC-M codes, in most countries (with exception of the USA), proving RPV integrity under the PTS load is still based on the deterministic fracture mechanics (DFM). As the maximum nil-ductility-transition temperature (RTNDT) of the beltline material for the 54 French RPVs after 40 years operation is higher than the critical values in the IAEA-TECDOC-1627 and European NEA/CSNI/R(99)3 reports (while still obviously lower than the “screening criteria” of the USA), it may conclude that the RPV will not be able to run in the LTO based on the DFM. In the FAVOR, the newest developments of fracture mechanics are applied, such as the warm pre-stress (WPS) effect, more accurate estimation of the flaw information and less conservation of the toughness (such as the three-parameter Weibull distribution of the fracture toughness). In this paper, the FAVOR software is first applied to show both the methodology and the results of the PFM, and then the limits in the current FAVOR software (Version 6.1, which represents the baseline for re-assessing the regulation of 10 CFR 50.61), lack of the impact of the constraint effect, ductile–tearing initiation failure analysis, elastic–plastic fracture mechanics analysis, safety assessment of the crack front interface points and interaction among multiple cracks, are also discussed.

Warm PreStress effect on highly irradiated reactor pressure vessel steel

This study investigates the Warm Prestress (WPS) effect on 16MND5 (A508 Cl3) RPV steel, irradiated up to a fluence of 13·1023n.m-2 (E>1MeV) at a temperature of 288°C, corresponding to more than 60years of operations in a French Pressurized Water Reactor (PWR). Mechanical properties, including tensile tests with different strain rates and tension–compression tests on notched specimens, have been characterized at unirradiated and irradiated states and used to calibrate constitutive equations to describe the mechanical behavior as a function of temperature and fluence. Irradiation embrittlement has been determined based on Charpy V-notch impact tests and isothermal quasi-static toughness tests. Assessment of WPS effect has been done through various types of thermomechanical loadings performed on CT(0.5T) specimens. All tests have confirmed the non-failure during the thermo-mechanical transients. Experimental data obtained in this study have been compared to both engineering-based models and to a local approach (Beremin) model for cleavage fracture. It is shown that both types of modeling give good predictions for the effective toughness after warm prestressing.

Methodology and Warm Prestressing Effect for Pressurized Thermal Shock Analysis in Nuclear Power Plant

One potential challenge to the integrity of the reactor pressure vessel (RPV) in a pressurized water reactor is posed by pressurized thermal shock (PTS). Therefore, the safety of the RPV with regard to neutron embrittlement has to be analyzed. In this paper, the procedure and method for the structural integrity analysis of RPV subjected to PTS is presented. The FAVOR code is applied to calculate the probabilities for crack initiation and failure by considering crack distributions based on cracks observed in the Shoreham and PVRUF RPVs in the U.S. A local approach to fracture, i.e. the σ*-A* model is used to predict the warm prestressing (WPS) effect on the RPV integrity. The results show that the remaining stress contributes to the WPS effect, whereas the increase of fracture toughness is not completely attributed to the remaining stress. The modeled load paths predict a material toughness increase of 30-100%.

Interaction of welding residual stresses and warm pre-stressing on brittle fracture of a pipe containing an internal semi-elliptical crack

Residual stresses (RS) due to welding process, may change the load bearing capacity of cracked components. These stresses can also affect the benefit of warm pre-stressing (WPS) cycles which are used for improving structure behaviour. RS are obtained from a two-passes welding simulation of a pipe and verified by experiments. A semi-elliptical internal crack at the weld line is considered. Redistribution of RS field after introducing the crack shows a significant tensile RS are remained at the crack tip. Two common WPS cycles, load-cool-fracture (LCF) and load–unload-cool-fracture (LUCF), are applied using the model at room and low temperature subjected to axial loading. Using local approach to fracture shows that welding RS dramatically raise the fracture probability. LCF has more influence on reducing the fracture probability in comparison with LUCF. The interaction of welding RS and WPS cycles still improves the fracture properties, however, welding RS cause to decrease the benefit of WPS. Comparing RS distributions on crack-tip shows that applying WPS cause to release a significant amount of welding RS and therefore, WPS can be very useful for welded structures. The near crack-tip opening stresses at a same fracture load are further studied for all cases.

Investigation of constraint and warm prestressing effects by means of a local approach to fracture

Brittle cleavage fracture, constraint effects as well as warm prestressing (WPS) effects play important roles in structural integrity analysis. A local approach to fracture, i.e. the σ∗−A∗ model is used to study the constraint and WPS effects on material toughness. The crack depth and loading determine the in-plane constraint while the thickness at the crack tip has an important impact on the out-of-plane constraint. Residual stress contributes to the WPS effect, whereas the increase of fracture toughness is not completely attributed to the residual stress. The magnitudes of preloading and unloading and the reloading temperature determine the increased fracture toughness.

Evaluating the Conditions When Warm Pre-stressing does not Produce a Benefit in Apparent Toughness

Warm pre-stressing (WPS) is the process of subjecting a pre-cracked component to a load cycle at a temperature higher than subsequent operating temperatures. This process is widely acknowledged as being able to enhance the load to fracture, especially in ferritic steels which exhibit lower shelf cleavage fracture. Although accurate estimates of the toughness distributions can be obtained, the accuracy of WPS predictions may be limited by the sample size. It can be argued that the experiments conducted to date are devised to show that the WPS enhancement is always successful, however there are circumstances when a specimen might fail prematurely during the WPS path. Focus is drawn on predicting the number of prematurely failed specimens at different temperatures and pre-load levels.

Monte Carlo Simulations of the effects of warm pre-stress on the scatter in fracture toughness

Warm pre-stressing (WPS) is the process of subjecting a pre-cracked component to a load cycle at a temperature higher than subsequent operating temperatures. This process is widely acknowledged as being able to enhance the load to fracture, especially in ferritic steels which exhibit lower shelf cleavage fracture. A WPS model developed by Chell and co-workers is reformulated to create a method of undertaking Monte Carlo Simulations to study the effects of WPS on brittle fracture. This requires the development of solution domains that are governed by the relative sizes of plastic zones ahead of the crack tip during the load and temperature cycles. Using recent experimental data it is shown that accurate estimates of the toughness distributions, after WPS, can be obtained. The scatter observed in this data is also replicated using these simulations. However, the accuracy of WPS predictions may be limited by the sample size. This analysis shows that a sample size smaller than 7 reduces the reliability of the results significantly. The enhancement caused by WPS can also be influenced by the presence of an existing residual stress field. It is shown that tensile residual stresses enhance the WPS effect (caused by an increase in both the pre-load magnitude and the unloaded state load level), provided that they do not cause premature fracture during the pre-load phase.

Deterministic and probabilistic analysis of a reactor pressure vessel subjected to pressurized thermal shocks

Both deterministic and probabilistic methods are used to analyze a reference reactor pressure vessel (RPV) subjected to pressurized thermal shocks (PTSs). The FAVOR code was applied to calculate the probabilities for crack initiation and failure of a RPV subjected to two PTS transients, by considering different crack types, sizes and orientations. The Master Curve methods are implemented in the FAVOR code for a more realistic consideration of fracture toughness of the irradiated RPV.The analysis shows that a postulated underclad crack is the most conservative crack assumption. Assuming shallower cracks can be more conservative than deeper ones due to the fact that both KI and KIC at the crack tip increase with crack depth. Considering the warm prestressing effect (WPS) reduces the failure probability by more than two orders of magnitude.In this analysis, the FAVOR model for the calculation of fracture toughness is more conservative than the Master Curve method. But the Master Curve method is more realistic than the FAVOR model and thus its application is recommended.

Integrity analysis of a reactor pressure vessel subjected to pressurized thermal shocks by considering constraint effect

The integrity analysis of a reactor pressure vessel subjected to pressurized thermal shocks is performed. Linear elastic analysis leads to a more conservative result than the elastic–plastic analysis if the warm prestressing effect is not considered. The stress intensity factor for the deepest point of a surface crack front is not always larger than that for a surface point, indicating that both the deepest and surface points of the crack front should be considered. The safety margin of the reactor pressure vessel is larger based on the K–T approach than that only based on a K approach.

Application of the Beremin Model for Cruciform Specimen to Determine the Fracture Probability in Case of WPS

Based on large-scale experimental tests performed on cruciform specimens made of French type RPV steel, the warm prestressing (WPS) effect for LCF (load–cool–fracture) and LCTF (load–cool–transient–fracture) load cycles is studied by finite element simulation. The modified Beremin model is used to determine the fracture probability of the specimens.

Toughness and failure of heat resistant steel before and after hydrogenation

Fracture toughness of heat-resistant steel can be increased by a preliminary thermomechanical loading called warm pre-stressing (WPS). The procedure creates a plastic deformed area around the crack tip and hence allows larger service loads to be tolerated by the cracked specimen. It is shown that a hydrogenation in the preloading stage decreases the fracture toughness of material. Investigations are also presented of the applicability of physical and mechanical approaches for the prediction of cleavage stress of materials after preliminary plastic deformation (PPD) effects and hydrogenation. Different schemes of the plastic deformation and influence of hydrogenation are considered in the preloading stage to provide different levels cleavage stress of steel 15Kh2MFA.

A calculation-experimental study of crack-tip opening displacement and residual stresses upon warm prestressing

For a reactor pressure vessel steel 15Kh2MFA(III) experiments and calculations have been carried out to study the factors that have an influence on the increase of the lower-shelf fracture toughness in the temperature dependence diagrams upon warm prestressing. Stereoscopic fractography and numerical investigation have demonstrated that after the warm prestressing the crack tip remains blunt. This reduces the stress singularity during subsequent loading and raises the material fracture toughness. The paper gives the calculated data on residual stresses and crack-tip opening displacement during warm prestressing and upon relief. The calculated results are shown to agree well with the known analytical relations and experimental data.

Fem prediction of the influence of warm prestressing on fracture toughness of heat-resistant steel

We present a procedure of prediction of the influence of warm prestressing combined with cycling on the brittle strength of steel 15Kh2MFA. Using a finite-element method, the effect of the combined warm prestressing on the stress-strain state at a fatigue crack tip is studied in an elastic-plastic statement. Electron microscopic observations of fracture surfaces have revealed that fracture is initiated at some distance from the fatigue crack front. Based on the pattern of influence of the plastic prestrain level on the cleavage stress of steel 15Kh2MFA and the experimental CID value, a method is put forward for finite-element modeling of the stress-strain state at a crack tip during the specimen fracture. Using the results of the finite-element modeling, the relevant curves have been plotted and an approximating formula has been proposed to represent the influence of the combined warm prestress level on the fracture toughness of steel 15Kh2MFA.

ICONE19-43233 Evaluation of Pressurized Thermal Shock in Korean PWR by using Probabilistic Fracture Mechanics

Plant-specific analyses of the typical pressurized water reactor in Korea are performed to assure the structural integrity of the reactor pressure vessel during transient which is expected to initiate pressurized thermal shock event. The deterministic analysis is performed to determine the critical time interval in the transient during which mitigating action can be effective. Also, the failure probability is obtained by performing probabilistic fracture mechanics analysis. The probabilistic reactor integrity evaluation code, named R-PIE code, is developed in this study. In addition, several sensitivity analyses are performed for warm pre-stressing, plastic zone correction and simulation option to calculate fracture toughness etc. to assess its effect on the failure probability. The critical crack depth and vessel failure probabilities from the deterministic and probabilistic fracture mechanics analyses are obtained, generating the following conclusions. (1) By including WPS effect, critical crack depth is not changed but critical time interval increased about 100 seconds, giving more time for the operator take some mitigating actions. (2) The warm prestressing has a significant effect on the failure probability for SBLOCA by lowering it by more than 50%. (3) Consideration of internal pressure and plastic zone corrections increases about 10% of stress intensity factor and generates more than 200% of increase in the failure probability depending on the fluence level. (4) The simulation option to calculate RTNDT is a very important factor to get the failure probability with the difference of more than 3 orders depending on the simulation option.