Dominique Moinereau

SMILE Project — The Effect of Warm Pre-Stress in RPV Assessment: Synthesis of Experimental Results and Analyses

Within framework of 5th EC European SMILE project, some R&D actions have been conducted to demonstrate, to model and to validate the beneficial warm pre-stress (WPS) effect regarding the risk of brittle failure in a RPV assessment. An important experimental work has been conducted including classical WPS type experiments on CT specimens on usual RPV steels, and one PTS type transient on a large cracked cylinder. All experimental results confirm the beneficial effect of warm pre-stress, with a significant increase of the materials resistance regarding the risk of brittle failure. The experiments have been analyzed using fracture mechanics, including both engineering methods (Chell, Haigh, Wallin) and more refined analyses based on local approach to fracture (‘modified Beremin’ model). Following a short description of WPS concept, the paper summarizes the main experimental results and presents a synthesis of mechanical analyses involving engineering approaches and numerical analyses.Copyright

Fracture Toughness of a Highly Irradiated Pressure Vessel Steel in Warm Pre-Stress Loading Conditions (WPS)

The warm pre-stress (WPS) of a flawed structure occurs when it is pre-loaded at high temperature in the ductile domain then cooled and loaded up to fracture in the brittle to ductile transition temperature domain. This load history is a feature of RPV accidental transients of LOCA type. Numerous tests on non irradiated specimens and structures have shown the favourable effect of WPS on fracture behaviour. Theorical knowledge let expect that the WPS effect occurs by the same way on irradiated material, but experimental approach had to be completed in such conditions. The experimental program presented in the present article consists in fracture toughness tests under WPS loading conditions performed on two RPV steels irradiated up to a fluence of 6,5.1019 n/cm2 . The CT12.5 specimens used for these tests had been irradiated in the capsules of the pressure vessel surveillance program of two french reactors. Different types of WPS load history have been applied to cover typical accidental transients. All the results obtained confirmed for an irradiated steel the two assumptions generally made about the WPS effect: no fracture occurred during the cooling step of the loading even at high load level and the mean fracture toughness value is higher than that measured with conventional mono-temperature tests.© 2011 ASME

The analysis of underclad cracks in large-scale tests using the local approach to cleavage fracture

Electricite de France has conducted a large program including experiments on large-size cladded specimens and their interpretations to evaluate different methods of fracture analysis used in French safety studies regarding the risk of fast fracture in reactor pressure vessels. Four specimens made of ferritic steel A508 C13 with stainless steel cladding, containing small artificial underclad defects, have been tested in four-point bending. Experiments have been conducted at very low temperature, and crack instability by cleavage fracture without crack arrest was obtained in base metal. The tests have been interpreted using the local approach to cleavage fracture (Weibull model) by two-dimensional finite element computations. The Weibull model parameters have been determined using axisymmetrical notched specimens. The probability of failure has been evaluated in each test using finite element analyses with varying mesh sizes. The results show an important effect of the size of the elements at the crack tip on the calculated probability of failure. Those effects are confirmed using the model of a CT specimen. Also discussed is the shallow flaw effect with the Weibull model.

STYLE Project: Assessment of Transferability of Fracture Material Properties From Specimens to Large Components by Local Approach to Fracture

Within the framework of the FP7 European project STYLE, a large scale experiment was performed at EDF R&D on a cladded ferritic pipe (Mock-Up 3). The objective of this experiment was to investigate the transferability of material properties from small specimens to large scale components.The large scale experiment involved applying 4-point bending under displacement control at room temperature to a clad ferritic steel pipe with an inner surface crack. The goal of the experiment was to initiate ductile crack growth and track the resulting stable crack growth until the surface flaw breaks through the wall.The pipe was representative of a surge line, consisting of a clad ferritic pipe with an outer diameter of 424 mm, and base metal wall thickness of 31 mm, with an austenitic stainless steel cladding layer 5 mm thick on the inner surface. The base metal is a low alloy 20 MnMoNi 5 5 steel (corresponding to the specifications of an SA 508 Grade 3, Class 1 steel).The pipe test was conducted in 2012 in the EDF R&D 4-point bending frame. Following the experiment, various specimens were taken from the mock-up to identify the material behavior and provide data to investigate the transferability of the material fracture properties.This paper recalls briefly the large scale experiment results and presents the main experimental results from the specimens. Then the results of the local approach finite element computations with the Rousselier model are presented and compared with the experimental results.Copyright

Effect of Warm Pre-Stress on Highly Irradiated Pressure Vessel Steel

The effect of warm pre-stress (WPS) is well known on cleavage fracture resistance of ferritic steels and has been confirmed on numerous tests on non irradiated materials. Theoretical knowledge let expect that this effect occurs by the same way on irradiated material but has to be demonstrated with experiments in such conditions. An experimental program has been conducted by EDF in Chinon hot cell laboratory on two RPV steels irradiated up to a fluence of 6,5*1019 n/cm2.The ½ TCT specimens (CT12.5) used in this programme have been irradiated in the capsules of the EDF RPV surveillance program of two PWR reactors. Different types of WPS loading path have been applied to cover typical postulated accidental transients. The results confirmed the effect on WPS on the cleavage fracture resistance of the irradiated materials. No fracture occurred during the cooling phase of the loading path and the fracture toughness values are higher than that measured with conventional isothermal tests.The analyses of the experiments have been conducted based on use of simplified engineering models and more refined fracture models based on local approach to cleavage fracture. These analyses are in agreement with the experimental results.Copyright

Reactor Pressure Vessel: EDF R&D Program to Support Lifetime Management

The Reactor Pressure Vessel (RPV) is with the concrete containment one of the two components of a NPP whose replacement is not considered as reasonably feasible. The RPV lifetime has thus an important impact on the lifetime of the whole NPP. One of the key issues concerning RPV lifetime is the radiation effect on the RPV steel in the core zones. The vessel steel becomes indeed more brittle in the RPV core region where radiation is high. Margins have been included at design and manufacturing stages taking into account the material’s embrittlement. Moreover, operating measures have been taken to manage ageing of RPV in order to extend lifetime. The challenge is to preserve high margins and to provide the safety studies showing these margins. A large R&D program has been developed to support lifetime extension. The objective of the program is to develop tools and provide input data for the demonstration of the safe operation of the reactor pressure vessel significantly over a 40-year lifetime. The aim of the paper is to present an overview of the R&D program to support lifetime management on the fields of materials, mechanics and thermalhydraulics. Experiments are indeed performed on irradiated material in order to improve the knowledge on embrittlement for high fluences and to be able to determine embrittlement correlations for materials representative of French RPV. Actions are also planned to improve evaluation of the RPV mechanical behaviour and to describe physical phenomena such as crack arrest or warm pre-stressing effect. Last, studies are realized to improve the thermal loadings evaluations under hypothetical accidental scenarios. These studies are supported by thermalhydraulic numerical simulations whose validation is obtained by comparison to experimental results from experimental hydraulic loops representative of French RPV.Copyright

Cleavage Fracture Micromechanisms Related to WPS Effect in RPV Steel

Since the first investigations four decades ago, a large number of experiments on ferritic steels has confirmed the existence of a warm pre-stress (WPS) effect, which describes the effective enhancement of the cleavage fracture toughness at low temperature following the application, at a higher temperature, of a stress intensity factor (SIF) which exceeds the fracture toughness of the virgin material at low temperature. These experiments allowed for the establishment of the so-called ‘conservative principle’, which states that no fracture will occur if the applied SIF decreases (or is held constant) while the temperature at the crack-tip decreases, even if the fracture toughness of the virgin material is exceeded. In structural integrity assessments involving a prior overload or a thermal transient, such as that of a nuclear pressure vessel subjected to a pressurized thermal shock (PTS) consecutive to a loss of coolant accident (LOCA), such a principle is of great importance in the evaluation of the safety margins. Three main reasons have been advanced to explain the WPS effect: the blunting of the crack tip at high temperature, the formation of high compressive stresses on elastic unloading, and a change in the cleavage fracture micromechanisms induced by plastic deformation. While all these factors certainly contribute to the effective toughness enhancement following WPS, their relative incidence on the fracture risk is not easily established. In this paper, we choose to mainly focus on the cleavage fracture micromechanisms following WPS, as a first step towards better quantifying the individual contributions of crack tip blunting and residual stresses.

SMILE: Validation of the Warm-Pre-Stress Effect With a Cylindrical Thick-Walled Specimen

The Reactor Pressure Vessel (RPV) is an essential component, which is liable to limit the lifetime duration of PWR plants. The assessment of defects in RPV subjected to pressurized thermal shock (PTS) transients made at an European level generally does not necessarily consider the beneficial effect of the load history (Warm Pre-stress, WPS). The SMILE project — Structural Margin Improvements in aged embrittled RPV with Load history Effects — aims to give sufficient elements to demonstrate, to model and to validate the beneficial WPS effect. It also aims to harmonize the different approaches in the national codes and standards regarding the inclusion of the WPS effect in a RPV structural integrity assessment. The project includes significant experimental work on WPS type experiments with C(T) specimens and a PTS type transient experiment on a large component. This paper deals with the results of the PTS type transient experiment on a component-like specimen subjected to WPS- loading, the so called Validation Test, carried out within the framework of work package WP4. The test specimen consists of a cylindrical thick walled specimen with a thickness of 40mm and an outer diameter of 160mm, provided with an internal fully circumferential crack with a depth of about 15mm. The specified load path type is Load-Cool-Unload-Fracture (LCUF). No crack initiation occurred during cooling (thermal shock loading) although the loading path crossed the fracture toughness curve in the transition region. The benefit of the WPS-effect by final reloading up to fracture in the lower shelf region, was shown clearly. The corresponding fracture load during reloading in the lower shelf region was significantly higher than the crack initiation values of the original material in the lower shelf region. The post test fractographic evaluation showed that the fracture mode was predominantly cleavage fracture also with some secondary cracks emanating from major crack.Copyright

SMILE: Interpretation of WP4 PTS Transient Type Experiment Performed on a Cracked Cylinder Involving Warm Pre-Stress

The Reactor Pressure Vessel (RPV) is an essential component liable to limit the lifetime duration of PWR plants. The assessment of defects in RPV subjected to PTS transients made at an European level generally not always account the beneficial effect of load history (warm pre-stress WPS) on vessel resistance regarding the risk of brittle failure. A 3-year Research & Development program — SMILE — has been started in January 2002 as part of the 5th Framework Program of the European Atomic Energy Community (EURATOM). The SMILE project — Structural Margin Improvements in aged embrittled RPV with Load history Effect — is one of a cluster of 5th framework projects in the area of Plant Life Management. It aims to give sufficient elements to demonstrate, to model and to validate the beneficial WPS effect in a RPV assessment. Within the framework of the project, an important experimental work has been conducted including WPS type experiments on CT specimens and one PTS type transient experiment on a large component. The WPS type experiment on the cylinder has been successfully conducted by MPA Stuttgart with a final brittle failure during the reloading. The present paper shortly describes the experiment and presents the corresponding analyses based on engineering methods, finite element elastic-plastic computations, and local approach to fracture. The results are in good agreement with the experimental observations. Very significant margins are underlined, with an effective important increase of the material resistance regarding the risk of brittle failure.Copyright

SMILE: A European R&D Program for the Inclusion of Warm Pre-Stress in RPV Assessment

The Reactor Pressure Vessel (RPV) is an essential component liable to limit the lifetime duration of PWR plants. The assessment of defects in RPV subjected to PTS transients made at an European level generally do not take necessary into account the beneficial effect of load history (warm pre-stress WPS). A 3-year Research & Development program — SMILE — has been started in January 2002 as part of the Fifth Framework of the European Atomic Energy Community (EURATOM). The SMILE project (“Structural Margin Improvements in aged-embrittled RPV with Load history Effects”) is one of a “cluster” of Fifth Framework projects in the area of Plant Life Management. It aims to give sufficient elements to demonstrate, to model and to validate the beneficial WPS effect in a RPV integrity assessment. Finally, this project aims to harmonize the different approaches in the European Codes and Standards regarding the inclusion of the WPS effect in a RPV structural integrity assessment. Within the framework of the project, an important experimental work has been conducted including WPS type experiments on CT specimens and also a PTS type transient experiment on a large component. The experimental results on CT specimens confirm the beneficial effect of warm pre-stress, with an effective significant increase of the material resistance regarding the risk of brittle failure. The WPS type experiment on the cylinder has been successfully conducted, with a final brittle failure during the reloading. The present paper describes the aims and objectives of the SMILE project, the main experimental results, and the corresponding analyses based on engineering methods, finite element elastic and elastic-plastic computations, and local approach to fracture.Copyright