Tutorial Irradiation Embrittlement and Life Management of RPVs RPV Mechanical properties evaluation Marta Serrano CIEMAT (Spain) Znojmo (CZ) 18 October 2010

Connecticut Yankee RPV Decommissioned RPV – The component RPV Dimensions Height =10-20 m Wall thickness = 15-30 cm Inner diameter= 4 - 5 m Operational conditions The normal operating pressure is 12.5 - 15.5 MPa for PWRs and 7-8 MPa for BWRs. The maximum coolant temperature is about 330°C. Radiation environment Design objectives; Lower stress Strong material High toughness Low brittle transition Assurance of zero defects Long life ~60 years Connecticut Yankee RPV Decommissioned

RPV – Neutron Irradiation Microstructure Vacancies & self-interstitial atoms Impurity segregation Mechanical properties Hardening Embrittlement  DBTT Shift Operation temperature Ductile Cu Cluster Matrix damage P Atoms Brittle

RPV - Required mechanical properties The RPV integrity is assessed using fracture toughness curves following a fracture mechanics approach A defect is allowed if do not reach a critical size The critical size is reached when the Stress Intensity Factor is equal to the Fracture Toughness Fracture toughness is affected by neutron irradiation K IC ,P T K IC Material K IC Material K IC Material K I Solicitation T

RPV Required mechanical properties How to determine fracture toughness values of RPV material ? For non irradiated material: Fracture toughness reference curves, indexed to reference temperature KJC = A + B exp (C(T-RT)) , A,B and C are constants RT is the reference temperature RTNDT – Defined by Charpy impact and Drop weight tests Tk – defined by Charpy impact test Fracture toughness tests For irradiated material Facture toughness reference curves Indexed to reference temperature obtained via Charpy impact test of specimens included on surveillance capsules Normative and Code approach Fracture toughness test Prometey Curve (B. Margolin) Master Curve (K. Walin)

RPV Mechanical tests In general the following tests have to be performed on RPV materials tensile test Charpy-V-notch test drop-weight test fracture mechanics test In order to obtain representative and conservative data for the component, requirements exist for specimen sampling and specimen orientation. Representativity of Materials: same fabrication and thermal treatment Location of Specimens: mechanization depth 1/4T (T = thickness of the forging or plate) Specimen Orientation In general, Charpy (and Fracture toughness) specimens are taken from the base material in the transverse direction (T-L) Tensile specimens are used in transverse direction (T) from the base material and in longitudinal direction (L) from the weld (longitudinal = welding direction). http://safelife.jrc.ec.europa.eu/ames/publications/docs/old-pubs/EUR16316EN.pdf

Tensile test The yield strength YS or yield point of a material is defined in engineering and materials science as the stress at which a material begins to deform plastically. A plastic strain of 0.2% is usually used to define the offset yield stress, although other values may be used depending on the material and the application. Ultimate tensile strength (UTS), is the maximum stress that a material can withstand before necking, which is when the specimen's cross-section starts to significantly contract Reduction of area is another measure of ductility and is obtained from the tensile test by measuring the original cross-sectional area of the specimen and relating it to the cross-sectional area after failure. Elongation. The increase in gauge length related to the original length times 100 is the percentage of elongation. ORNL/TM-2003/63 7

Tensile – Effect of irradiation The yield and ultimate strengths increase while both the uniform and total elongations decrease. Also, the work hardening decreases with decreases in the ultimate yield strength ratio. The increase in yield strength is likewise reflected in increasing hardness as the material experiences a reduction in the capacity for plastic flow. ORNL/TM-2003/63 EPRI PWRMRP-26 2000

Zwick drop weight tester The result from the drop-weight test is one of the criteria to determine the Reference Temperature RTNDT of the unirradiated initial material state. In all countries, except Russia, drop-weight tests are performed The drop-weight test was developed at the Naval Research Laboratory in 1952 and has been used extensively to investigate the conditions required for initiation of brittle fractures in structural steels. ASTM E 208 Standard Test Method for Conducting Drop-Weight Test to Determine Nil-Ductility Transition Temperature of Ferritic Steels The drop-weight test employs simple beam specimens specially prepared to create a material crack in their tensile surfaces at an early time interval of the test. The test is conducted by subjecting each of a series (generally four to eight) of specimens of a given material to a single impact load at a sequence of selected temperatures to determine the maximum temperature at which a specimen breaks Zwick drop weight tester

Drop weight test The usual test sequence is as follows: After the preparation and temperature conditioning of the specimen, the initial drop-weight test is conducted at a test temperature estimated to be near the NDT temperature. nil-ductility transition (NDT) temperature—the maximum temperature where a standard drop-weight specimen breaks when tested according to the provisions of this method Depending upon the results of the first test, tests of the other specimens are conducted at suitable temperature intervals to establish the limits within 10°F (5°C) for break and no-break performance. A duplicate test at the lowest no-break temperature of the series is conducted to confirm no-break performance at this temperature. Steele NED 1966 Typical drop - weight - test series, illustrating NDT at 10ºF

TNDT values for IAEA JRQ material Drop weight test TNDT values for IAEA JRQ material www.iaea.org

Charpy test The Charpy test consist on break by a pendulum a notched specimens and to measure the energy absorbed In the nuclear field the properties to be measured area the upper shelf energy, USE and the temperature at with the energy absorbed is 41 J, T41J

Charpy specimens tested at different temperatures Charpy test Charpy test parameters Absorbed energy Lateral expansion Percent of shear fracture Charpy specimens tested at different temperatures

Charpy test – Effect of irradiation Lower USE Higher DBTT ORNL

Fracture toughness test A fracture toughness test measures the resistance of a material to crack extension Single value of fracture toughness - Cleavage Resistance curve, where a toughness parameters are K, J of CTOD are plotted against crack extension - Ductile All fracture toughness test have several common features Specimen geometries –Size to assure plain strain Pre-crack by fatigue Basic instrumentation – Load, displacement Anderson 1995

Fracture toughness test Fracture toughness parameters K (stress intensity factor) can be considered as a stress-based estimate of fracture toughness. Depends on geometry (the flaw depth, together with a geometric function, which is given in test standards for each test specimen geometry). CTOD or (crack-tip opening displacement) can be considered as a strain-based estimate of fracture toughness. However, it can be separated into elastic and plastic components. The elastic part of CTOD is derived from the stress intensity factor, K. The plastic component is derived from the crack mouth opening displacement (measured using a clip gauge). J (the J-integral) is an energy-based estimate of fracture toughness. It can be separated into elastic and plastic components. As with CTOD, the elastic component is based on K, while the plastic component is derived from the plastic area under the force-displacement curve. www.twi.org

Fracture toughness tests KIC test (ASTM) Plain strain is necessary condition for a valid KIC, but also the specimen must also behave in linear elastic manner The pre-cracked specimen is loaded to failure at a constant displacement rate (mm/min). The resulting load –displacement curve can be type I, II, or III Definition of critical load PQ PQ=P5 for Type I PQ is defined at pop-in for Type II PQ=Pmax for Type III KIC=KQ only if

Fracture toughness tests JIC test (ASTM) The R curve (J.vs.crack growth) for JIC measurement can be generated by Multiple specimens: a serie of nominally identical specimens are loaded to various levels and then unloaded. Each specimen is broken after the tests and the crack extension is measures Single specimen technique. The crack growth is monitoring during the test by elastic compliance method or the direct current potential drop technique Regardless of the method for monitoring the crack growth, the J value is computed for each point of the R curve JIC=JQ only if

Fracture toughness reference curves – ASME approach Since the adoption of the KIC and KIR/KIa reference curves by ASME Section XI in the 1970s, there have been significant improvements in the theory and practice of fracture mechanics. The original curves were developed using data from linear-elastic fracture mechanics tests. Advances in fracture mechanics technology during the past 25 years have made it possible to improve on this approach. The development of elastic-plastic fracture mechanics has made it possible to determine fracture toughness values using much smaller specimens and utilizing J-integral techniques, that is, measuring values of KJC. EPRI PWRMRP-26 EPRI MRP 101

Fracture toughness reference curves – ASME approach Definition of a new reference temperature – Master Curve ASME Code Case N-631 (Section III) defines RTT0 for unirradiated reactor vessel material, while ASME Code Case N-629 (Section XI) defines RTT0 for unirradiated and irradiated reactor vessel material. This new reference temperature is defined as: RTT0 = T0 + 35ºF The ASME KIC reference curve indexed to RTT0 has the form KIC = 36.5 + 3.083exp[0.036 (T–RTT0 + 55.6)] T0 is the Master Curve reference temperature Can be determined by testing the Charpy specimens included in the surveillance capsule Pre-cracking ASTM E1921

Fracture toughness Master Curve Master Curve (Wallin 1984) Treatment the scatter of fracture toughness data in the transition region for ferritic steels Weibull distribution Thickness correction Universal shape Reference temperature T0 Ferritic steels Kmin = 20 MPam; b= 4; B0=25 mm The use of the Master Curve allows to obtain valid fracture toughness data by testing the pre-cracked Charpy specimens included in the surveillance capsules

Fracture toughness Master Curve NUREG 1807

Fracture toughness Master Curve Master Curve is validated for VVER 440 and VVER 1000 RPV materials VERLIFE For VVER RPV type steels and their welds, the following relation is suggested: KJC5%=min25.2+36.6 exp(0.019(T-RTT0)); 200 where RTT0 is defined as T0+T0.

Fracture toughness – effect of irradiation 450 ORNL 72W Irradiated at 2x10 19 n/cm 2 400 Non Irradiated 350 300 250 Fracture toughness MPam 200 150 100 50 - 200 - 150 - 100 - 50 50 100 150 200 Temperature (ºC)

Fracture toughness Master Curve T0 determination- ASTM E1921 Fracture toughness tests of pre-cracked specimens in the transition range. Load (kN) Displacement (mm)

Fracture toughness Master Curve One temperature testing Multi-temperature testing  = 1 valid y =0 invalid

Fracture toughness Master Curve Determination of T0 for JRQ material with pre-cracked charpy specimens (CIEMAT data) KJCmed(1T)= 30 + 70 exp(0,019(T-T0))

Fracture toughness – Instrumented Charpy test The ability to instrument the Charpy striker, or tup, has led to the ability to better measure the material response in terms of general yielding and potential fast fracture response. The actual loading of the Charpy specimen can be observed as a function of time and, more recently, as a function of direct specimen deflection for the three-point bend test. The evaluation of instrumented behavior at different test temperatures can provide a better understanding of the ductile-to brittle transition response than merely meeting either Charpy energy or other related ductility criteria [Server 2007] Load Fgy Fmax Fc Fa Crack arrest fracture toughness can be obtained by testing the Charpy v-notch specimens Fa Dynamic fracture toughness can be obtained by testing pre-cracked charpy specimens Fc Time

Fracture toughness – Instrumented Charpy test The positive temperature gradient and attenuation of neutron radiation across the pressure vessel wall provides a mechanism by which initiated cracks may be arrested before they propagate through the wall. Crack arrest Master Curve KIa = 30+70exp(0.019(T-TKIa)) KIa follows a log-normal distribution TKIa = TFa4kN + 11ºC Planman 1997

Fracture toughness – Instrumented Charpy test In general RPV do not experience crack-tip loading rates as high as 105 MPam /s. However, during the short time interval immediately following a crack arrest event, the crack tip loading rates can be between 104 - 105 MPam /s and temperatures of NDT + 42ºC It is possible under these conditions for the arrested crack to reinitiate Load Fgy Fmax Fc Fa IAEA TE 1631 Dynamic fracture toughness determination by instrumented impact test – Pre-cracked specimens [IAEA CRP8 RR]

Conclusions Mechanical properties evaluation is needed to assure the integrity of the reactor pressure vessel. Mechanical test more usual on irradiated material are: Tensile Charpy Fracture toughness New techniques can be used to have a better knowledge of the status of the RPV Master Curve, Prometey Curve Instrumented impact test

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BACKGROUND SLIDES

Fracture toughness reference curves – Russian approach Recently fracture toughness temperature dependence KJC(T) for base and weld metals of RPV is determined according to the procedures given in Russian Standard MKc-KR-2000. This standard is divided in two: Part I for the case when the lateral shift condition is valid - Basic Curve approach KIC = 23+48exp[0.019(T-Tk)] Part II for the case when the lateral shift condition is invalid, i.e. the shape of KJC(T) curve varies due to the irradiation effect - Prometey local approach to brittle fracture  depends on the degree of embrittlement

Fracture toughness – ASME approach The use of correlations between drop weight nil-ductility transition temperature (NDT temperature) and Charpy V-notch properties to establish fracture resistance criteria for vessels dates from the 1963 version of the Code. The ASME Code developed Code Case 1514 in early 1972, revisions to Section III, NB-2300 were made, and Appendix G was added ASME III Appendix G relates to design, A maximun postulated flaw size is assumed and the mode I stress intensity factor, KI is compared to KIR KIR material toughness curve – Lower bound of static KIC, arrest KIa and dynamic KId KIR = 26.78+1.233 exp (0.0145 (T-RTNDT+160)) [ksiin, ºF] ASME XI Appendices A and G: similar fracture mechanics based procedures Appendix A Flaws discovered during In-Service Inspections Appendix G – fracture toughness criteria for protection against failure KIR reference curve The fracture toughness basis for ASME Code Sections III and XI is still that of WRC Bulletin 175, issued in 1972. Code Case N-640, which was approved in 1999, changed the fracture toughness curve used for development of P-T limit curves from KIR to KIC. KIC = 44.2+20.734exp (0.02 (T-RTNDT)) [ksiin, ºF] ksiin-2 ºF EPRI PWRMRP-26 2000

Fracture toughness reference curves Fracture toughness reference curves are indexed to a reference temperature RTNDT (ASME and others) TK (Russian approach) RTNDT is defined in accordance with ASME NB2331, as MAX (TNDT, T35/50-60) TNDT is the nil ductility temperature determined by drop weight test specimens in accordance with ASTM E208. T35/50 is the charpy transition temperature at which lateral expansion is at least 0.035-in. (0.89-mm) and absorbed energy is at least 50 ft-lbs (68J). Tk is determined by a combination of absorbed energy required (impact test) and 50% ductile fracture appearance determined from the broken specimen fracture surface. http://safelife.jrc.ec.europa.eu/ames/publications/docs/old-pubs/EUR16279EN.pdf

Dynamic & Crack arrest fracture toughness The fracture toughness of ferritic steels depends on temperature and strain rate. This behaviour is taked into account in design codes such as the ASME code. The KIR ASME curve was developed from measurements of the dynamic and crack arrest fracture toughness Most of the dynamic data was developed at a loading rate of 105 MPam /s In general RPV do not experience crack-tip loading rates as high as 105 MPam /s. However, during the short time interval immediately following a crack arrest event, the crack tip loading rates can be between 104 - 105 MPam /s and temperatures of NDT + 42ºC It is possible under these conditions for the arrested crack to reinitiate NUREG 6512

Crack arrest fracture toughness The positive temperature gradient and attenuation of neutron radiation across the pressure vessel wall provides a mechanism by which initiated cracks may be arrested before they propagate through the wall. Crack arrest fracture toughness, KIa is defined as the maximum stress intensity at which the propagation of a running crack is arrested However, the transferability to components is not well established, and the applicability of KIa as a material parameter is still questioned In the ASME code crack arrest reference fracture toughness curve is included and used to determine the pressure-temperature limit curve of the vessel. This type of reference curve is not included in the Soviet Code [Jhung 2009]

Crack arrest fracture toughness test ASTM E1221 - 06 Standard Test Method for Determining Plane-Strain Crack-Arrest Fracture Toughness, KIa, of Ferritic Steels The procedure involves testing of modified compact specimens that have been notched by machining Opening displacement measurements in conjunction with crack size and compliance calibrations are used for calculating Ko and Ka. Starting notch to produce crack initiation at an opening displacement (or wedging force) that will permit an appropriate length of crack extension prior to crack arrest The recommended starter notch for low- and intermediate-strength steels is a notched brittle weld, produced by depositing a weld across the specimen thickness

Crack arrest fracture toughness test ASTM E1221 - 06 Standard Test Method for Determining Plane-Strain Crack-Arrest Fracture Toughness, KIa, of Ferritic Steels The test method calls for the use of a cyclic loading technique. In this technique, load is applied to the wedge until a rapid crack initiates If a rapid fracture has not initiated prior to the recommended maximum displacement being reached, the specimen is unloaded until the wedge loses contact with the split-pin. The specimen is then reloaded in the same manner as before and load application is once again terminated either by initiation of a rapid crack or upon the opening displacement reaching a specified value. Successively higher values of the recommended maximum opening displacement are allowed on each loading cycle, until a rapid crack initiates or until the test is discontinued. The load applied to the specimen is obtained from measurements of the crack-mouth opening displacement AREVA CARISMA ASTM E1221

MC Application Comparison 36ºC ASME curve indexed to RTT0 is less conservative than indexed to RTNDT. These 36ºC represents up to 10 year more operating at full power. -100 -50 50 100 150 200 250 300 KJC (MPam) Temperature (ºC) ARTNDT ARTT0 36ºC