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

2. Connecticut Yankee RPV Decommissioned
RPV – The component
RPV Dimensions
Height =10-20 m
Wall thickness = cm
Inner diameter= m
Operational conditions
The normal operating pressure is 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

3. 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

4. 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

5. 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)

6. 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).

7. 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

8. 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

9. 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

10. 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

11. TNDT values for IAEA JRQ material
Drop weight test
TNDT values for IAEA JRQ material

12. 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

13. 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

14. Charpy test – Effect of irradiation
Lower USE
Higher DBTT
ORNL

15. 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

16. 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.

17. 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

18. 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

19. 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

20. 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 = exp[0.036 (T–RTT )]
T0 is the Master Curve reference temperature
Can be determined by testing the Charpy specimens included in the surveillance capsule
Pre-cracking
ASTM E1921

21. 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

22. Fracture toughness Master Curve
NUREG 1807

23. 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 exp(0.019(T-RTT0)); 200
where RTT0 is defined as T0+T0.

24. 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)

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

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

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

28. 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

29. 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

30. 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 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]

31. 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

32. Thank you and do not hesitate to joint the nuclear club

33. BACKGROUND SLIDES

34. 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

35. 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 = exp ( (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 = exp (0.02 (T-RTNDT)) [ksiin, ºF]
ksiin-2 ºF
EPRI PWRMRP

36. 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 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.

37. 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 MPam /s and temperatures of NDT + 42ºC
It is possible under these conditions for the arrested crack to reinitiate
NUREG 6512

38. 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]

39. Crack arrest fracture toughness test
ASTM E 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

40. Crack arrest fracture toughness test
ASTM E 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

41. 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