Tutorial Irradiation Embrittlement and Life Management of RPVs. Structural Integrity Issues F. Gillemot

Transcription

1 Tutorial Irradiation Embrittlement and Life Management of RPVs Structural Integrity Issues F. Gillemot

2 What is structural integrity? No answer in the WEB or in other documents! Safe operation of passive components in normal and non-normal conditions. The structural integrity means that the sructure, or component not only safe, but survives the service and environmetal effects without any serious damage.

3 Passive components Passive components: all pressurized or loaded structures Examples: Reactor Pressure Vessel Steam generator Pressurizer House of main coolant pump Pipes Crane structures etc.

4 b Safety whats that? Safety means protection of the environment and populations from radioactive contamination, or from other harness The deteministic safety assessment methodology uses a technique in with a defence in depth assessment assure success in each level of the defence. Design/safety limits are specified for each level of defence. The probabilistic safety assessment uses a methodology to calculate the risk of failure, and determines the acceptable risk level. No 100% safety, too high safety requirements are damaging the society. Very high responsibility for the engineers.

5 Structures, systems components (SSC) integrity

6 IAEA Safety Standards and Guidelines on PLiM and AM Safety Requirement Safety of NPP Design NS R-1 Safety Guide SG on AMP Safety Guide on PSR Safety Guide on MSI Safety Guide on Personal Qualification Tech. Guidelines Programmatic Guidelines Component Specific Guidelines (13) AMP Review guideline RPV and PLiM Human Ageing Guideline NSNI NENP

7 Steam generators (TECDOC-981) Concrete containment buildings (TECDOC-1025) PWR pressure vessels (TECDOC-1120) PWR vessel internals (TECDOC-1119) Metal components of BWR containment (TECDOC- 1181) In-containment I&C Cables (TECDOC-1188) Volume I In-containment I&C Cables (TECDOC-1188) Volume II PWR primary piping (TECDOC-1361) BWR Reactor Pressure Vessel (TECDOC-1470) BWR Rector Pressure Vessel Internals (TECDOC- 1471)

8 Ageing effects Design safety level Operating strategy II Operating strategy III Operating strategy I 50% failure probability Safe operating life I Safe operating life II Operating time

9 Ageing mechanism - Radiation embrittlement - Thermal embrittlement - General corrosion - Stress corrosion cracking - Pitting corrosion - Irradiation assissted corrosion - Hidrogen embrittlement - Liquid metal embrittlement - Wear - Fatigue and low-cycle fatigue - Creep - High temperature rupture - Errosion - Etc...

10 Event selection (PSA Probabilistic Safety analyses) High pressure, safety valve open Safety valve mailfunction, coolant pressure drop Safety valve closed, normal shut down Probability is below of the acceptance level Reactor stop, emergency core coolant pumps operating Rapid cooling PTS Structural integrity assessment Event tree Failure probability is below of the acceptance level No integrity

11 Failure Analysis Failure of a component indicates it has become completely or partially unusable or has deteriorated to the point that it is undependable or unsafe for normal sustained service. Typical Root Cause Failure Mechanisms 1. Fatigue failures 2. Corrosion failures 3. Stress corrosion cracking 4. Ductile and brittle fractures 5. Hydrogen embrittlement 6. Liquid metal embrittlement 7. Creep and stress rupture

12 Fatigue Failures Metal fatigue is caused by repeated cycling of of the load. It is a progressive localized damage due to fluctuating stresses and strains on the material. Metal fatigue cracks initiate and propagate in regions where the strain is most severe. The process of fatigue consists of three stages: Initial crack initiation Progressive crack growth across the part Final sudden fracture of the remaining cross section Schematic of S-N Curve, showing increase in fatigue life with decreasing stresses.

13 Stress Ratio The most commonly used stress ratio is R, the ratio of the minimum stress to the maximum stress (S min /S max ). 1. If the stresses are fully reversed, then R = If the stresses are partially reversed, R = a negative number less than If the stress is cycled between a maximum stress and no load, R = zero. 4. If the stress is cycled between two tensile stresses, R = a positive number less than 1. Variations in the stress ratios can significantly affect fatigue life. The presence of a mean stress component has a substantial effect on fatigue failure. When a tensile mean stress is added to the alternating stresses, a component will fail at lower alternating stress than it does under a fully reversed stress.

14 Preventing Fatigue Failure The most effective method of improving fatigue performance is improvements in design: 1. Eliminate or reduce stress raisers by streamlining the part 2. Avoid sharp surface tears resulting from punching, stamping, shearing, or other processes 3. Prevent the development of surface discontinuities during processing. 4. Reduce or eliminate tensile residual stresses caused by manufacturing. 5. Improve the details of fabrication and fastening procedures Fatigue Failure Analysis Metal fatigue is a significant problem because it can occur due to repeated loads below the static yield strength. This can result in an unexpected and catastrophic failure in use. Because most engineering materials contain discontinuities most metal fatigue cracks initiate from discontinuities in highly stressed regions of the component. The failure may be due the discontinuity, design, improper maintenance or other causes. A failure analysis can determine the cause of the failure.

15 High Temperature Failure Analysis Creep occurs under load at high temperature. Boilers, gas turbine engines, and ovens are some of the systems that have components that experience creep. An understanding of high temperature materials behavior is beneficial in evaluating failures in these types of systems. Failures involving creep are usually easy to identify due to the deformation that occurs. Failures may appear ductile or brittle. Cracking may be either transgranular or intergranular. While creep testing is done at constant temperature and constant load actual components may experience damage at various temperatures and loading conditions. Creep of Metals High temperature progressive deformation of a material at constant stress is called creep. High temperature is a relative term that is dependent on the materials being evaluated. A typical creep curve is shown below:

16 In a creep test a constant load is applied to a tensile specimen maintained at a constant temperature. Strain is then measured over a period of time. The slope of the curve, identified in the above figure, is the strain rate of the test during stage II or the creep rate of the material. Primary creep, Stage I, is a period of decreasing creep rate. Primary creep is a period of primarily transient creep. During this period deformation takes place and the resistance to creep increases until stage II. Secondary creep, Stage II, is a period of roughly constant creep rate. Stage II is referred to as steady state creep. Tertiary creep, Stage III, occurs when there is a reduction in cross sectional area due to necking or effective reduction in area due to internal void formation.

17 Stress Rupture Stress rupture testing is similar to creep testing except that the stresses used are higher than in a creep test. Stress rupture testing is always done until failure of the material. In creep testing the main goal is to determine the minimum creep rate in stage II. Once a designer knows the materials will creep and has accounted for this deformation a primary goal is to avoid failure of the component.

18 Corrosion Failures Corrosion is chemically induced damage to a material that results in deterioration of the material and its properties. This may result in failure of the component. Several factors should be considered during a failure analysis to determine the affect corrosion played in a failure. Examples are listed below: Type of corrosion Corrosion rate The extent of the corrosion Interaction between corrosion and other failure mechanisms Corrosion is is a normal, natural process. Corrosion can seldom be totally prevented, but it can be minimized or controlled by proper choice of material, design, coatings, and occasionally by changing the environment. Various types of metallic and nonmetallic coatings are regularly used to protect metal parts from corrosion.

19 Stress Corrosion Cracking Stress corrosion cracking is a failure mechanism that is caused by environment, susceptible material, and tensile stress. Temperature is a significant environmental factor affecting cracking. For stress corrosion cracking to occur all three conditions must be met simultaneously. The component needs to be in a particular crack promoting environment, the component must be made of a susceptible material, and there must be tensile stresses above some minimum threshold value. An externally applied load is not required as the tensile stresses may be due to residual stresses in the material. The threshold stresses are commonly below the yield stress of the material. Stress Corrosion Cracking Failures Stress corrosion cracking is an insidious type of failure as it can occur without an externally applied load or at loads significantly below yield stress. Thus, catastrophic failure can occur without significant deformation or obvious deterioration of the component. Pitting is commonly associated with stress corrosion cracking phenomena.

20 Pitting Corrosion Pitting is a localized form of corrosive attack. Pitting corrosion is typified by the formation of holes or pits on the metal surface. Pitting can cause failure due to perforation while the total corrosion, as measured by weight loss, might be rather minimal. The rate of penetration may be 10 to 100 times that by general corrosion. Pits may be rather small and difficult to detect. In some cases pits may be masked due to general corrosion. Pitting may take some time to initiate and develop to an easily viewable size. Pitting occurs more readily in a stagnant environment. The aggressiveness of the corrodent will affect the rate of pitting. Some methods for reducing the effects of pitting corrosion are listed below: Reduce the aggressiveness of the environment Use more pitting resistant materials Improve the design of the system

21 Mihama accident Orifice

22 Structural Integrity Issues

23 Structural Integrity Issues Davies-Besse NPP

24 Structural Integrity Issues Fracture Analysis 1. sizes of flaws which must be detected during nondestructive examinations of components 2. needs to replace or repair structures and components which are found to have flaws present. 3. remaining years of operating life of degraded components.

25 Structural Integrity Issues Ductile and Brittle Metal Characteristics Ductile metals experience observable plastic deformation prior to fracture. Brittle metals experience little or no plastic deformation prior to fracture. At times metals behave in a transitional manner - partially ductile/brittle. Ductile fracture has dimpled, cup and cone fracture appearance. The dimples can become elongated by a lateral shearing force, or if the crack is in the opening (tearing) mode. Brittle fracture displays either cleavage (transgranular) or intergranular fracture. This depends upon whether the grain boundaries are stronger or weaker than the grains. The fracture modes (dimples, cleavage, or intergranular fracture) may be seen on the fracture surface and it is possible all three modes will be present of a given fracture face. Schematics of typical tensile test fractures are displayed above.

26 Structural Integrity Issues Brittle Fractures Brittle fracture is characterized by rapid crack propagation with low energy release and without significant plastic deformation. The fracture may have a bright granular appearance. The fractures are generally of the flat type and chevron patterns may be present. Ductile Fractures Ductile fracture is characterized by tearing of metal and significant plastic deformation. The ductile fracture may have a gray, fibrous appearance. Ductile fractures are associated with overload of the structure or large discontinuities

27 Structural Integrity Issues No defectless material, or at least we cannot prove that it exist We have to live with it. Material behaviour Toughness Depends on the three state facors: -Temperature - Stress state - Strain rate Temperature Brittle Ductile Static load Dynamic load

28 Structural Integrity Issues Fracture toughness Plain stress Plain strain Thickness

29 Structural Integrity Issues Stress intensity: ratio of the stress at crack tip/normal stress Denoted with K (K1 tensile K2 shear stress) Stress at crack tip Crack tip Normal stress

30 Structural Integrity Issues K1c = fracture toughness that is the K1 value where crack start to propagate in an absolute brittle material No or negligable plastic deformation =valid K1c Jc=Je+Jp Limited plastic zone= J integral Kjc=(E*Jc)/(1-ν)2)1/2 MPa m General yielding, no fracture mechanics, leak before break E=Young modulus V=Poissons ratio

31 Structural Integrity Issues Stress Intensity Factor and Crack Tip Stresses Crack tips produce a singularity. The stress fields near a crack tip of an isotropic linear elastic material can be expressed as a product of and a function of with a scaling factor K where the superscripts and subscripts I, II, and III denote the three different modes that different loadings may be applied to a crack. The factor K is called the Stress Intensity Factor.

32 Structural Integrity Issues Infinite Plate with a Center Through Crack under Tension Infinite Plate with a Hole and Symmetric Double Through Cracks under Tension

33 Structural Integrity Issues b Semi-infinite Plate with an Edge Through Crack under Tension Infinite Stripe with a Center Through Crack under Tension or

34 Structural Integrity Issues Charpy testing

35 Structural Integrity Issues THE MASTER CURVE APPROACH TYPICAL RAW DATA 95 % 5% SMALL SPECIMENS 5% KJC [MPa m] KJC [MPa m] 95 % MASTER CURVE ANALYSIS STATISTICAL THICKNESS 95 % ADJUSTMENT 5% SMALL SPECIMENS LARGE SPECIMENS LARGE SPECIMENS T [0C] 0 T [ C] THEORETICAL SCATTER DESCRIPTION STATISTICAL SIZE ADJUSTMENT UNIFIED TEMPERATURE DEPENDENCE

36 Structural Integrity Issues PROBABILITY OF INITIATION (WEAKEST LINK) CUMULATIVE FAILURE PROBABILITY OF A VOLUME ELEMENT P 1 1 Pr I / O f N Pr{I/O} = Pr{I} (1- Pr{V/O}) = Cleavage initiation without prior void initiation.. Pr{I/O} = ( d, D, T,. etc.)

37 Structural Integrity Issues Master Curve Kjc= 30+70*exp[0.019(T-T0)] Median Kjc(5%) =25,4+37.8*exp[0.019*(T-T0)] Lower bound Measurement: three point bend (precracked Charpy) or CT specimens (8-10 pc minimum) at low temperature. ASTM Evaluation T0 Margin=10-16 C Size adjustment included, small specimens can be used Guidelines for Application of the Master Curve Approach to Reactor Pressure Vessel Integrity in Nuclear Power Plants Details Technical Reports Series No , English, Full Text, (File Size: 1377 KB) Euro. Date of Issue: 22 April Free download, 39Euro in printed form