Terminal Learning Objectives

Operator Generic Fundamentals Brittle Fracture and Vessel Thermal Stress

Terminal Learning Objectives
At the completion of this training session, the trainee will demonstrate mastery of this topic by passing a written exam with a grade of ≥ 80 percent on the following topics (TLOs): Describe stress and strain and their metallurgical effects. Explain points on the stress-strain curves and the differences between brittle and ductile materials. Describe the causes, consequences, and methods of preventing brittle fracture. Describe how thermal stresses and shock affect brittle fracture. TLOs

Stress and Strain TLO 1 - Describe stress and strain, and their metallurgical affects. 1.1 Describe the following terms: Stress Tensile stress Compressive stress Shear stress Strain Plastic deformation TLO 1

Characteristics of Stress
ELO Describe the following terms: stress, tensile stress, compressive stress, shear stress, strain, and plastic deformation. Any component, simple or complex, transmits or sustains a mechanical load of some sort When a force is applied to an object A stress is normally felt in the opposite direction When stress is applied to an object Movement (strain) is possible Load may be one of the following types Dead load - a load that is applied steadily Live load - a load that fluctuates, with slow or fast changes in magnitude Shock load - a load that is applied suddenly Impact - a load due to impact in some form ELO 1.1

Measuring Stress The external load and the area where stress is applied are measurable 𝑆𝑡𝑟𝑒𝑠𝑠=𝜎= 𝐹 𝐴 Where: σ = stress (Pascals, MegaPascals, psi or lbs of force per in.2) F = applied force (Newtons or pound-force per in2) A = cross-sectional area (m2, mm2 or in2) Stress (σ) equals the load per unit area or the force (F) applied per cross-sectional area (A) perpendicular to the force. ELO 1.1

Types of Stress The six main classifications of stress include:
Residual stress Structural stress Pressure stress Flow stress Thermal stress Fatigue stress ELO 1.1

Residual and Structural Stress
Residual stress results from manufacturing processes For example, welding Structural stress is because of the weight supported Pipe hangers, etc. ELO 1.1

Pressure and Flow Stress
Pressure stress is stress induced in vessels containing pressurized materials Pressure loading produces a tensile stress For example, reactor vessel Flow stress occurs when a mass of flowing fluid induces dynamic pressure on a pipe wall For example - reaction to momentum change through an elbow This type of stress loading has fluctuating flow rates and is unsteady ELO 1.1

Thermal and Fatigue Stress
Thermal stress exists when temperature gradients are present in a material Thick-walled pressure vessels with heatup and cooldown conditions Fatigue stress results from the cyclic application of a stress Due to vibration or thermal cycling Vibration – safety valves or low pressure turbine blades Thermal – PZR spray nozzle or RCS penetrations ELO 1.1

Flaws and Cyclic Loading
The significance of stress increases when the stressed material is flawed Flaws concentrate or amplify stresses at the flaw location Unsteady load stresses affect a material more severely Stress associated with flaws and unsteady load combine and may exceed the material’s failure point ELO 1.1

Types of Applied Stress
Stress intensity expressed as one of three basic types of internal load: tensile, compressive, or shear. Mathematically, there are only two types of internal load because tensile and compressive stress provide positive and negative versions of the same type of normal loading. Component response to stress differs. It is better and safer to think of both as separate types of stress. The plane of a tensile or compressive stress lies perpendicular to the axis of operation of the force from which it originates. The plane of a shear stress lies in the plane of the force from which it originates. Figure: Types of Applied Stress ELO 1.1

Tensile Stress Tensile stress happens when the two sections of a material on either side of a stress plane pull apart or elongate Pressure stress is tensile Cooldown on inner wall of reactor vessel is also tensile Heatups and cooldown discussed in later slides. Figure: Example of Tensile Stress ELO 1.1

Compressive Stress Compressive stress is the reverse of tensile stress
Adjacent parts of the material press against each other through a typical stress plane Heatup on inner wall of reactor vessel is compressive Heatups and cooldown discussed in later slides. Figure: Example of Compressive Stress ELO 1.1

Shear Stress Shear stress exists when two parts of a material slide across each other in any typical plane of shear on application of force parallel to that plane Figure: Example of Shear Stress ELO 1.1

Characteristics of Strain
A metal subjected to a load or force will distort or deform to some degree If load is small, the distortion may disappear when the load is removed Intensity, or degree, of distortion defines the strain If a metal returns to its original dimensions when load removed No permanent deformation When metal experiences strain, its volume remains constant For example, apply tensile stress to a wire Wire length increases, but diameter decreases No related KA for this objective. ELO 1.1

Characteristics of Strain
Strain is the total elongation per unit length of material due to some applied stress 𝑆𝑡𝑟𝑎𝑖𝑛=𝜀= 𝛿 𝐿 Where: ε = strain (m/m, mm/mm, or in./in.) δ = total elongation (m, mm, or in.) L = original length (m, mm, in.) When using metal for mechanical engineering purposes, a state of stress usually exists in a large amount of the material Reaction of the atomic structure manifests itself on a macroscopic scale ELO 1.1

Types of Strain – Elastic
Elastic strain also called elastic deformation Stress causes atoms to move from their equilibrium position All atoms maintain their relative geometry They do not displace equally When stress removed Atoms return to their original positions no permanent deformation Elastic strain is a transitory dimensional change existing only while the initiating stress is applied and disappearing immediately on removal of the stress ELO 1.1

Types of Strain – Plastic
If stress exceeds “elastic” limit permanent deformation will occur Called Plastic deformation or plastic strain Normally seen in ductile materials Ductile and Brittle materials explained in upcoming slides ELO 1.1

Stress and Strain Curves
TLO 2 Explain points on the stress-strain curves and the differences between brittle and ductile materials. 2.1 Explain Hooke's Law and Young's Modulus related to stress and elastic materials. 2.2 Explain the following points on a stress-strain curve: Proportional limit Yield point Ultimate strength Fracture point 2.3 Describe the differences in the shape of stress-strain curves for brittle and ductile materials. No Direct KA tie to TLO 2/ELOs, but comprehension necessary to understand other KA’s. The relationships between stress and strain, brittle and ductile properties, and the stress-strain curves are included in this module. TLO 2

Hooke’s Law and Young’s Modulus
ELO Explain Hooke's Law and Young's Modulus related to stress and elastic materials. Hooke's Law states: the strain of a material is directly proportional to the stress that is induced in the material only applies to the proportional region of the curve The change in stress divided by the change in strain in this region called Young's Modulus No related KA for this objective. Hooke discovered that the elongation of a metal bar is directly proportional to the tensile force applied and the length of the bar, and inversely proportional to the cross sectional area of the bar and the bar’s elastic constant (or Modulus of Elasticity) ELO 2.1

Hooke’s Law and Young’s Modulus
Young’s Modulus (E), equals the ratio of the unit stress to the unit strain Modulus of elasticity of the material in tension or compression Strain (ε) is proportional to applied stress (σ) inversely proportional to Young’s Modulus (E) 𝜀= 𝜎 𝐸 Where: E = Young's Modulus (N/m2, N/mm2, or lbf/in.2) σ = stress (MPa, Pa, or psi) ε = strain (m/m, mm/mm, or in./in.) E = elastic constant of the material, called the Modulus of Elasticity, or Young's Modulus (N/m2, N/mm2 or lbf/in2) It measures the ratio of stress to strain and resistance to elastic deformation Equals stress (below the proportional limit) divided by the corresponding strain Slope of the straight-line portion of the stress-strain curve ELO 2.1

Stress-Strain Curve Terms
ELO Explain the following points on a stress-strain curve: Proportional limit, Yield point, Ultimate strength, Fracture point. The movement (strain) caused by an applied stress Shown on a Stress-Strain Curve Different curves for Ductile and Brittle materials Ductile curve shown in this section Differences between Ductile and Brittle in next ELO Basically a function of whether any elastic/plastic deformation occurs before fracturing No related KA for this objective. ELO 2.2

Proportional Limit The region where the deformation follows a linear relationship Slope of the curve is equal to Young’s Modulus Stress level corresponding to elastic strain For load intensities beyond the proportional limit Stress-strain relationship not proportional, but still elastic Up to the Yield Stress point Although the last bullet is a true statement it is misleading. Elastic deformation can occur above the proportional limit without plastic deformation. A rubber band is a good example where there is a lot of elastic deformation and not necessarily any plastic deformation. ELO 2.2

Points 1 and 2 is the proportional region Young’s Modulus applies Point 2 is the proportional limit Strain increase not proportional between Points 2 and 3 Point 3 is the yield strength or yield point Where elastic deformation ends and plastic deformation begins NOTE: “Elongation %” is STRAIN Proportional Limit definition provided in previous ELO Figure: Ductile Material Stress-Strain Curve ELO 2.2

Point 4 is the point of ultimate tensile strength maximum stress while being stretched before failing Point 5 is the fracture point where failure of the material occurs The area between Points 3 and 5 called - plastic region material will not return to its original length UTS and Fracture Point explained in further detail on next two slides Figure: Ductile Material Stress-Strain Curve ELO 2.2

Ultimate Tensile Strength
The ultimate tensile strength (UTS) is the maximum stress while being stretched before failing 𝑈𝑇𝑆= 𝑚𝑎𝑥𝑖𝑚𝑢𝑚 𝑙𝑜𝑎𝑑 𝑖𝑛 2 =𝑝𝑠𝑖 The stress coordinate value of the highest point on the curve ELO 2.2

Figure: Ductile Material Stress-Strain Curve
Fracture Point Strain creates reduction in the cross-sectional area Material tends to “neck” down Material carries less load in the final stages of the tensile test Materials usually fracture at stresses lower than designed to carry Figure: Ductile Material Stress-Strain Curve ELO 2.2

Brittle and Ductile Material Stress-Strain Curves
ELO Describe the differences in the shape of stress-strain curves for brittle and ductile materials Same terms previously discussed apply to both curves Strain/Stress ratio differs Brittle material can handle greater stresses with minimal strain The terms “strength” and “fracture toughness” are synonymous with each other Ductile materials have lower strength, but higher fracture toughness Brittle materials have a higher strength, but lower fracture toughness No related KA for this objective. ELO 2.3

Ductile Material Stress-Strain Curves
Stress-strain curve typical of a ductile material Strength is smaller Plastic region is greater Material bears more strain or deformation before fracture Figure: Ductile Material Stress-Strain Curve ELO 2.3

Ductility Determination
Ductility of a material measured by: Determining initial unstrained length Applying tensile stress until fracturing through necking Fitting pieces back together to determine final length Figure: Elongation After Failure ELO 2.3

Brittle Material Stress-Strain Curves
Stress-strain curve typical of a brittle material plastic region is small strength of the material is high Little or no deformation before fracture occurs Fracture point still below UTS Figure: Brittle Material Stress-Strain Curve ELO 2.3

Brittle Fracture TLO 3 – Describe the causes, consequences, and methods of preventing brittle fracture. 3.1 Explain the following terms: Ductile fracture Brittle fracture Nil-ductility Transition Temperature (NDTT) Reference Temperature for Nil-Ductility Transition (RTNDT) 3.2 Describe the effects that neutron irradiation has on NDTT and RTNDT. 3.3 Explain the conditions necessary for brittle fracture to occur. 3.4 Describe the use of the following to prevent the occurrence of brittle fracture: Minimum Pressurization-Temperature (MPT) Curves Heatup and cooldown limitations K1.02 State the definition of Nil-Ductility Transition Temperature. K1.03 Define reference temperature. K1.04 State how the possibility of brittle fracture is minimized by operating limitations. K1.05 State the effect of fast neutron irradiation on reactor vessel metals. K1.06 Define pressurized thermal shock (PTS) K1.07 State the operational concerns of uncontrolled cooldown. TLO 3

Ductile and Brittle Fracture Terms
ELO Explain the following terms: ductile fracture, brittle fracture, nil- ductility transition (NDT) temperature, reference temperature for nil- ductility transition (RTNDT). Metals can fail by ductile or brittle fracture Ductile fracture sustain substantial plastic strain or deformation before fracturing occurs at higher temperatures Brittle fracture little or no plastic deformation occurs at low temperatures Relates KAs K1.01 State the brittle fracture mode of failure , K1.02 State the definition of Nil-Ductility Transition Temperature , K1.03 Define reference temperature , K1.05 State the effect of fast neutron irradiation on reactor vessel metals ELO 3.1

Ductile Fracture Any material that allows reasonable amounts of strain before fracturing is called “ductile” Ductility is relative measure of how much something stretches before it breaks Ductile fracture occurs after some plastic deformation Necking of the material Usually occurs at higher temperatures ELO 3.1

Brittle Fracture Brittle fracture is the sudden catastrophic failure with little or no plastic deformation cracks propagate rapidly failure results from cleavage splitting along definite planes Usually occurs at low temperatures ELO 3.1

Ductile and Brittle Fracture
Ductile fracture is preferable to brittle fracture Ductile fracture occurs over time, providing some chance to discover failure before it happens. Brittle fracture happens quickly, without warning, and occurs in the presence of pre-existing flaws at much lower stress levels than a ductile fracture. Figure: Metal Fracture Modes ELO 3.1

Nil-Ductility Transition Temperature
Nil-Ductility Transition (NDT) temperature Temperature above which a material is ductile Temperature below which a material is brittle When the reactor vessel is below this temperature Tensile stress should be limited (pressure) Small grain size tends to increase ductility results in a decrease in NDT temperature Impurities (copper) lowers temperature below which brittle fracture can occur Fast Neutron Embrittlement raises temperature below which brittle fracture can occur NDT Temperature is not precise. It varies according to prior mechanical and heat treatment as well as the nature and amounts of impure elements present in the metal alloy. NDT values are determined using drop-weight tests. Examples include the Izod or Charpy tests. ELO 3.1

Reference Temperature for Nil-Ductility Transition (RTNDT)
The RTNDT is the temperature above which plastic deformation accompanies all fractures Brittle failure generally occurs because a flaw or crack propagates throughout the material As the temperature decreases, a lower stress is required for a crack to propagate As the temperature increases, a higher stress is required for a crack to propagate ELO 3.1

Material Toughness Tests
Various tests available to determine NDTT or RTNDT Charpy V-Notch Test Specimen has pre-existing notch Pendulum swing breaks specimen, continues upward the smaller the upward swing, the tougher the material The maximum energy developed by the hammer is 120 ft-lb in the Izod Test and 240 ft-lb in the Charpy Test. Figure: Charpy V-Notch Test ELO 3.1

Ductile and Brittle Fracture Terms
Knowledge Check The nil-ductility transition temperature for a reactor vessel is the temperature... below which the probability of brittle fracture significantly increases. determined by fracture mechanics to be equivalent to the reference transition temperature. determined by Charpy V-notch test to be equivalent to the reference transition temperature. below which the yield stress of the metal is inversely proportional to Young's modulus of elasticity. Correct answer is A. Correct Answer: A NRC Bank Question – P98 Analysis: While operation below the nil-ductility temperature results in aloss of a material’s ductile properties, making brittle fracture more likely, operation below this temperature does not directly result in brittle fracture; the tensile stress may be significant to result in brittle fracture. ELO 3.1

Neutron Embrittlement
ELO Describe the effects that neutron irradiation has on NDTT and RTNDT. As the reactor vessel ages through multiple refuelings Neutron embrittlement causes the vessel to become stronger less ductile raise the NDT temperature Must heat up to higher temperature before you can increase pressure High energy neutrons can cause permanent localized damage Displace atoms from equalized position(s) Called Point Imperfections K1.01 State the brittle fracture mode of failure K1.05 State the effect of fast neutron irradiation on reactor vessel metals Dissipation of energy of neutron or fission fragments from elastic collisions produce large thermal spikes Distortion of lattice usually occurs In rapid cooling due to conduction of heat to surroundings, certain amount of lattice distortion is frozen permanently changes structures of materials ELO 3.2

Point Imperfections Divided into three main categories:
Vacancy defects Substitutional defects Interstitial defects Point defects enhance or lessen a material’s usefulness for construction, depending on the intended use Figure: Point Defects ELO 3.2

When exposed to neutron flux, many elements undergo (n, γ) reaction No real damage is done to metals since new substance is isotope of the initial material New isotope might Beta-minus decay to become new element This causes impurity within metal If enough of these impurities are created, properties of metals can be significantly affected ELO 3.2

Knowledge Check Prolonged exposure of a reactor vessel to a fast neutron flux will cause the nil-ductility transition temperature to... decrease, due to the propagation of existing flaws. increase, due to the propagation of existing flaws. decrease, due to changes in the material properties of the vessel wall. increase, due to changes in the material properties of the vessel wall. Correct answer is D. Correct answer is D. NRC Bank Question – P298 Analysis: Fast neutron bombardment of the reactor vessel randomly knocks metal atoms out of their lattice structure and can cause pre-existing flaws to grow larger. RTNDT changes over core life due to neutron embrittlement: High energy neutrons physically displace atoms in the metal lattice structure, resulting in “embrittlement stress” and a corresponding rise in the RTNDT. (meaning a material now loses its ductile properties at higher temperatures). ELO 3.2

Brittle Fracture Conditions
ELO Explain the conditions necessary for brittle fracture to occur. Three conditions necessary for brittle fracture: A pre-existing flaw, such as a crack A stress of sufficient intensity to develop a small deformation at the crack tip Tensile stress (RCS pressure) A temperature low enough to promote brittle fracture Related KA K1.01 State the brittle fracture mode of failure ELO 3.3

Brittle Fracture Details
As the temperature lowers tensile strength (Curve A) increases ultimate tensile strength yield strength (Curve B) increase Curve B slope > Curve A slope beginning of plastic deformation Graph representative of crack arrest curve for carbon steel (reactor vessel) Figure: Stress-Temperature Diagram for Crack Initiation and Arrest ELO 3.3

At some low temperature, approximately 10°F yield strength and tensile strength coincide NDT temperature no yielding when a failure occurs therefore, the failure is brittle Figure: Stress-Temperature Diagram for Crack Initiation and Arrest ELO 3.3

Curves A and C identical at high temperatures Curves B and C identical at low temperatures With flaw present As temperature lowered Curve C drops At the point where Curves C and B meet new (higher) NDT temperature 10oF with no flaw Now, 60oF with flaw Recall, this does NOT mean that below 60oF brittle fracture will occur. Since a tensile stress (pressure) is required to propagate the flaw, it just means you have to raise temperature even higher before you are allowed to raise pressure. Figure: Stress-Temperature Diagram for Crack Initiation and Arrest ELO 3.3

Crack Initiation and Propagation
The start of a fracture at low stresses occurs by the cracking tendencies at the tip of the crack At high temperatures metal mass surrounding the crack supports the stress to prevent further crack propagation At low temperatures (with a tensile stress) crack initiates and propagates through the material rapidly at the speed of sound As stated, brittle failure usually happens because a flaw or crack propagates throughout the material. ELO 3.3

Grain Structure and Boundary
Grain structure refers to the arrangement of the grains in a metal Each grain has a particular crystal or lattice structure Each of the light areas is a grain, or crystal Each region of space within a border is occupied by a continuous crystal lattice Grain boundaries are the dark lines surrounding the grains Imperfections at grain boundaries can lead to flaws in material Examination of a thin section of a common metal under a microscope would show a grain structure similar to that shown above in the illustration. Figure: Grain Structure ELO 3.3

Lower Fracture Propagation Stress
There is a minimum stress level in order to propagate a crack regardless of the temperature Called lower fracture propagation stress As temperature increases, a higher stress is required for a crack to propagate The crack arrest curve plots the relationship between the temperature and the stress required for a crack to propagate, shown on Curve D of the above graphic. At temperatures above those indicated on Curve D, crack propagation does NOT occur. Figure: Stress-Temperature Diagram for Crack Initiation and Arrest ELO 3.3

Brittle Fracture Details Conclusion
Thick-walled vessels and components require operation above the NDT temperature to ensure no brittle fracture occurs For additional margin, operation should be limited to above RTNDT (NDT + 60°F) NO brittle fracture occurs for purely elastic loads Neutron embrittlement raises this “minimum” temperature over life of vessel Vessel test specimens tested each cycle to determine new “higher” RTNDT ELO 3.3

Brittle Fracture Knowledge Check
Which one of the following statements describes the relationship between brittle fracture and the nil-ductility transition temperature? Operation below the nil-ductility transition temperature will result in brittle fracture. Operation above the nil-ductility transition temperature will result in brittle fracture. Operation below the nil-ductility transition temperature will increase the probability of brittle fracture. Operation above the nil-ductility transition temperature will increase the probability of brittle fracture. Correct answer is C. Correct answer is C. NRC Bank Question – P1396 Analysis: While operation below the nil-ductility temperature results in a loss of a material’s ductile properties, making brittle fracture more likely, operation below this temperature does not directly result in brittle fracture. It only increases the probability of brittle fracture. A tensile stress (pressure) is still required. ELO 3.3

Brittle Fracture Prevention
ELO 3.4 – Describe the use of the following to prevent the occurrence of brittle fracture; minimum pressurization-temperature (MPT) curves, heatup and cooldown limitations. Administrative/operational limits placed on heatup/cooldown rates Minimizes the potential for brittle fracture to occur Minimizes thermal stresses placed on reactor vessel K1.04 State how the possibility of brittle fracture is minimized by operating limitations K1.07 State the operational concerns of uncontrolled cooldown * ELO 3.4

Minimum Pressurization-Temperature Curves
Reactor vessels usually constructed of low carbon steel Susceptible to brittle fracture under the right conditions Low temperature and high pressure Fast neutron embrittlement tends to: Increase material strength, but, lower the fracture toughness Strict temperature and pressure operational limitations exist Brittle fracture prevention is of the utmost importance to the nuclear power industry. ELO 3.4

Minimum Pressurization-Temperature Curves
MPT curve - ensures adequate operating margin from brittle fracture Safe operating region is to the right of the reactor vessel MPT curve Example of pressure-temperature operating curves for a Pressurized Water Reactor (PWR). Curves incorporate instrument error, providing adequate safety margin. Because of neutron embrittlement, the curves shift to the right over the facility’s life Figure: Example Pressurized Water Reactor (PWR) Minimum Pressurization Temperature Curve ELO 3.4

Exceeding MPT Limits During Operations
If the limits are exceeded during plant operation: Immediately bring plant pressure and temperature parameters within limits of the MPT curves Conduct an engineering evaluation to identify if any damage occurred to limit future operations Reports to the Nuclear Regulatory Commission (NRC) may also be required ELO 3.4

Heatup and Cooldown Limitations
Heatups and cooldown cause thermal stresses across reactor vessel wall Faster the rate, the greater the stress Heatups and cooldown are considered “cyclic” stresses Usually tracked IAW plant procedure requirements Stresses caused by heatups vary from those caused by cooldowns Heatup stresses are compressive on inner wall More limiting on outer wall Cooldown stresses are tensile on inner wall More limiting on inner wall Also more limiting that heatups (for same rate) Heatup and Cooldown stress curves shown on next couple of slides ELO 3.4

Figure: Heatup Stress Profile
During heatup, vessel outer wall temperature lags the inner wall temperature Compressive on inner wall At ¼ T Compressive heatup stress offsets tensile pressure stress Lower total stress At ¾ T Both stresses tensile Higher total stress Heatup – total stress on outer wall slightly closer to allowable stress than on inner wall Stress Profile Standards: Pressure stress ALWAYS tensile (and more tensile on inner wall) Residual and Embrittlement stresses (not shown) also ALWAYS tensile The faster the heatup, the greater the slope of the “Temperature Induced Stress” curve Each crack is ¼ of the wall thickness deep; the 1/4T crack starts from the inside and the 3/4T crack starts from the outside. During a heatup, the inner wall of the reactor vessel wants to expand. However, the outer wall doesn’t see this higher temperature yet so it tries to keep the inner wall from expanding. This is why the inner wall has a compressive stress on it from a heatup. Figure: Heatup Stress Profile ELO 3.4

Figure: Cooldown Stress Profile
During cooldown, vessel outer wall temperature lags the inner wall temperature Tensile on inner wall At ¼ T Tensile cooldown stress adds to tensile pressure stress Higher total stress At ¾ T Compressive cooldown stress offsets tensile pressure stress Lower total stress Cooldown more limiting on inner wall And more limiting than heatup Stress Profile Standards: Pressure stress ALWAYS tensile (and more tensile on inner wall) Residual and Embrittlement stresses (not shown) also ALWAYS tensile The faster the cooldown, the greater the slope of the “Temperature Induced Stress” curve During a cooldown, the inner wall of the reactor vessel wants to contract. However, the outer wall doesn’t see this lower temperature yet so it tries to keep the inner wall from contracting. This is why the inner wall has a tensile stress on it from a cooldown. Figure: Cooldown Stress Profile ELO 3.4

Heatup and Cooldown Rate Limitations
Faster cooldown rates require more limiting pressure-temperature conditions Large thick-walled components are the limiting components Notice the temperature and pressure coordinates. Faster cooldown rates require more limiting pressure-temperature conditions. Thermal stresses are higher with faster cooldown or heatup rates. The opposite also holds true. SOAK TIME is explained on the next slide Figure: Example Heatup and Cooldown Rate Limits Versus Pressure-Temperature ELO 3.4

Soak Times A SOAK is when you allow equalizations of temperature across thick- walled components Reactor vessel wall during a normal cooldown Upper vessel head during natural circulation cooldown Soak times minimize thermal stresses Smaller slope of temperature stress curve Soak times allow the metal in a large component to heat more evenly from the hot side to the cold side, limiting the thermal stress across the component ELO 3.4

Brittle Fracture Prevention
Knowledge Check An uncontrolled cooldown is a brittle fracture concern because it creates a large __________ stress at the __________ wall of the reactor vessel. tensile; inner tensile; outer compressive; inner compressive; outer Correct answer is A. Correct answer is A. NRC Bank Question – P1000 Analysis: Cooldown temperature stress is tensile at the inner wall and compressive on the outer wall. This can be attributed to the differential cooldown rate: the inner wall cools down faster than the outer wall, resulting in a stress that changes from tensile to compressive across the wall. ELO 3.4

Thermal Shock and Stress
TLO 4 – Describe how thermal stresses and shock affect brittle fracture. 4.1 Explain why thermal stress is a major concern when rapidly heating or cooling a thick-walled vessel and methods for limiting its severity. 4.2 Describe the term pressurized thermal shock including: Factors that affect its severity Plant transients of greatest impact This chapter addresses thermal shock (stresses), pressure stresses, and their effects on plant materials, components and systems. TLO 4

Thick-Walled Vessel Thermal Stresses
ELO 4.1 – Explain why thermal stress is a major concern when rapidly heating or cooling a thick-walled vessel and methods for limiting its severity. Heatup and cooldown transients lead to excessive thermal gradients on materials, particularly on thick-walled reactor vessels Results in excessive stresses Can be tensile or compressive in nature Thermal stresses are cyclic in nature Heating followed by cooling, followed by heating, etc. Causes fatigue failure of the materials or components subjected to the stresses Related KA K1.07 State the operational concerns of uncontrolled cooldown * Recall that tensile stress tends to pull an object apart, whereas compressive stress tends to compress or push in on an object. ELO 4.1

Thick-Walled Vessel Thermal Stress
“Thick or Thin-walled” is a comparison of vessel’s thickness to its radius Thin-walled vessel thickness of less than 1% of the vessel’s radius Thick-walled vessel thickness is more than 5-10% of the vessel’s radius When rapidly heating (or cooling) a thick-walled vessel One part of the wall tries to restrain the other side Therefore, both sides are under stress compressive on one side, and tensile stress on the other Thermal stress is a major concern in thick-walled vessels and components because of the magnitude of the stress involved. ELO 4.1

Thick-Walled Vessel Thermal Stress
Tensile Compressive When cold water enters the vessel inside wall (left side) cools before the metal on the outside When the metal on the inside wall cools, it contracts hot metal on the outside wall continues expanding This sets up a thermal stress cold side in tensile stress hot side in compressive stress These stresses can cause cracks in the cold side of the wall When cold water enters the vessel, the cold water causes the metal on the inside wall (left side) to cool before the metal on the outside. When the metal on the inside wall cools, it contracts, while the hot metal on the outside wall continues expanding. This sets up a thermal stress, placing the cold side in tensile stress and the hot side in compressive stress. These stresses can cause cracks in the cold side of the wall. Figure: Thermal Stresses on a Thick-Walled Vessel ELO 4.1

Limiting the Severity of Thermal Stress
Minimize cyclic thermal stresses with Slow controlled heating and cooling of systems Controlled makeup water addition rates Plant operating procedures limit Heatup and cooldown rates for components Temperatures for placing systems in operation Pressures for specific temperatures during system operation Some plants use design features Flow restricting nozzles on main steam lines Limit rate of blowdown (cooldown) of SG on a break ELO 4.1

Thermal Shock and Stress
Knowledge Check During an uncontrolled cooldown of a reactor coolant system, the component most susceptible to brittle fracture is the... reactor vessel. steam generator tube sheet. cold leg accumulator penetration. loop resistance temperature detector penetration. Correct answer is A. Correct answer is A. NRC Bank Question – P1099 Analysis: The major component of concern regarding fracture is the reactor vessel. This is due to its large size and wall thickness, resulting in significant heatup and cooldown thermal stresses due to the inner wall being affected faster than the outer wall. Also, significant neutron embrittlement results in a lower allowable stress at the reactor vessel inner wall. ELO 4.1

Pressurized Thermal Shock
ELO 4.2 – Describe the term pressurized thermal shock including: Factors that affect its severity, Plant transients of greatest impact. Pressurized thermal shock (PTS) is a rapid cooldown followed by a repressurization Factors that affect its severity: Low temperature and high pressure K1.06 Define pressurized thermal shock (PTS) K1.07 State the operational concerns of uncontrolled cooldown * Pressurized thermal shock (PTS) is the trauma experienced by a thick-walled vessel due when combined stresses from temperature changes for the given conditions result in added stress Non-uniform temperature distribution and subsequent compressive and tensile stresses add stress from pressure and create overstress conditions with the potential for material failure ELO 4.2

Plant transients of greatest impact Main steam line break Initiates reactor trip and SI Rapid cooldown of RCS Once SG blows dry, decay heat will heat up cold SI water RCS could repressurize if not controlled Procedure might direct you maintain RCS temperature at Tsat of intact SG(s) RCS LOCA will also initiate a trip/SI However, RCS should not repressurize because of break ELO 4.2

Procedural/Administrative protections Low Temperature Overpressure Protection or Cold Overpressure Protection (LTOP or COPs) Lowers PZR relief setpoints when RCS is cold Tagging of Safety-related equipment When in Cold Shutdown, required safety injection equipment operability is less Might de-energize and/or tag out some high pressure SI pumps Minimize possibility of raising pressure at low temps LTOPs or COPs are two of many possible acronyms for systems used in Cold Shutdown to protect from Pressurized Thermal Shock. These are standard terms used in many Westinghouse and/or CE plants. If your plant system/term is different, you may interject that information here. ELO 4.2

Knowledge Check A nuclear power plant is shutdown with the reactor coolant system at 1,200 psia and 350°F. Which one of the following would be most likely to cause a pressurized thermal shock to the reactor vessel? Rapid depressurization followed by a rapid heatup. Rapid depressurization followed by a rapid cooldown. Rapid cooldown followed by a rapid pressurization. Rapid heatup followed by a rapid pressurization. Correct answer is C. Correct answer is C. NRC Bank Question – P99 Analysis: The major component of concern regarding fracture is the reactor vessel. To prevent brittle fracture, vessel must not be stressed too heavily while it is cool. In other words, do NOT allow an overpressurization condition to occur at low temperatures (i.e. PREVENT pressurized thermal shock)! Pressurized thermal shock occurs at low temperatures and high pressures. ELO 4.2

NRC KA to ELO Tie KA # KA Statement RO SRO ELO K1.01
State the brittle fracture mode of failure. 2.8 3.2 1.1, 3.1 K1.02 State the definition of Nil-Ductility Transition Temperature. 2.4 2.5 3.1 K1.03 Define reference temperature. 2.0 K1.04 State how the possibility of brittle fracture is minimized by operating limitations. 3.3 3.7 1.1, 3.3 K1.05 State the effect of fast neutron irradiation on reactor vessel metals. 2.9 3.0 K1.06 Define pressurized thermal shock (PTS) 3.6 3.8 4.2 K1.07 State the operational concerns of uncontrolled cooldown. 4.1 4.1, 4.2

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