Tài liệu Qualification Standard Reference Guide SEPTEMBER 2008 ppt

Mechanical systems personnel shall demonstrate a working level knowledge of steady-state heat transfer.. Mechanical systems personnel shall demonstrate a working level knowledge of therm

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Table of Contents

LIST OF FIGURES iii

LIST OF TABLES iv

ACRONYMS v

PURPOSE 1

SCOPE 1

PREFACE 1

ACKNOWLEDGEMENTS 2

TECHNICAL COMPETENCIES 3

General Technical 3

1 Mechanical systems personnel shall demonstrate a working level knowledge of steady-state heat transfer 3

2 Mechanical systems personnel shall demonstrate a working level knowledge of thermodynamics 6

3 Mechanical systems personnel shall demonstrate a working level knowledge of fluid mechanics 12

4 Mechanical systems personnel shall demonstrate a working level knowledge of the concepts, theories, and principles of basic material science 19

5 Mechanical systems personnel shall demonstrate a working level knowledge concerning the selection of appropriate components and materials in support of a mechanical system design or modification 32

6 Mechanical systems personnel shall demonstrate a working level knowledge of mechanical diagrams, including: 41

7 Mechanical systems personnel shall demonstrate a working level knowledge of installed mechanical equipment 45

8 Mechanical systems personnel shall demonstrate a working level knowledge of a typical diesel generator, including support systems 46

9 Mechanical systems personnel shall demonstrate a working level knowledge of the construction and operation of heat exchangers 54

10 Mechanical systems personnel shall demonstrate a working level knowledge of the theory and operation of heating, ventilation, and air conditioning (HVAC) systems 57

11 Mechanical systems personnel shall demonstrate working level knowledge of general piping systems 76

12 Mechanical systems personnel shall demonstrate a working level knowledge of the general construction, operation, and theory of valves 87

13 Mechanical systems personnel shall demonstrate a working level knowledge of safety and relief devices 97

14 Mechanical systems personnel shall demonstrate a working level knowledge of pump theory and operation 100

15 Mechanical systems personnel shall demonstrate a working level knowledge of strainers and filters 112

16 Mechanical systems personnel shall demonstrate a working level knowledge of the basic components, operations, and theory of hydraulic systems 120

Table of Contents

17 Mechanical systems personnel shall demonstrate a working level knowledge of the

components, operation, and theory of pneumatic systems 123

18 Mechanical systems personnel shall demonstrate a working level knowledge of the basic design, construction, and operation of glovebox systems 129

19 Mechanical systems personnel shall demonstrate a working level knowledge of the

principles of lubrication 146

20 Mechanical systems personnel shall demonstrate a familiarity level knowledge of

reading and interpreting electrical diagrams and schematics 152

21 Mechanical systems personnel shall demonstrate a familiarity level knowledge of

reading and interpreting electrical logic diagrams 158

Regulatory 161

22 Mechanical systems personnel shall demonstrate a working level knowledge of the

requirements of DOE O 420.1B, Facility Safety, and the associated guidance of DOE

G 420.1-1, Nonreactor Nuclear Safety Design Criteria and Explosives Safety Criteria

Guide for Use with DOE O 420.1, Facility Safety; and DOE G 420.1-2, Guide for the

Mitigation of Natural Phenomena Hazards for DOE Nuclear Facilities and Nonnuclear

Facilities 161

23 Mechanical systems personnel shall demonstrate a working level knowledge of safety in design as described and required in DOE O 413.3A, Program and Project Management for the Acquisition of Capital Assets, and DOE M 413.3-1, Project Management for the Acquisition of Capital Assets, and DOE-STD-1189-2008, Integration of Safety into the Design Process 169

24 Mechanical systems personnel shall demonstrate a working level knowledge of the

following standards related to natural phenomena hazards: 173

25 Mechanical systems personnel shall demonstrate a working level knowledge of DOE

maintenance management requirements as defined in DOE O 433.1A, Maintenance

Management Program for DOE Nuclear Facilities 175

26 Mechanical systems personnel shall demonstrate a working level knowledge of DOE

standard DOE-STD-1073-2003, Configuration Management 178

27 Mechanical systems personnel shall demonstrate a familiarity level knowledge of the

codes and standards of the American Society for Testing and Materials (ASTM)* 180

28 Mechanical systems personnel shall demonstrate a working level knowledge of the codes and standards of the American Society of Mechanical Engineers 182

29 Mechanical systems personnel shall demonstrate a familiarity level knowledge of the

following organizations’ non-mechanical systems-specific codes and standards: 189

30 Mechanical systems personnel shall demonstrate a familiarity level knowledge of the

codes and standards of the American Society of Heating, Refrigeration, and Air

Conditioning Engineers (ASHRAE) 191

31 Mechanical system personnel shall demonstrate a working level knowledge of the

quality control inspection techniques described in sections V and XI of the ASME Boiler and Pressure Vessel code and the verification of mechanical system integrity, including: 192

Management, Assessment, and Oversight 202

Table of Contents

32 Mechanical systems personnel shall demonstrate a working level knowledge of problem analysis principles and the ability to apply techniques necessary to identify problems,

determine potential causes of problems, and identify corrective action(s) 202

33 Mechanical systems personnel shall demonstrate a working level knowledge of assessment techniques (such as the planning and use of observations, interviews, and document reviews) to assess facility performance and contractor design and construction activities, report results, and follow up on actions taken as the result of assessments 215

Other 220

34 Mechanical systems personnel shall demonstrate a working level knowledge of the safety and health fundamentals of mechanical systems and/or components 220

35 Mechanical systems personnel shall demonstrate a working level knowledge of the following engineering design principles: 236

36 Mechanical systems personnel shall demonstrate a working level knowledge of maintenance management practices related to mechanical systems 243

Selected Bibliography and Suggested Reading 247

Figures Figure 1 Mollier diagram 8

Figure 2 Carnot cycle representation 9

Figure 3 Typical steam plant cycle 10

Figure 4 Ideal Otto cycle 11

Figure 5 Pascal’s law 15

Figure 6 Pressure-volume diagram 18

Figure 7 Effects of gamma radiation on different types of hydrocarbons 28

Figure 8 Idealization of unloaded region near crack flanks 30

Figure 9 The fracture energy balance 31

Figure 10 Charpy test equipment 36

Figure 11 Material toughness test 37

Figure 12 Successive stages of creep with increasing time 39

Figure 13 Valve conditions 41

Figure 14 Title block 42

Figure 15 Notes and legend 44

Figure 16 Natural convection cooling tower 56

Figure 17 Vapor-pressure curve 58

Figure 18 Simplified centrifugal pump 62

Figure 19 Balanced heat recovery ventilation schematic 71

Figure 20 Orifice plate 83

Figure 21 Gate valve 91

Figure 22 Globe valve 92

Figure 23 Ball valve 94

Figure 24 Swing check valve 95

Figure 25 Butterfly check valve 96

Figure 26 Variable reducing valve 97

Table of Contents

Figure 27 Centrifugal pump 103

Figure 28 Single and double volutes 103

Figure 29 Reciprocating positive displacement pump operation 104

Figure 30 Single-acting and double-acting pumps 108

Figure 31 Two screw, low-pitch screw pump 109

Figure 32 Three screw, high-pitch screw pump 110

Figure 33 Rotary moving vane pump 111

Figure 34 Typical multi-cartridge filter 113

Figure 35 Pneumatic actuator 124

Figure 36 Arrangement of indicating devices in glovebox ventilation system 134

Figure 37 Typical local mounting for differential pressure gauge 135

Figure 38 Indicating a pressure drop through a filter 135

Figure 39 Velocity measurements 136

Figure 40 Orifice meter method of measuring volume flow rate in small ducts 137

Figure 41 Arrangement of sharp-edge concentric orifice in small duct 138

Figure 42 Typical glovebox with major features 141

Figure 43 Characteristics of gloveboxes 142

Figure 44 Glovebox with multiple gloveports to facilitate access 143

Figure 45 Methods of injecting test aerosol and extracting samples (Methods A and B) 145

Figure 46 Methods of injecting aerosols and extracting samples (Methods C and D) 145

Figure 47 Basic transformer symbols 153

Figure 48 Transformer polarity 154

Figure 49 Switches and switch symbols 154

Figure 50 Switch and switch status symbology 155

Figure 51 Three-phase and removable breaker symbols 155

Figure 52 Common electrical component symbols 156

Figure 53 Large common electrical components 157

Figure 54 Basic logic symbols 159

Figure 55 Conventions for depicting multiple inputs 159

Figure 56 Truth tables 160

Figure 57 ISM functions and the B-level causal analysis tree branches 205

Figure 58 C-level causal analysis tree codes 206

Figure 59 CAT branch A3 matrix (continued on next page) 207

Figure 60 Six steps of change analysis 210

Figure 61 Mini-MORT analysis chart 211

Figure 62 MORT-based root cause analysis form 212

Figure 63 Cause and effect chart process 213

Figure 64 Example of cause and effect charting 214

Tables Table 1 Radiolytic decomposition of polyphenyls at 350 °C 28

Table 2 Loss of head for various d/D ratios 138

Table 3 Nondestructive testing personnel training requirements 201

AGS American Glovebox Society

AISC American Institute of Steel Construction

ALARA as low as reasonably achievable

ANS American Nuclear Society

ANSI American National Standards Institute

API American Petroleum Institute

ASCE American Society of Civil Engineers

ASHRAE American Society of Heating, Refrigeration, and Air-Conditioning Engineers ASME American Society of Mechanical Engineers

ASTM ASTM International (formerly American Society for Testing and Materials) B&PV ASME Boiler and Pressure (B&PV) Vessel Code

BMEP brake mean effective pressure

BNA baseline need assessment

cfm cubic feet per minute

cfs cubic feet per second

CIPT contractor integrated project team

COR code of record

CPVC chlorinated polyvinyl chloride

CRD contractor requirements document

CSE cognizant system engineer

dB decibel

DDESB DoD Explosives Safety Board

DMG directives management group

DoD U.S Department of Defense

DOE U.S Department of Energy

ACRONYMS

DSA documented safety analysis

EGSA Electrical Generating Systems Association EPA U.S Environmental Protection Agency ESF engineered safety feature

fpm feet per minute

fps feet per second

LC50 median lethal concentration

LMTD log mean temperature difference

ACRONYMS

LTA less than adequate

M&TE measuring and test equipment

MeV million electron volts

MIT Massachusetts Institute of Technology

MJ megajoule

mm millimeter

MORT management oversight risk tree

MSDS material safety data sheet

NASA National Aeronautics and Space Administration

NEHRP National Earthquake Hazards Reduction Program

NFPA National Fire Protection Association

NIST National Institute of Standards and Technology

NLGI National Lubricating Grease Institute

NNSA National Nuclear Security Administration

NPH natural phenomena hazard

NPSH net positive suction head

NQA National Quality Assurance

NRC Nuclear Regulatory Commission

OMB Office of Management and Budget

OPMO Organizational Property Management Office

ORPS Occurrence Reporting and Processing System

OSH Act Occupational Safety and Health Act

OSHA Occupational Safety and Health Administration

P&ID piping and instrumentation diagram

PAFT Programme for Alternative Fluorocarbon Toxicity Testing

PEP project execution plan

ppm parts per million

psi pounds per square inch

psia pounds per square inch absolute

psig pounds per square inch gage

ACRONYMS

PTFE polytetrafluorethylene

rad radiation absorbed dose

RAMI reliability, availability, maintainability, and inspectability

RTD resistance temperature detector

sec second

SEI Structural Engineering Institute

SSCs structures, systems, and components

TSR technical safety requirements

USDA U.S Department of Agriculture

PURPOSE

The purpose of this reference guide is to provide a document that contains the information

required for a Department of Energy (DOE)/National Nuclear Security Administration (NNSA) technical employee to successfully complete the Mechanical Systems Functional Area

Qualification Standard (FAQS) Information essential to meeting the qualification requirements

is provided; however, some competency statements require extensive knowledge or skill

development Reproducing all the required information for those statements in this document is not practical In those instances, references are included to guide the candidate to additional resources

SCOPE

This reference guide addresses the competency statements in the June 2008 edition of

DOE-STD-1161-2008, Mechanical Systems Functional Area Qualification Standard The

qualification standard contains 36 competency statements

Please direct your questions or comments related to this document to the NNSA Learning and Career Development Department

PREFACE

Competency statements and supporting knowledge and/or skill statements from the qualification standard are shown in contrasting bold type, while the corresponding information associated with each statement is provided below it

A comprehensive list of acronyms and abbreviations is found at the beginning of this document

It is recommended that the candidate review the list prior to proceeding with the competencies,

as the acronyms and abbreviations may not be further defined within the text unless special emphasis is required

The competencies and supporting knowledge, skill, and ability (KSA) statements are taken directly from the FAQS Most corrections to spelling, punctuation, and grammar have been made without remark, and all document-related titles, which variously appear in roman or italic type or set within quotation marks, have been changed to plain text, also mostly without remark

Capitalized terms are found as such in the qualification standard and remain so in this reference guide When they are needed for clarification, explanations are enclosed in brackets

Every effort has been made to provide the most current information and references available as

of September 2008 However, the candidate is advised to verify the applicability of the

information provided It is recognized that some personnel may oversee facilities that utilize predecessor documents to those identified In those cases, such documents should be included in local qualification standards via the Technical Qualification Program

In the cases where information about an FAQS topic in a competency or KSA statement is not available in the newest edition of a standard (consensus or industry), an older version is

referenced These references are noted in the text and in the bibliography

Only significant corrections to errors in the technical content of the discussion text source

material are identified Editorial changes that do not affect the technical content (e.g.,

grammatical or spelling corrections, and changes to style) appear without remark

ACKNOWLEDGEMENTS

Thanks to Del Kellogg (Pantex Site Office) for participating in the development and/or review of this reference guide, including providing some of the content for the responses to the knowledge, skill, and ability statements

TECHNICAL COMPETENCIES

General Technical

1 Mechanical systems personnel shall demonstrate a working level knowledge of

steady-state heat transfer

a Define the following terms:

Convection

Convection involves the transfer of heat by the motion and mixing of macroscopic portions

of a fluid (that is, the flow of a fluid past a solid boundary) The term “natural convection” is used if this motion and mixing is caused by density variations resulting from temperature differences within the fluid The term “forced convection” is used if this motion and mixing

is caused by an outside force, such as a pump The transfer of heat from a hot water radiator

to a room is an example of heat transfer by natural convection The transfer of heat from the surface of a heat exchanger to the bulk of a fluid being pumped through the heat exchanger is

an example of forced convection

Radiation

Radiant heat transfer involves the transfer of heat by electromagnetic radiation that arises due

to the temperature of a body Most energy of this type is in the infrared region of the

electromagnetic spectrum, although some of it is in the visible region The term “thermal radiation” is frequently used to distinguish this form of electromagnetic radiation from other forms, such as radio waves, x-rays, or gamma rays The transfer of heat from a fireplace across a room in the line of sight is an example of radiant heat transfer

Thermal Conductivity

Thermal conductivity is a measure of a substance’s ability to transfer heat through a solid by conduction

b Discuss Fourier’s law

The following is taken from DOE-HDBK-1012/2-92

In conduction heat transfer, the most common means of correlation is through Fourier’s law

of conduction The law, in its equation form, is used most often in its rectangular or

cylindrical form (pipes and cylinders), both of which are presented below:

where

Q = rate of heat transfer (Btu/hr [British thermal unit/hour])

A = cross-sectional area of heat transfer (ft2 [square feet])

Δx = thickness of slab (ft [feet])

Δr = thickness of cylindrical wall (ft)

ΔT = temperature difference (°Fahrenheit [°F])

k = thermal conductivity of slab (Btu/ft-hr-°F)

The equations are used in determining the amount of heat transferred by conduction

c Describe the factors that contribute to the coefficient of thermal conductivity

According to DOE-HDBK-1012/2-92, the heat transfer characteristics of a solid material are measured by a property called the thermal conductivity (k) measured in Btu/hr-ft-°F It is a measure of a substance’s ability to transfer heat through a solid by conduction The thermal conductivity of most liquids and solids varies with temperature For vapors, it depends upon pressure

Mandatory Performance Activities:

a Calculate the heat flux for one-dimensional, steady-state heat transfer through the following types of walls:

 Composite

 Series

 Parallel

Mandatory performance activities are performance based The Qualifying Official will evaluate the completion of this activity The following information from DOE-HDBK-1012/2-92 may be useful

Heat Flux

The rate at which heat is transferred is represented by the symbol Common units for heat transfer rate is Btu/hr Sometimes it is important to determine the heat transfer rate per unit area, or heat flux, which has the symbol  Units for heat flux are Btu/hr-ft2 The heat flux can be determined by dividing the heat transfer rate by the area through which the heat is being transferred

Q

Q

 = Q

AQ

where

Q = heat flux (Btu/hr-ft2)

Q = heat transfer rate (Btu/hr)

A = area (ft2)

b Given data, calculate total heat transfer and local heat flux in a laminar flow system

Mandatory performance activities are performance based The Qualifying Official will evaluate the completion of this activity

c Given data, calculate the log mean temperature difference for heat exchangers

Mandatory performance activities are performance based The Qualifying Official will evaluate the completion of this activity The following information from DOE-HDBK-1012/2-92 may be useful

In heat exchanger applications, the inlet and outlet temperatures are commonly specified based on the fluid in the tubes The temperature change that takes place across the heat exchanger from the entrance to the exit is not linear A precise temperature change between two fluids across the heat exchanger is best represented by the log mean temperature

difference (LMTD or Tlm), defined in the following equation:

ΔTlm =  

 22/ 11

T T

ΔT2 = the larger temperature difference between the two fluid streams at either the entrance

or the exit to the heat exchanger

ΔT1 = the smaller temperature difference between the two fluid streams at either the entrance

or the exit to the heat exchanger

2 Mechanical systems personnel shall demonstrate a working level knowledge of

The following is taken from Britannica Online Encyclopedia

Compression is a decrease in volume of any object or substance resulting from applied stress

b Discuss entropy and enthalpy as they relate to mechanical systems

The following is taken from DOE-HDBK-1012/1-92

Entropy

Entropy (represented by the letter S) is a property of a substance, as are pressure,

temperature, volume, and enthalpy Because entropy is a property, changes in it can be determined by knowing the initial and final conditions of a substance Entropy quantifies the energy of a substance that is no longer available to perform useful work Because entropy tells so much about the usefulness of an amount of heat transferred in performing work, the steam tables include values of specific entropy (s = S/m) as part of the information tabulated Entropy is sometimes referred to as a measure of the inability to do work for a given heat transferred Entropy can be defined as ΔS in the following relationships:

ΔS=

abs

T Q



ΔS = the change in entropy of a system during some process (Btu/°R)

ΔQ = the amount of heat transferred to or from the system during the process (Btu)

Tabs = the absolute temperature at which the heat was transferred (°R)

Δs = the change in specific entropy of a system during some process (Btu/pound-mass [lbm]

- °R)

Δq = the amount of heat transferred to or from the system during the process (Btu/lbm).Like enthalpy, entropy cannot be measured directly Also, like enthalpy, the entropy of a substance is given with respect to some reference value For example, the specific entropy of water or steam is given using the reference that the specific entropy of water is zero at 32 °F The fact that the absolute value of specific entropy is unknown is not a problem, because it is the change in specific entropy (s) and not the absolute value that is important in practical problems

Enthalpy

Specific enthalpy (h) is defined as h = u + Pv, where u is the specific internal energy

(Btu/lbm) of the system being studied, P is the pressure of the system (pound-force [lbf]/ft2), and v is the specific volume (cubic feet [ft3]/lbm) of the system Enthalpy is usually used in connection with an open system problem in thermodynamics Enthalpy is a property of a substance, like pressure, temperature, and volume, but it cannot be measured directly

Normally, the enthalpy of a substance is given with respect to some reference value For example, the specific enthalpy of water or steam is given using the reference that the specific enthalpy of water is zero at 01 °C and normal atmospheric pressure The fact that the

absolute value of specific enthalpy is unknown is not a problem, however, because it is the change in specific enthalpy (Δh) and not the absolute value that is important in practical problems Steam tables include values of enthalpy as part of the information tabulated

c Given a Mollier diagram, read and interpret it

Element c is performance based The Qualifying Official will evaluate its completion The following information from DOE-HDBK-1012/1-92 may be useful

The Mollier diagram shown in figure 1 is a chart on which enthalpy (h) versus entropy (s) is plotted It is sometimes known as the h-s diagram The chart contains a series of constant temperature lines, a series of constant pressure lines, a series of constant moisture or quality lines, and a series of constant superheat lines The Mollier diagram is used only when quality

is greater than 50% and for superheated steam

Source: DOE-HDBK-1012/1-92

Figure 1 Mollier diagram

d Define and discuss the following cycles:

The following is taken from DOE-HDBK-1012/1-92

With the practice of using reversible processes, Sadi Carnot in 1824 advanced the study of the second law of thermodynamics by disclosing a principle consisting of the following propositions:

 No engine can be more efficient than a reversible engine operating between the same high temperature and low temperature reservoirs Here the term heat reservoir is taken to mean either a heat source or a heat sink

 The efficiencies of all reversible engines operating between the same constant

temperature reservoirs are the same

 The efficiency of a reversible engine depends only on the temperatures of the heat source and heat receiver

The above principle is best demonstrated with a simple cycle (shown in figure 2) and an example of a proposed heat power cycle The cycle consists of the following reversible processes:

 1-2: adiabatic compression from TC to TH due to work performed on fluid

 2-3: isothermal expansion as fluid expands when heat is added to the fluid at

This cycle is known as a Carnot cycle The heat input (QH) in a Carnot cycle is graphically represented on figure 2 as the area under line 2-3 The heat rejected (QC) is graphically

represented as the area under line 1-4 The difference between the heat added and the heat rejected is the net work (sum of all work processes), which is represented as the area of rectangle 1-2-3-4

Rankine Cycle

If a fluid passes through various processes and then eventually returns to the same state it began with, the system is said to have undergone a cyclic process One such cyclic process used is the Rankine cycle The Rankine cycle is the hypothetical cycle of a steam engine in which all heat transfers take place at constant pressure and in which expansion and

compression occur adiabatically Figure 3 shows a typical steam plant cycle

Source: DOE-HDBK-1012-1-92

Figure 3 Typical steam plant cycle

The main feature of the Rankine cycle is that it confines the isentropic compression process

to only the liquid phase This minimizes the amount of work required to attain operating pressures and avoids the mechanical problems associated with pumping a two-phase mixture

Vapor-Refrigeration Cycle

The following is taken from Kettering University, “Refrigeration and Heat Pump Systems.”

In a vapor-refrigeration cycle heat is transferred to the working fluid (refrigerant) in the evaporator and then compressed by the compressor Heat is transferred from the working fluid in the condenser, and then its pressure is suddenly reduced in the expansion valve A

refrigeration cycle is used to extract energy from a fluid (air) in contact with the evaporator

It is normally assumed that kinetic and potential energy are negligible, and that the expansion process through the valve is a throttling process

Source: NASA, Glenn Research Center

Figure 4 Ideal Otto cycle

During the cycle, work is done on the gas by the piston between stages 2 and 3 Work is done by the gas on the piston between stages 4 and 5 The difference between the work done by the gas and the work done on the gas is the area enclosed by the cycle curve and is the work produced by the cycle The work multiplied by the rate of the cycle (cycles per second) is equal to the power produced by the engine

Gas Standard Cycle

The following is taken from McGraw-Hill Dictionary of Scientific and Technical Terms

A gas standard cycle is a sequence in which a gaseous fluid undergoes a series of

thermodynamic phases, ultimately returning to its original state The Brayton cycle is an example of a gas standard cycle This cycle, also called the Joule or complete expansion diesel cycle, consists of two constant-pressure, isobaric, processes interspersed with two reversible adiabatic, isentropic, processes

Mandatory Performance Activities:

a Given data from a steady-state system, calculate the following:

 Entropy change

 Enthalpy change

 Pressure

 Temperature

Mandatory performance activities are performance based The Qualifying Official will

evaluate the completion of this activity

3 Mechanical systems personnel shall demonstrate a working level knowledge of fluid mechanics

a Define the following:

Temperature

Temperature is defined as the relative measure of how hot or cold a material is It can be used

to predict the direction that heat will be transferred

Pressure

Pressure is defined as the force per unit area Common units for pressure are pounds force

per square inch (psi)

Viscosity

Viscosity is a fluid property that measures the resistance of the fluid to deforming due to a shear force Viscosity is the internal friction of a fluid, which makes it resist flowing past a solid surface or other layers of the fluid Viscosity can also be considered to be a measure of the resistance of a fluid to flowing A thick oil has a high viscosity; water has a low viscosity

Specific Volume

The specific volume of a substance is the volume per unit mass of the substance Typical units are ft3/lbm

Specific Gravity

Specific gravity is a measure of the relative density of a substance as compared to the density

of water at a standard temperature Physicists use 39.2 °F as the standard, but engineers ordinarily use 60 °F In the International System of Units, the density of water is 1.00 g/cm3

at the standard temperature Therefore, the specific gravity (which is dimensionless) for a liquid has the same numerical value as its density in units of g/cm3 Since the density of a fluid varies with temperature, specific gravities must be determined and specified at

particular temperatures

Capillarity

According to the National Oceanic and Atmospheric Administration’s National Weather Service Glossary, capillarity is the degree to which a material or object containing minute openings or passages, when immersed in a liquid, will draw the surface of the liquid above the hydrostatic level Unless otherwise defined, the liquid is generally assumed to be water

Cavitation

When the liquid being pumped enters the eye of a centrifugal pump, the pressure is

significantly reduced The greater the flow velocity through the pump the greater this

pressure drop If the pressure drop is great enough, or if the temperature of the liquid is high enough, the pressure drop may be sufficient to cause the liquid to flash to steam when the local pressure falls below the saturation pressure for the fluid that is being pumped These vapor bubbles are swept along the pump impeller with the fluid As the flow velocity

decreases the fluid pressure increases This causes the vapor bubbles to suddenly collapse on the outer portions of the impeller The formation of these vapor bubbles and their subsequent collapse is cavitation

Laminar Flow

Laminar flow is also referred to as streamline or viscous flow These terms are descriptive of the flow because, in laminar flow, (1) layers of water flow over one another at different speeds with virtually no mixing between layers, (2) fluid particles move in definite and observable paths or streamlines, and (3) the flow is characteristic of viscous (thick) fluid or is one in which viscosity of the fluid plays a significant part

Turbulent Flow

Turbulent flow is characterized by the irregular movement of particles of the fluid There is

no definite frequency as there is in wave motion The particles travel in irregular paths with

no observable pattern and no definite layers

Uniform Flow

Uniform flow and varied flow describe the changes in depth and velocity with respect to distance If the water surface is parallel to the channel bottom, flow is uniform and the water surface is at normal depth Varied flow, or non-uniform flow, occurs when depth or velocity changes over a distance, as in a constriction or over a riffle Gradually varied flow occurs when the change is small, and rapidly varied flow occurs when the change is large, for example a wave, a waterfall, or the rapid transition from a stream channel into the inlet of a culvert

Surface Tension

According to the National Institute of Standards and Technology (NIST), Surface Tension, surface tension influences the growth of bubbles in nucleate boiling and the drainage of condensate from certain enhanced condenser surfaces It is a fluid property required in many two-phase heat transfer correlations

b Describe the bulk modulus of elasticity and compressibility

The following is taken from Halliday, Resnick, Walker, Fundamentals of Physics, as found

at http://hyperphysics.phy-astr.gsu.edu/hbase/permot3.html

The bulk elastic properties of a material determine how much it will compress under a given amount of external pressure The ratio of the change in pressure to the fractional volume compression is called the bulk modulus of the material The reciprocal of the bulk modulus is called the compressibility of the substance

c Describe the effects characterized by Pascal’s law of fluid pressure

The following is taken from DOE-HDBK-1012/3-92

Liquid pressures may result from the application of external forces on the liquid Consider the following examples Figure 5 represents a container completely filled with liquid A, B,

C, D, and E represent pistons of equal cross-sectional areas fitted into the walls of the vessel There will be forces acting on the pistons C, D, and E due to the pressures caused by the different depths of the liquid Assume that the forces on the pistons due to the pressure

caused by the weight of the liquid are as follows: A = 0 lbf, B = 0 lbf, C = 10 lbf, D = 30 lbf, and E = 25 lbf Now let an external force of 50 lbf be applied to piston A This external force will cause the pressure at all points in the container to increase by the same amount Since the pistons all have the same cross-sectional area, the increase in pressure will result in the forces

on the pistons all increasing by 50 lbf So if an external force of 50 lbf is applied to piston A, the force exerted by the fluid on the other pistons will now be as follows: B = 50 lbf, C =

60 lbf, D = 80 lbf, and E = 75 lbf

Source: DOE-HDBK-1012/3-92

Figure 5 Pascal’s law

This effect of an external force on a confined fluid was first stated by Pascal in 1653

d Explain the equation of continuity as it applies to fluid flow

The following is taken from U.S Department of Agriculture (USDA) Forest Service, Stream Systems Technology Center

One of the fundamental principles used in the analysis of uniform flow is known as the continuity of flow This principle is derived from the fact that mass is always conserved in fluid systems regardless of the pipeline complexity or direction of flow

If steady flow exists in a channel and the principle of conservation of mass is applied to the system, there exists a continuity of flow such that the mean velocities at all cross sections having equal areas are then equal, and if the areas are not equal, the velocities are inversely proportional to the areas of the respective cross sections Thus if the flow is constant in a reach of channel the product of the area and velocity will be the same for any two cross sections within that reach Looking at the units of the product of area (sq ft) and velocity (feet per second [fps]) leads to the definition of flow rate (cubic feet per second [cfs]) This is expressed in the continuity equation:

Q = A1V1 = A2V2

where

Q = the volumetric flow rate

A = the cross sectional area of flow

V = the mean velocity

Calculation of flow rate is often complicated by the interdependence between flow rate and friction loss Each affects the other and often these problems need to be solved iteratively

e Discuss the Reynolds number, including how it is used

The following is taken from Britannica Online Encyclopedia

In fluid mechanics, the Reynolds number is a criterion of whether fluid (liquid or gas) flow is absolutely steady (streamlined, or laminar) or on the average steady with small unsteady fluctuations (turbulent) Whenever the Reynolds number is less than about 2,000, flow in a pipe is generally laminar, whereas, at values greater than 2,000, flow is usually turbulent The Reynolds number can be expressed as follows:

f Discuss pressurized and non-pressurized flow

According to the U.S Environmental Protection Agency (EPA) Terminology Reference System definition for the term “gravity flow,” all open channels as well as most storm

sewers, sanitary sewers, and combined sewers in which the pipes are less than completely full operate based on non-pressurized flow and are thus considered gravity systems

However, segments of a sewer system may at times flow under surcharged conditions

whereby the water level is above the crown of the pipe causing pressurized flow in these segments

g Discuss Bernoulli’s equation as it applies to steady-state flow rate calculations

The following is taken from DOE-HDBK-1012/3-92

Bernoulli’s equation results from the application of the general energy equation and the first law of thermodynamics to a steady flow system in which no work is done on or by the fluid,

no heat is transferred to or from the fluid, and no change occurs in the internal energy (i.e.,

no temperature change) of the fluid Under these conditions, the general energy equation is

z = height above reference (ft)

v = average velocity (ft/second [sec])

g = acceleration due to gravity (32.17 ft/sec2)

gc = gravitational constant (32.17 ft-lbm/lbf-sec2)

Note: The factor gc is only required when the English System of measurement is used and mass is measured in pound mass It is essentially a conversion factor needed to allow the units to come out directly No factor is necessary if mass is measured in slugs or if the metric system of measurement is used

Each term in the equation represents a form of energy possessed by a moving fluid (potential, kinetic, and pressure related energies) In essence, the equation physically represents a

balance of the KE, PE, PV energies so that if one form of energy increases, one or more of the others will decrease to compensate and vice versa

h Discuss the ideal gas law as it applies to pressure, volume, and temperature relationships

The following is taken from DOE-HDBK-1012/1-93

The results of certain experiments with gases at relatively low pressure led Robert Boyle to formulate a well-known law It states that the pressure of a gas expanding at constant

temperature varies inversely to the volume, or (P1)(V1) = (P2)(V2) = (P3)(V3) = constant

Jacques Alexandre Charles, also as the result of experimentation, concluded that the pressure

of a gas varies directly with temperature when the volume is held constant, and the volume varies directly with temperature when the pressure is held constant, or

V1/V2 = T1/T2 or P1/P2 = T1/T2

By combining the results of Charles’s and Boyle’s experiments, the relationship Pv/T

constant may be obtained The constant in the above equation is called the ideal gas constant and is designated by R Thus the ideal gas equation becomes

where the pressure and temperature are absolute values

The individual gas constant (R) may be obtained by dividing the universal gas constant (Ro)

by the molecular weight of the gas No real gases follow the ideal gas law or equation

completely At temperatures near a gas’s boiling point, increases in pressure will cause condensation and drastic decreases in volume At very high pressures, the intermolecular forces of a gas are significant However, most gases are in approximate agreement at

pressures and temperatures above their boiling point

The ideal gas law is utilized by engineers working with gases because it is simple to use and approximates real gas behavior Most physical conditions of gases used by man fit the above description Perhaps the most common use of gas behavior studied by engineers is that of the compression process using ideal gas approximations Such a compression process may occur

at constant temperature (pV = constant), constant volume, or adiabatic (no heat transfer) Whatever the process, the amount of work that results from it depends upon the process The compression process using ideal gas considerations results in work performed on the system and is essentially the area under a P-V curve (see figure 6)

Source: DOE-HDBK-1012/1-93

Figure 6 Pressure-volume diagram

i Discuss the Darcy-Weisbach equation

According to E Shashi Menon’s Piping Calculations Manual, the Darcy-Weisbach equation

is used for calculating the friction loss in fire protection piping The following form of the equation is the simplest used by engineers for a long time In this version the head loss in feet (as opposed to pressure drop in psi) is given in terms of the pipe diameter, pipe length, and the flow velocity

H = f

g D

LV

2

2

where

H = frictional head lost (ft)

f = Darcy friction factor (dimensionless)

L = pipe length (ft)

V = average flow velocity (ft/sec2)

g = acceleration due to gravity (ft/sec2)

4 Mechanical systems personnel shall demonstrate a working level knowledge of the concepts, theories, and principles of basic material science

The information for the KSAs in this competency is taken from DOE-HDBK-1012/3-92, 1015/1-93, 1017/1-93, and /2-93, unless stated otherwise

a State the five types of bonding that occur in material and the characteristics of those bonds

Ionic Bond

Ionic bonding is the type of bond where one or more electrons are wholly transferred from an atom of one element to an atom of another, and the elements are held together by the force of attraction due to the opposite polarity of the charge

Covalent Bond

A covalent bond is a bond formed by shared electrons Electrons are shared when an atom needs electrons to complete its outer shell and can share those electrons with its neighbor The electrons are then part of both atoms and both shells are filled

Metallic Bond

A metallic bond is a type of bond where the atoms do not share or exchange electrons to bond together Instead, many electrons (roughly one for each atom) are more or less free to move throughout the metal so that each electron can interact with many of the fixed atoms

Molecular Bond

A molecular bond occurs when the electrons of neutral atoms spend more time in one region

of their orbit A temporary weak charge will exist The molecule will weakly attract other molecules This is sometimes called the van der Waals or molecular bond

Hydrogen Bond

A hydrogen bond is a bond that is similar to the molecular bond and occurs due to the ease with which hydrogen atoms are willing to give up an electron to atoms of oxygen, fluorine,

or nitrogen

b Compare and contrast the properties, characteristics, and applications of

stainless steel and those of carbon steel

Stainless steel is an alloy with other metals added to it as it is made; carbon steel is iron with carbon added to it Stainless steel is stronger and less ductile, has more uses than carbon steel, and is used heavily in nuclear operations, where a more durable, corrosion-resistant metal is needed

c Discuss the process of general corrosion of iron and steel when they are exposed

to water

The oxidation and reduction half-reactions in the corrosion of iron are as follows

Fe → Fe-2 + 2e- (oxidation)

H3O + e- → H + H2O (reduction) The overall reaction is the sum of these half-reactions

Fe + 2 H3O+ → Fe-2 + 2H + 2H2O The Fe ions readily combine with OH ions at the metal surface, first forming Fe(OH), which decomposes to FeO

Fe-2 + 2OH- → Fe(OH)2 → FeO + H2O Ferrous oxide (FeO) then forms a layer on the surface of the metal Below about 1,000 °F, however, FeO is unstable and undergoes further oxidation

2FeO + H2O → Fe2O3 + 2H Atomic hydrogen then reacts to form molecular hydrogen and a layer of ferric oxide builds

up on the FeO layer

d Discuss the conditions that can cause galvanic corrosion

Galvanic corrosion occurs when two dissimilar metals with different potentials are placed in electrical contact in an electrolyte It may also take place with one metal with heterogeneities

(dissimilarities) (for example, impurity inclusions, grains of different sizes, difference in composition of grains, or differences in mechanical stress) A difference in electrical

potential exists between the different metals and serves as the driving force for electrical current flow through the corrodant or electrolyte This current results in corrosion of one of the metals The larger the potential difference, the greater the probability of galvanic

corrosion Galvanic corrosion only causes deterioration of one of the metals The less

resistant, more active one becomes the anodic (negative) corrosion site The stronger, more noble one is cathodic (positive) and protected If there were no electrical contact, the two metals would be uniformly attacked by the corrosive medium This would then be called general corrosion

e Discuss the following types of specialized corrosion:

Pitting corrosion occurs where the anodic site becomes fixed in a small area, and the

formation of holes (deep attack) in an otherwise unaffected area takes place

Stress Corrosion Cracking

Stress corrosion cracking occurs in susceptible alloys when the alloy is exposed to a specific environment when the alloy is in a stressed condition Stress corrosion cracking appears to be relatively independent of general uniform corrosion processes Thus, the extent of general corrosion can be essentially nil, and stress cracking can still occur Most pure metals are immune to this type of attack

f Explain the ion exchange process

Ion exchange is a process used extensively in nuclear facilities to control the purity and pH of water by removing undesirable ions and replacing them with acceptable ones Specifically, it

is the exchange of ions between a solid substance (called a resin) and an aqueous solution (reactor coolant or makeup water) Depending on the identity of the ions that a resin releases

to the water, the process may result in purification of water or in control of the concentration

of a particular ion in a solution An ion exchange is the reversible exchange of ions between a

liquid and a solid This process is generally used to remove undesirable ions from a liquid and substitute acceptable ions from the solid (resin)

The devices in which ion exchange occurs are commonly called demineralizers This name is derived from the term demineralize, which means the process whereby impurities present in the incoming fluid (water) are removed by exchanging impure ions with H+ and OH- ions, resulting in the formation of pure water H+ and OH- are present on the sites of resin beads contained in the demineralizer tank or column

There are two general types of ion exchange resins: those that exchange positive ions, called cation resins, and those that exchange negative ions, called anion resins A cation is an ion with a positive charge Common cations include Ca, Mg, Fe, and H A cation resin is one that exchanges positive ions An anion is an ion with a negative charge Common anions include

Cl, SO, and OH An anion resin is one that exchanges negative ions Chemically, both types are similar and belong to a group of compounds called polymers, which are extremely large molecules that are formed by the combination of many molecules of one or two compounds

in a repeating structure that produces long chains

A mixed-bed demineralizer is a vessel, usually with a volume of several cubic feet, that contains the resin Physically, ion exchange resins are formed in the shape of very small beads, called resin beads, with an average diameter of about 0.005 millimeters Wet resin has the appearance of damp, transparent, amber sand and is insoluble in water, acids, and bases Retention elements or other suitable devices in the top and bottom have openings smaller than the diameter of the resin beads The resin itself is a uniform mixture of cation and anion resins in a specific volume ratio depending on their specific gravities The ratio is normally

2 parts cation resin to 3 parts anion resin

In some cases, there may be chemical bonds formed between individual chain molecules at various points along the chain Such polymers are said to be cross-linked This type of

polymer constitutes the basic structure of ion exchange resins In particular, cross-linked polystyrene is the polymer commonly used in ion exchange resins However, chemical treatment of polystyrene is required to give it ion exchange capability, and this treatment varies depending on whether the final product is to be an anion resin or a cation resin Each such group is covalently bonded to the polymer, but each also contains an atom that is

bonded to the radical group by a predominantly ionic bond

g Discuss the following terms:

Compressibility

Compressibility is the measure of the change in volume a substance undergoes when a

pressure is exerted on the substance Liquids are generally considered to be incompressible Gases, on the other hand, are very compressible The volume of a gas can be readily changed

by exerting an external pressure on the gas

Permanent deformation takes place when a material is exposed to stress greater than its yield stress

h Given the stress-strain curves for ductile and brittle material, identify the following points on the curves:

 Proportional limit

 Ultimate strength

 Yield point

 Fracture point

Element h is performance based The Qualifying Official will evaluate its completion

i Discuss the following terms:

 Strength

 Malleability

 Ductility

Ultimate Tensile Strength

Ultimate tensile strength is the maximum resistance to fracture It is equivalent to the

maximum load that can be carried by one square inch of cross-sectional area when the load is applied as simple tension

j Describe the adverse effects of welding on metal, including the types of stress

Welding can induce internal stresses that will remain in the material after the welding is completed In stainless steels, such as type 304, the crystal lattice is face-centered cubic (FCC) (austenite) During high-temperature welding, some surrounding metal may be

elevated to between 500 F and 1,000 F In this temperature region, the austenite is

transformed into a body-centered cubic (BCC) lattice structure (bainite) When the metal has cooled, regions surrounding the weld contain some original austenite and some newly formed bainite A problem arises because the “packing factor” (PF = volume of atoms/volume of unit cell) is not the same for FCC crystals as for BCC crystals The bainite that has been formed occupies more space than the original austenite lattice This elongation of the material causes residual compressive and tensile stresses in the material Welding stresses can be minimized by using heat sink welding, which results in lower metal temperatures, and by annealing

k Discuss the phenomenon of thermal shock

Thermal shock can lead to excessive thermal gradients on materials, which lead to excessive stresses These stresses can be comprised of tensile stress, which is stress arising from forces acting in opposite directions tending to pull a material apart, and compressive stress, which is stress arising from forces acting in opposite directions tending to push a material together These stresses, cyclic in nature, can lead to fatigue failure of the materials Thermal shock is caused by nonuniform heating or cooling of a uniform material, or uniform heating of

nonuniform materials Suppose a body is heated and constrained so that it cannot expand When the temperature of the material increases, the increased activity of the molecules causes them to press against the constraining boundaries, thus setting up thermal stresses

l Discuss the following terms, including their relationship to material failure:

 Ductile fracture

 Brittle fracture

 Nil-ductility transition (NDT) temperature

Ductile Fracture

Metals can fail by ductile or brittle fracture Metals that can sustain substantial plastic strain

or deformation before fracturing exhibit ductile fracture Usually, a large part of the plastic flow is concentrated near the fracture faces

Brittle Fracture

Metals that fracture with a relatively small or negligible amount of plastic strain exhibit brittle fracture Cracks propagate rapidly Brittle failure results from cleavage (splitting along definite planes) Ductile fracture is better than brittle fracture because ductile fracture occurs over a period of time, whereas brittle fracture is fast and can occur (with flaws) at lower stress levels than a ductile fracture

Nil-Ductility Transition Temperature

The NDT temperature, which is the temperature at which a given metal changes from ductile

to brittle fracture, is often markedly increased by neutron irradiation The increase in the NDT temperature is one of the most important effects of irradiation from the standpoint of nuclear power system design

m Explain fatigue failure and work hardening with respect to material failure

Fatigue Failure

Fatigue failure is defined as the tendency of a material to fracture by means of progressive, brittle cracking under repeated alternating or cyclic stresses of intensity considerably below the normal strength Although the fracture is of a brittle type, it may take some time to

propagate, depending on both the intensity and frequency of the stress cycles Nevertheless, there is very little, if any, warning before failure if the crack is not noticed The number of cycles required to cause fatigue failure at a particular peak stress is generally quite large, but

it decreases as the stress is increased For some mild steels, cyclical stresses can be continued

indefinitely provided the peak stress (sometimes called fatigue strength) is below the

endurance limit value

Work Hardening

Work hardening occurs when a metal is strained beyond the yield point An increasing stress

is required to produce additional plastic deformation, and the metal apparently becomes stronger and more difficult to deform Therefore, a metal with a high shear modulus will have a high strain or work hardening coefficient Grain size will also influence strain

hardening A material with small grain size will strain harden more rapidly than the same material with a larger grain size However, the effect only applies in the early stages of plastic deformation, and the influence disappears as the structure deforms and grain structure breaks down Work hardening is closely related to fatigue This is the result of work or strain hardening Work hardening reduces ductility, which increases the chances of brittle failure Work hardening can also be used to treat material Prior work hardening (cold working) causes the treated material to have an apparently higher yield stress Therefore, the metal is strengthened

n Discuss the effects of radiation on the structural integrity of metals

Incident gamma and beta radiation cause very little damage in metals, but will break the chemical bonds and prevent bond recombination of organic compounds and cause permanent damage Ionization is the major damage mechanism in organic compounds Ionization effects are caused by the passage through a material of gamma rays or charged particles such as beta and alpha particles Even fast neutrons, producing fast protons on collision, lead to ionization

as a major damage mechanism For thermal neutrons, the major effect is through (n,gamma) reactions with hydrogen, with the 2.2 MeV gamma producing energetic electrons, and

ionization Ionization is particularly important with materials that have either ionic or

covalent bonding

Ion production within a chemical compound is accomplished by the breaking of chemical bonds This radiation-induced decomposition prevents the use of many compounds in a reactor environment Materials such as insulators, dielectrics, plastics, lubricants, hydraulic fluids, and rubber are among those that are sensitive to ionization Plastics with long-chain-type molecules having varying amounts of cross-linking may have sharp changes in

properties due to irradiation In general, plastics suffer varying degrees of loss in their

properties after exposure to high radiation fields Nylon begins to suffer degradation of its toughness at relatively low doses, but suffers little loss in strength

High-density (linear) polyethylene marlex 50 loses both strength and ductility at relatively low doses In general, rubber will harden upon being irradiated However, butyl or Thiokol rubber will soften or become liquid with high radiation doses

Oils and greases should be evaluated for their resistance to radiation if they are to be

employed in a high-radiation environment Liquids that have the aromatic ring-type structure show an inherent radiation resistance and are well suited to be used as lubricants or hydraulic fluids

For a given gamma flux, the degree of decomposition observed depends on the type of chemical bonding present The chemical bond with the least resistance to decomposition is the covalent bond In a covalent bond, the outer, or valence, electrons are shared by two atoms rather than being firmly attached to any one atom Organic compounds, and some inorganic compounds such as water, exhibit this type of bonding There is considerable variation in the strength of covalent bonds present in compounds of different types and therefore a wide variation in their stability under radiation The plastics discussed above can show very sharp property changes with radiation, whereas polyphenyls are reasonably stable One result of ionization is that smaller hydrocarbon chains will be formed (lighter

hydrocarbons and gases) as well as heavier hydrocarbons by recombination of broken chains into larger ones This recombination of broken hydrocarbon chains into longer ones is called

polymerization

Polymerization is one of the chemical reactions that take place in organic compounds during irradiation and is responsible for changes in the properties of this material Some other chemical reactions in organic compounds that can be caused by radiation are oxidation, halogenation, and changes in isomerism The polymerization mechanism is used in some industrial applications to change the character of plastics after they are in place; for example, wood is impregnated with a light plastic and then cross-bonded (polymerized) by irradiating

it to make it more sturdy This change in properties, whether it be a lubricant, electrical insulation, or gaskets, is of concern when choosing materials for use near nuclear reactors One of the results of the Three Mile Island accident is that utilities have been asked to

evaluate whether instrumentation would function in the event of radiation exposure being spread because of an accident

Because neutrons and gamma rays (and other nuclear radiations) produce the same kind of decomposition in organic compounds, it is common to express the effects as a function of the energy absorbed One way is to state the energy in terms of a unit called the rad (radiation absorbed dose) The rad represents an energy absorption of 100 ergs per gram of material As

an example of the effects of radiation, figure 7 shows the increase in viscosity with radiation exposure (in rads) of three organic compounds that might be considered for use as reactor moderators and coolants

The ordinates represent the viscosity increase relative to that of the material before

irradiation (mostly at 100 F), so that they give a general indication of the extent of

decomposition due to radiation exposure This figure illustrates that aromatic hydrocarbons (n-butyl benzene) are more resistant to radiation damage than are aliphatic compounds (hexadecane) The most resistant of all are the polyphenyls, of which diphenyl is the simplest example

The stability of organic (and other covalent) compounds to radiation is frequently expressed

by means of the “G” value, which is equal to the number of molecules decomposed, or of product formed, per 100 eV of energy dissipated in the material As an example of the use of

G values, the data in table 1 are for a number of polyphenyls exposed to the radiation in a thermal reactor

Source: DOE-HDBK-1017/2-93

Figure 7 Effects of gamma radiation on different types of hydrocarbons

Table 1 shows the number of gas molecules produced, G (gas), and the number of

polyphenyl molecules, G (polymer), used to produce higher polymers per 100 eV of energy

deposited in the material Note that this adds up to approximately 1,000 atoms of gas and

10,000 atoms forming higher polymers per each 1 MeV particle It is also of interest to note

that the terphenyls are even more resistant to radiation than diphenyl and, since they have a

higher boiling point, a mixture of terphenyls with a relatively low melting temperature was

chosen as the moderator coolant in organic-moderated reactors

Table 1 Radiolytic decomposition of polyphenyls at 350 °C

Ortho-terphenyl 0.108 0.70 Meta-terphenyl 0.081 0.64 Para-terphenyl 0.073 0.54

Notes:

* A mixture of the three terphenyls plus a small amount of diphenyl

Source: DOE-HDBK-1017/2-93

o Discuss the need for fracture mechanics and the use of associated mathematical relations

The following is taken from Introduction to Fracture Mechanics, D Roylance,

Massachusetts Institute of Technology (MIT)

The strength of structural metals – particularly steel – can be increased to very high levels by manipulating the microstructure so as to inhibit dislocation motion Unfortunately, this renders the material increasingly brittle, so that cracks can form and propagate

catastrophically with very little warning An unfortunate number of engineering disasters are related directly to this phenomenon, and engineers involved in structural design must be aware of the procedures now available to safeguard against brittle fracture

The central difficulty in designing against fracture in high-strength materials is that the presence of cracks can modify the local stresses to such an extent that the elastic stress analyses done so carefully by the designers are insufficient When a crack reaches a certain critical length, it can propagate catastrophically through the structure, even though the gross stress is much less than would normally cause yield or failure in a tensile specimen The term

“fracture mechanics” refers to a vital specialization within solid mechanics in which the presence of a crack is assumed, and we wish to find quantitative relations between the crack length, the material’s inherent resistance to crack growth, and the stress at which the crack propagates at high speed to cause structural failure

When A A Griffith (1893–1963) began his pioneering studies of fracture in glass in the years just prior to 1920, he was aware of C E Inglis’s work in calculating the stress

concentrations around elliptical holes, and naturally considered how it might be used in developing a fundamental approach to predicting fracture strengths However, the Inglis solution poses a mathematical difficulty: in the limit of a perfectly sharp crack, the stresses approach infinity at the crack tip This is obviously nonphysical (actually the material

generally undergoes some local yielding to blunt the crack tip), and using such a result would predict that materials would have near zero strength: even for very small applied loads, the stresses near crack tips would become infinite, and the bonds there would rupture Rather than focusing on the crack-tip stresses directly, Griffith employed an energy-balance

approach that has become one of the most famous developments in materials science

The strain energy per unit volume of stressed material is

U =  fdx V

1

=

L

dx A

f

If the material is linear (σ = E), then the strain energy per unit volume is

U = 2

2



 =

2

2



When a crack has grown into a solid to a depth a, a region of material adjacent to the free

surfaces is unloaded, and its strain energy released Using the Inglis solution, Griffith was able to compute just how much energy this is

A simple way of visualizing this energy release, illustrated in figure 8, is to regard two

triangular regions near the crack flanks, of width a and height βa, as being completely

unloaded, while the remaining material continues to feel the full stress σ

Source: D Roylance, MIT, Introduction to Fracture Mechanics

Figure 8. Idealization of unloaded region near crack flanks The parameter β can be selected so as to agree with the Inglis solution, and it turns out that for plane stress loading β = π The total strain energy U released is then the strain energy per unit volume times the volume in both triangular regions:

U =

2

in forming the crack, bonds must be broken, and the requisite bond energy is in effect

absorbed by the material The surface energy S associated with a crack of length a (and unit

depth) is

S = 2 a

where γ is the surface energy (e.g., Joules/meter2) and the factor 2 is needed since two free surfaces have been formed As shown in figure 9, the total energy associated with the crack is then the sum of the (positive) energy absorbed to create the new surfaces, plus the (negative) strain energy liberated by allowing the regions near the crack flanks to become unloaded