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High Temperature Material Properties for Power Systems Richard N. Wright High Temperature Materials R&D Technical Lead May 2012

Nuclear Power Electricity Generation - Sustainable Energy? • Electricity from nuclear power

provides low CO2 baseload • New plants are under construction –

the majority of the US fleet was built before 1990

• Cheap natural gas and high capital cost work against new nuclear capacity

• Post-Yucca Mountain study concludes nuclear waste storage/disposal is political not technical issue

• To apply materials for nuclear power we must know enough to support standards (ASTM), codes (ASME) and regulatory (NRC) requirements

Beyond Electricity – Applications of HTGRs: Any Interesting Materials Research?

• High Temperature Gas Reactors can provide energy that supports the spectrum of industrial applications including the petrochemical and petroleum industries

• High temperatures and long service lives re-open a host of materials questions that were first addressed fifty or more years ago

Design Data Needs for New Subsection NH Material

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Relaxation Strengths • Provide limits to ensure that

shakedown takes place so ratcheting does not occur

• Stress relaxation test data from 50°F below the creep threshold to 100°F above the maximum use temperature at 100°F intervals

• Alternatively, use general constitutive equations, or special equations for uniaxial stress relaxation, to perform a pure uniaxial relaxation analysis to determine the relaxation strengths

Huddleston Multiaxial Failure Criterion • Needed for using

inelastic analysis approach for creep-fatigue analysis

• Require creep-rupture test data under different combinations of tension, torsion and internal pressurization to determine the Huddleston constants

• Close to 100 high temperature materials in non-nuclear sections of ASME code

• Only five in Subsection NH • Significantly more data, and types of data, are

required for Subsection NH applications

NGNP Design Drivers • The NGNP is big: reference

pressure vessel design is on the order of 8m diameter by 23m tall and up to 200mm thick

• The vessel can operate hot – up to 500 C and must not creep

• The heat exchanger operates above 850 C

• The desired license period is 60 years (500,000 hours)

Control Rod Drive Assembly

Refueling Stand Pipe

Control Rod Guide tubes

Cold leg Core Coolant Upper Plenum

Central Reflector Graphite

Annular shaped Active Core

Outer Side Reflector Graphite

Core Exit Hot Gas Plenum

Graphite Core Support Columns

Reactor Vessel

Upper Plenum Shroud

Shutdown Cooling System Module Hot Duct

Insulation Module

Cross Vessel Nipple

Hot Duct Structural Element

Metallic Core Support Structure

Core Inlet Flow

Core Outlet Flow

Insulation Layer for Metallic Core Support Plate

Upper Core Restraint Structure

Control Rods

7m(23 ft)

23.7m(78ft)

2.2m(7ft)

8.2m(27ft) Dia Vessel Flange

ASME Code Rules for Conventional Pressure Vessel Steel • Code Section NB

allows A508/533 steel up to 371 C with negligible creep

• Code Case 499 established time dependent stresses which indicate significant creep below NB limits

A533B - Code Case N-499 Database

100

125

150

175

200

225

250

340 345 350 355 360 365 370 375 380 385 390

Temperature (C)

Prim

ary

Stre

ss L

imits

(MP

a )

St @ 600,000 hrSt @ 300,000 hrSt @ 100,000 hrTime Independent Primary Stress, Sm

A508/533 RPV Steel – Damage + Creep Rupture • LCF damage treatment: 427°C, 180 cycles, 1% strain

range, hold time = 0, 10, 30 min.

Solar Thermal Electricity: Creep-Fatigue • Solar energy is focused on a steam generator that spins a conventional

turbine to produce electricity • Steam generator is thermally cycled at dawn and dusk – but also every time a

cloud passes • Creep-fatigue of steam generator tubes is life limiting in the design • Creep-fatigue is also life limiting for NGNP heat exchanger

Creep-Fatigue is Typically More Damaging Than Superposition of Creep and Fatigue Mechanisms

Creep-fatigue interaction

Fatigue dominated

Creep dominated

Laboratory test cycle Cartoon of creep-fatigue in austenitic stainless steel

Creep Behavior of Alloy 617

• Alloy 617 creep behavior not well described by conventional model – prolonged primary creep and inflection at minimum creep rate

• Creep cavitation is not initiated until strain >10%

Textbook Stainless Steel Alloy 617

0 5

10 15 20 25 30 35 40 45 50

0 1000 2000 3000 4000

Cre

ep S

trai

n, %

Creep Time, hours

Aging Results in Microstructural Instability

Stress Axis

•Thermal aging results in precipitation of additional phases

•Oxidation causes redistribution of phases

• Aging under load results in precipitate redistribution and cavitation

• The same instability occurs in solder joints in electronics near room temperature

Crept Specimen Specimen aged in oxidizing environment

Low Cycle Fatigue Behavior • Fatigue data from the current experiments are in good agreement with

previously published data for Alloy 617 compiled from numerous sources

5 5

Temperature Effect on Cycles to Failure • Temperature has a small influence on the fatigue life for the conditions

tested • A crossover occurs at about 1% total strain range

– Low strain ranges→long total test times →significant environmental effect→increased oxidation kinetics at 950°C →shorter fatigue lives

– High strain ranges→ greater deformation→increased ductility at 950°C →longer fatigue lives

5

Coffin-Manson Relation • Log-log plot of inelastic strain range as a function of cycles to failure

typically results in a straight line with a slope of -0.5 • For Alloy 617 the slope is closer to -1.0 • Deformation in the creep regime or the environmental component of

crack growth may account for the different slope

Dynamic Strain Aging of Alloy 617 • Serrated yielding observed for Alloy 617 in temperature regime of

750ºC to 850ºC • Serrated flow is associated with reduced ductility • Serrated yielding introduces creep-fatigue testing challenges • Mechanism and potential mitigation yet to be determined

800°C Tensile Stress Strain Curves

Flow Stress of Alloy 617 – Strain Rate Sensitivity

Strain rate jumps from 10-6 to 10-2 s-1 Low Cycle Fatigue Tests – Hysteresis Loops, 2% Total Strain, 950°C

Influence of Tensile Hold Time on Cycles to Failure 950 C

• Addition of a hold time during tensile part of fully reversed cycle significantly reduces cycles to failure – increasing the hold time past some minimum has little or no further effect

0 200 400 600 800 1000 12000

50

100

150

200

250 p y

Time (Seconds)

Stre

ss (M

Pa)

G-53 after 1e-3 Strain RateG-53 after 1e-4 Strain RateG-53 after 1e-5 Strain RateG-57 after 1e-3 Strain RateG-57 after 1e-4 Strain RateG-57 after 1e-5 Strain Rate

σ = 122.1061 *(t + 0.1214)-0.2457

Creep-Fatigue Interaction Diagram for Alloy 617 • Effort to verify Alloy 617 Draft

Code Case D-Diagram recommendation

• Interaction curve is constructed as an average curve of the creep-fatigue failure data

∆+ ≤

∑ ∑

Creep DamageCyclic Damage

j kd dj k

n t DN T

Carroll, J. Wright, Sham (2010)

Creep-fatigue data in the “creep-dominant” part of the D-diagram are needed

Summary • Things We Think We Know

– Ni based alloys behave differently than Type 304/316 stainless and will need new Code Rules

– We will have to address impact of welding and heat affected zone on properties to Code qualify alloys

– Strain rate has a large effect on flow stress; stress relaxation is rapid

– There will be unexpected failure mechanisms

• Things We Need to Learn – How do we model creep/fatigue – What is the mechanism and impact of dynamic strain aging – Is there some clever way to examine negligible creep issue