03/15/2017 maintaining safety in nuclear components - nrc. · maintaining safety in nuclear...

Maintaining Safety inNuclear Components

Rob TregoningMark Kirk

Christopher HovanecMatthew Mitchell

Nuclear Regulatory Commission

Regulatory Information ConferenceMarch 15, 2017

Maintaining Safety in Nuclear Components:Outline and Objectives

• Summarize the nexus of the three physical attributes (i.e., stress, cracks, and material toughness) that is required for component failure

• Discuss the attributes that exist in the beltline region of the reactor pressure vessel (RPV) shell

• Compare the likelihood of failure in the RPV shell with failure in RPV head or steam generator (SG) channel head components, which may be affected by carbon macrosegregation

• Summarize the measures intended to prevent failures in the RPV shell and discuss how several of these measures also protect against RPV and SG head failures

• Identify current and planned NRC activities to assess the significance of carbon macrosegregation in U.S. plants and continue to provide assurance of RPV integrity

• Principal conclusion: The safety significance to U.S. plants due to carbon macrosegregation appears to be negligible based on knowledge of the U.S. material qualification process, qualitative analysis, and the results of preliminary structural evaluations.

Attributes of Component Failure

High Stress

Deep Crack

Low Toughness

Component Failure: Critical combination of three attributes

Bel

tlin

e

Photo Credit: https://www.asme.org/about-asme/who-we-are/engineering-history/landmarks/47-shippingport-nuclear-

power-station

Possible Failure Location:Reactor Pressure Vessel (RPV) Beltline

• U.S. focus over last 50+ years has been guarding against failures in the RPV shell at the beltline, near the inner vessel wall

Image credit: http://jolisfukyu.tokai-sc.jaea.go.jp/fukyu/mirai-en/2012/6_2.html

Pressurized Thermal Shock



• Why is this location of concern?– Stresses

• Generally higher in shell, especially during postulated accident conditions (e.g., pressurized thermal shock)

Crack Distribution in Thick-Section Steels





– Cracks• Welds used to join plates and forgings

have a higher likelihood of fabrication defects

Temperature

Toug

hnes

s

Initial

Irradiated

∆RTNDT





– Cracks• Welds used to join plates and forgings

have a higher likelihood of fabrication defects

– Material toughness• Decreases over time due to radiation

embrittlement• Radiation embrittlement is greatest at the

inner wall• Carbon macrosegregation is less likely in

the shell and is not expected to contribute to reduced toughness

RPV Head and SG Channel Head• Failure Location: Near outer surface• Highest stresses: Start-up or heat-up• Normal operations: Start-up

– Pressure stresses – Similar by design– Thermal stresses – Mixing near head

reduces temperature gradient– Cladding stresses – Insignificant near

outer surface– Residual stresses – No weld stresses

• Hypothetical accident scenarios – Stresses remain low near the outer

component surface

• Effect of Carbon Macrosegregation– Insignificant effect on either the global

or local stresses within components

RPV Shell in Beltline Region• Failure location: Near inner vessel wall• Highest stresses: Shutdown or cooldown• Normal operations: Shutdown

– Pressure stresses – Similar by design – Thermal stresses – Thicker component

and steeper temperature gradient – Cladding stresses – Only significant

near the cladding interface– Residual stresses – Additional

contribution to stresses near inner wall in welds

• Hypothetical accident scenarios– More severe cooldown events– Increases the thermal and cladding

stresses compared to normal cooldown transients.

Comparison of Possible Failure Locations:Stresses

≈

Crack Distribution in Thick-Section Steels

Comparison of Possible Failure Locations:Cracks

RPV Shell in Beltline Region• Failure location: Near inner vessel wall• Welds

– Significantly greater crack density and crack size

• Plates and forgings– Similar flaw distributions– Categorized as base metal flaws

• Near cladding interface– Small flaws can form in the cladding

heat affected zone

RPV Head and SG Channel Head• Failure Location: Near outer surface• Welds

– No welds in head component where carbon macrosegation may occur

• Plates and forgings– Flaw distribution near inner and outer

surfaces is expected to be similar• Near cladding interface

– No cladding near outer surface

• Effect of Carbon Macrosegregation– Not expected to significantly affect

fabrication flaw distribution

Comparison of Possible Failure Locations:Cracks

RPV Shell in Beltline Region• Failure location: Near inner vessel wall• Welds

– Significantly greater crack density and crack size

• Plates and forgings– Similar flaw distributions– Categorized as base metal flaws

• Near cladding interface– Small flaws can form in the cladding

heat affected zone

≈

RPV Head and SG Channel Head• Failure Location: Near outer surface• Initial material toughness

– Decreases as carbon level increases– Limited data indicates that 60oF RTNDT

shift may be possible for every 0.1% increase in carbon above nominal levels

Comparison of Possible Failure Locations:Material Toughness

Car

bon

(wt%

)To

ughn

ess

(J)

Thickness

Toughness Measurements at 0oC

Image obtained from CODEP-DEP-2016-0019209




RPV Shell in Beltline Region• Failure location: Near inner vessel wall• Initial material toughness

– Unaffected by carbon segregation


Toughness as a Function of Operation Time

23 46 60 (Years of Reactor Operation)

0

Surveillance Data

Trend Curves

RPV Shell in Beltline Region• Failure location: Near inner vessel wall

• Toughness degradation during service– Irradiation embrittlement

• Maximum in beltline where fluence is highest

• Accumulates quickly during early years of reactor operation


RPV Shell in Beltline Region• Failure location: Near inner vessel wall




• RTNDT can shift by more than 200oF by 60 years of operation

• However, even large RTNDT shifts are unlikely to cause RPV failure


0

50

100

150

200

250

300

350

0 10 20 30 40 50 60 70 80∆R

T NDT

(o F)

Plant Number

60 years of operation

Possible shift due to Max Carbon of 0.3% to 0.4%

< 1x10-6 yr-1 failure likelihood due to PTS





• Toughness decreases as a function of temperature, fluence, and other material constituents (e.g., Cu, Ni, P, Mn), but is not affected by carbon levels

• Insignificant effect due to low fluenceaccumulated during service

RPV Shell in Beltline Region• Failure location: Near inner vessel wall• Initial material toughness

– Unaffected by carbon segregation




• RTNDT can shift by more than 200oF by 60 years of operation

• However, even large RTNDT shifts are unlikely to cause RPV failure


RPV Head and SG Channel Head• Failure Location: Near outer surface• Stresses

– Reactor start-up and accident scenarios

• Cracks– Base material

• Material toughness– Initial toughness may be lower but there

is insignificant additional toughness degradation as components age

Comparison of Possible Failure Locations:Summary

RPV Shell in Beltline Region• Failure location: Near inner vessel wall• Stresses

– Reactor shutdown and accident scenarios

• Cracks– Welds, base material and near cladding

interface • Material toughness

– Initial toughness may be higher but toughness degrades rapidly as the RPV ages

• Summary– The location with the greatest likelihood

of failure is the RPV shell within the beltline region.

– However, failure at this location is highly unlikely

Comparison of Possible Failure Locations:Failure Prevention Measures

RPV Shell in Beltline Region• U.S. regulations provide reasonable

assurance that failures will not occur– Design requirements

• Limit operating stresses and stresses associated with hypothetical accidents

– Preservice fabrication and inspection• Ensure initial material properties are adequate • Inspect for presence of unacceptable cracks

– Inservice inspection • Periodically inspect for cracks over plant life

– Surveillance capsule monitoring• Periodically test properties over plant life

– Operating restrictions• Restrict allowable combination of pressure and

temperature during operations• Require toughness monitoring and

assessment to guard against failure due to hypothetical accidents

Photo credit: http://www.wermac.org/others/ndt_pressure_testing.html

Hydrotest of Pressure Vessel

Nozzle Failure

Comparison of Possible Failure Locations:Failure Prevention Measures

– Design requirements• Limit operating stresses and stresses

associated with hypothetical accidents

– Preservice fabrication and inspection• Ensure initial material properties are adequate • Inspect for presence of unacceptable cracks

– Operating restrictions• Restrict allowable combination of pressure and

temperature during operations• Require toughness monitoring and

assessment to guard against failure due to hypothetical accidents

RPV Head and SG Channel Head• Components protected by measures

adopted to prevent RPV shell failures

Maintaining Safety in Nuclear Components:Current and Planned NRC Activities

• RPV and SG Heads: Carbon Macrosegregation– Monitor the French investigation

• Mapping carbon levels in various affected components• Measuring initial fracture toughness as a function of carbon content

– Have identified the U.S. components forged at Creusot Forge– Have not identified any U.S. components forged at Japan Casting and

Forging Corporation (JCFC)– Continue to accumulate material processing and mechanical property

data– Finalize safety assessment once all this information is available

• RPV Shell: Radiation Embrittlement– Continue to monitor radiation embrittlement of critical beltline materials at

each plant, over its entire operating life– Continue to ensure that existing regulations provide reasonable

assurance that a failure within the beltline region will not occur

Maintaining Safety in Nuclear Components: Summary

• Component failure requires the confluence of low toughness, high stresses, and the presence of a critical flaw(s)

• Carbon macrosegregation degrades the material’s initial toughness while minimally impacting any toughness degradation that may occur due to aging during the plant’s operating life

• Carbon macrosegregation may be present in RPV and SG channel head components but is not expected to exist in RPV shell components

• RPV and SG channel heads have lower stresses, less likelihood of cracking, and higher material toughness as the plant ages than the beltline region of the RPV shell. Because an RPV shell failure is very unlikely, these attributes imply that an RPV or SG head failure is even less likely.

• Existing regulations intended to prevent failure in the RPV shell also provide adequate protection for components that may be affected by carbon segregation

• Research and evaluation will continue to evaluate the severity of carbon segregation and ensure the adequacy of existing regulations over the life of the nuclear plants

03/15/2017 maintaining safety in nuclear components - nrc. · maintaining safety in nuclear...

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