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
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
• 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
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
• Why is this location of concern?– Stresses
• Generally higher in shell, especially during postulated accident conditions (e.g., pressurized thermal shock)
– Cracks• Welds used to join plates and forgings
have a higher likelihood of fabrication defects
Temperature
Toug
hnes
s
Initial
Irradiated
∆RTNDT
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
• Why is this location of concern?– Stresses
• Generally higher in shell, especially during postulated accident conditions (e.g., pressurized thermal shock)
– 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 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
RPV Shell in Beltline Region• Failure location: Near inner vessel wall• Initial material toughness
– Unaffected by carbon segregation
Comparison of Possible Failure Locations:Material Toughness
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
Comparison of Possible Failure Locations:Material Toughness
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
• RTNDT can shift by more than 200oF by 60 years of operation
• However, even large RTNDT shifts are unlikely to cause RPV failure
Comparison of Possible Failure Locations:Material Toughness
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
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
• Toughness degradation during service– Irradiation embrittlement
• 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
• Toughness degradation during service– Irradiation embrittlement
• Maximum in beltline where fluence is highest
• Accumulates quickly during early years of reactor operation
• RTNDT can shift by more than 200oF by 60 years of operation
• However, even large RTNDT shifts are unlikely to cause RPV failure
Comparison of Possible Failure Locations:Material Toughness
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