Methodology research on prediction for operating lifetime

Methodology research on prediction Methodology research on prediction for operating lifetime of PWR RPVfor operating lifetime of PWR RPV

Shanghai Nuclear Engineering Research and Shanghai Nuclear Engineering Research and Design Institute (SNERDI), Shanghai, ChinaDesign Institute (SNERDI), Shanghai, China

Oct 2007, ShanghaiOct 2007, ShanghaiOct 2007, ShanghaiOct 2007, ShanghaiOct 2007, ShanghaiOct 2007, ShanghaiOct 2007, ShanghaiOct 2007, Shanghai

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上海核工程研究设计院上海核工程研究设计院上海核工程研究设计院上海核工程研究设计院

Shanghai Nuclear Engineering Research and Design Institute

• Introduction• Codes, Regulations and Working Scheme• Determination of increment of RT• Pressure – temperature limits• Pressurized thermal shock• Charpy upper shelf energy • Residual lifetime assessment• Conclusions

Contents

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Contents

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• The residual lifetime of the operating PWR NPP is always concerned by the owners and the RPV plays a critical role in the lifetime prediction of PWR NPP due to irradiation induced embrittlement of the active core beltline and its non-replaceability.

• The PWR NPP in question (we call it as specific NPP) have been operating over half of the initial design lifetime and overall survey and investigation focusing on the Time Limited Aging Analysis (TLAA) for the RPV have been carried out.

Introduction

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• It has been found that the irradiation embrittlement is the most significant aging mechanism comparing with others, for example the fatigue damage etc.

• The research were focusing on:�Shift RT: R.G 1.99�P-T limits: App.G to ASME XI and App.G to 10CFR50�PTS: IAEA-EBP-WWER-08, 10CFR50.61�USE: App.G to 10CFR50

Introduction

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Contents

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• The specific PWR NPP was mainly designed, manufactured, constructed and inspected in accordance with the US codes or equivalent ones.

• The assessment of RPV integrity and of its residual lifetime prediction were performed following the relevant national nuclear safety laws, US CFR, NRC R.G, ASME Code and IAEA guidelines.

Codes, Regulations and Working Scheme

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• The increment of reference temperature (ΔΔΔΔRTNDT) is a part of appearances of embrittled RPV material due to neutron irradiation.

• There are two approaches to calculate ΔΔΔΔRTNDT:�approach recommended in R.G 1.99 Rev.2 using the chemistry factor listed in the tables. �for the specific plants there are more than two withdrawn capsule specimens, the specific approach is available for getting the ΔΔΔΔRTNDT related to the fast neutron (E > 1MeV) fluence and Charpy tests of the capsule specimens.

Determination of increment of RT

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• Approach using RG table


ART=RTNDT(U)+ ΔRTNDT+M(0.28 0.10log )( ) f

NDTRT CF f −∆ =2 22 UM σ σ∆= +

CF is chemistry factor related to copper and nickel content and given in R.G 1.99. The maximum fluence on inner surface of RPV at EOL: f = 4.97×1019 n/cm2 and ART=32℃

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• Approach using surveillance results


Ai is the ΔT41J for capsule i;fi is the fluence for capsule i.The ART in EOL (or RTPTS) is 24℃ using the parameters from capsule specimen tests. It is observed that the ARTusing CF table in R.G 1.99 is higher than that from specific plant tests and more conservative.

(0.28 0.10log )

1(0.56 0.20log )

1

i

i

nf

i ii

nf

ii

A fCF

f

−

=

−

=

× =

∑∑

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• The 10CFR50.60 put forward the acceptance criteria for fracture prevention measures for NPPs during normal operation. Appendix G to 10CFR50 provides the general basis for pressure – temperature limits and requires that P-T limits must be at least as conservative as the value obtained according to Appendix G to Section XI of the ASME Code.

Pressure – temperature limits

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• The specific RPV was designed about 20 years ago in accordance with codes and regulations of the version of that time. P-T limits were compiled following Appendix G to Section III of the ASME Code and KIRwas used for the fracture estimation.

• The P-T limits for the specific RPV have been recalculated using KIC as per appendix G to Section XI of the ASME Code and the comparison has been made.

• It is noticed that the new curve is on the left side of the old one around 15 ℃℃℃℃ as shown in the next slide.


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15ºC

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• The P-T limits from KIC is especially beneficial for the NPP operation during approaching designed EOL and extended lifetime.

• The new operating window can be derived combining the P-T limits of RPV, saturation curve and operation limits for RCP etc.

• The revised P-T limits can be put into use after getting national nuclear regulatory body’s permission.


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• The projected values of RTPTS have been determined for the beltline of the specific NPP RPV in previous slides and are less than the screening criterion of 132 ℃℃℃℃ specified by 10CFR50.61.

• That means the risk due to PTS events is acceptable for US NRC if the expected penetration frequency (probability) of through-wall crack is less than 5××××10-6 per reactor year.

Pressurized thermal shock

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• The problem is that the regulation for PTS has not been established in China until now.

• After the detailed investigation and research it will be determined whether the USA screening criteria are fully available for the RPV of NPP in china.

• Facing this status in China, the PTS analysis procedures recommended by IAEA are employed to the specific RPV for PTS analysis.


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models and methodsmodels and methods• There are no welds in the active core beltline of the

shell of RPV.• The axisymmetrical loading conditions are assumed

without considering the plume effect.• The through cladding axial surface semi-elliptical

was postulated in the beltline wall. • The flaw depth is taken as 1/10 of wall thickness and

the aspect ratio is one third. The illustration sketch is shown in next slides.


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models and methodsmodels and methods


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Material propertiesMaterial properties• Since the lead factors of capsule specimens are not

great than two and the fluences of all the withdrawn specimens have not reached the level equivalent to the fluence in EOL yet till now.

• The postulated irradiated materials are applied to the PTS structural analysis assuming the yield strength of beltline will enhance by 20% in EOL based on the tested material properties of initial unirradiatedspecimens.

• The fracture toughness of beltline in EOL are obtained by shifting the RTPTS from unirradiated fracture toughness curve.


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PTS eventsPTS events• The PTS events are mainly characterized by a rapid

cooldown in RCS with a high primary system pressure or re-pressurization. Such events depend strongly on the specific plant and system configurations, systems operation, and operator actions.

• The best way to determine the PTS events for RPV integrity assessment is to perform the event sequences analysis based on PRA technology.


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PTS eventsPTS events• For the specific NPP, such a PRA for PTS purpose

has not been fulfilled. • Alternatively, the PTS events are selected referring to

the international research achievements and the experience feedback from the other NPPs.

• The SB-LOCA and LB-LOCA are chosen from the NPP design transients as the typical PTS events.

• Additionally, a typical PTS with re-pressurization is supplemented.


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Fracture analysisFracture analysis• The virtual crack extension (VCE) method are

employed to obtain the J-integral at the deepest and 2mm below interface points. The effects of thermal loads and pressure loads on the faces of the crack are considered.


EJK J =

21 ν−=

EJK JFor plane strain:

For plane stress:

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Integrity assessmentIntegrity assessment• The integrity assessment for RPV are performed

using the maximum allowable reference temperature [RTPTS] which is obtained by tangent approach.

• The different [RTPTS] can be determined for the SB-LOCA, LB-LOCA and typical PTS event with re-pressurization and shown in next slides.


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Integrity assessmentIntegrity assessment


[RTPTS] for SB-LOCA

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[RTPTS] for LB-LOCA

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[RTPTS] for typical PTS event with re-pressurization

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Integrity assessmentIntegrity assessment• The integrity assessment for RPV are performed

using the maximum allowable reference temperature [RTPTS] which is obtained by tangent approach.

• The different [RTPTS] can be determined for the SB-LOCA, LB-LOCA and typical PTS event with re-pressurization and shown in next slides.


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Integrity assessmentIntegrity assessment• According to the PTS analysis the critical [RTPTS] is

44.8 ℃℃℃℃. As calculated in Section 3 the ART in EOL (or RTPTS) is 32 ℃℃℃℃ or 24.12 ℃℃℃℃ are less than 44.8 ℃℃℃℃.

• So the structural integrity of RPV within its design lifetime can be maintained relied on the current test data and calculation.


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• Introduction• Codes, Regulations and Working Scheme• Determination of increment of RT• Pressure – temperature limits• Pressurized thermal shock• Charpy upper shelf energy• Residual lifetime assessment• Conclusions

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• According to the results from Charpy impact tests of the several withdrawn capsule specimens, the decrement of upper shelf energy is a small amout(about 16J) and the USE are far higher than the limit of 68J which is required in App.G to 10CFR50.

Charpy upper shelf energy

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• Based on the measurement and calculation of fast neutron flux, the modified extrapolation factor can be derived and applied to calculate the peak fast neutron flux on inner surface of base metal for the balance fuel recycle.

• The relationship between ART and fluence is shown in next slides.

Residual lifetime assessment

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Relationship between ART and fluence


5.58××××1019n/cm2

27ºC

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• Considering the average 80% power factor, the relationship between fluence and operation years also can be expected as shown as below:


50years

4.39××××1019n/cm2

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• The current test and calculation results show that the structure integrity of the specific NPP RPV will be maintained within throughout design lifetime and the potential extended lifetime.

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Conclusions

• The test results of the withdrawn surveillance capsule specimens of the specific NPP have been applied to re-estimate P-T limits, USE and PTS.

• From the view point of the integrity assessment for RPV, the specific NPP has revealed the attractive potential to extend its operating lifetime.

• Some supplementary research subjects related to extend NPP lifetime should be taken into account and launched as early as possible.

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Thank you for your Thank you for your attention!attention!

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Methodology research on prediction for operating lifetime