PTS re-evaluation projectfor Czech NPPs

Vladislav Pištora, Miroslav Žamboch,Pavel Král, Ladislav Vyskočil

Fourth International Conference on Nuclear Power Plant Life Management

23–27 October 2017

Lyon, France

Introduction

� Reactor pressure vessel (RPV) is a key component of nuclear power plant (NPP) that can limit the NPP lifetime.

� Radiation embrittlement due to neutron fluence is the most significant aging mechanism for RPV.

� For the RPV lifetime assessment, material degradation limit has to be established.

� This limit is established on the basis of pressurised thermal shock (PTS) analyses:

� PTS is an event in NPP that is characterized by rapid cooldown in the primary coolant system with (usually) high primary pressure.

� PTS analyses are a multidisciplinary task. They consist of a series of thermal hydraulic and structural analyses.

�Two NPPs are operated in the Czech Republic by the ČEZ company:

� Dukovany NPP (4 units of WWER 440/213 type, start of operation 1985),

� Temelín NPP (2 units of WWER 1000/320 type, start of operation 2000).

� The original assessment of PTS was performed

� for NPP Dukovany during 1996 – 2004,

� for NPP Temelín during 2001 – 2004.

1

General scheme of PTS analyses according to VERLIFE

2

TH system analyses

FE model of RPV

Temperature and stress fields

Fracture mechanics

stress intensity factor KI

Maximum allowable critical

temperature of brittleness Tk

a

or

maximum allowable Master curve

reference temperature T0

a

Neutron fluence prediction

Material degradation

prediction

(standards, surveillance

programme)

Tk < Tk

a

or

T0 < T0

a

Input data

for

structural

analysis

Postulated

cracks in RPV

Prediction of

critical temperature of

brittleness Tk

or Master curve reference

tamperature T0

RPV

embrittlement

TH mixing analyses

NDE qualification,

standards

Selection of PTS scenarios

Normative documents used for the PTS evaluation

3

� Guidelines on Pressurized Thermal Shock Analysis for WWER Nuclear PowerPlants, Revision 1, IAEA-EBP-WWER-08 (Rev. 1), IAEA, Vienna, 2006

� Unified Procedure for Lifetime Assessment of Components and Piping in VVERNPPs, VERLIFE, ver. 2003 (the basis for the first PTS Temelín project)

� Normative Technical Documentation of Association of Mechanical Engineers(NTD ASI), Section IV, ver. 2004 (Czech equivalent of VERLIFE, ver. 2003)

� Unified Procedure for Lifetime Assessment of Components and Piping in VVERNPPs, VERLIFE, ver. 2008

� Normative Technical Documentation of Association of Mechanical Engineers(NTD ASI), Section IV, ver. 2013 (Czech equivalent of VERLIFE, ver. 2008)

� IAEA – NULIFE Guidelines for Integrity and Lifetime Assessment of Componentsand Piping in WWER NPPs During Operation (VERLIFE), to be published

� Normative Technical Documentation of Association of Mechanical Engineers(NTD ASI), Section IV, ver. 2017 (based on IAEA – NULIFE VERLIFE)

Note: UJV Rez was a co-author of the IAEA, the NTD ASI and the VERLIFE guidelines.

Overview of PTS events analysed for NPP Dukovany within the original PTS assessment

4

Overview of TH analyses performed in PTS evaluation:

1) Main steam line break (MSLB) or other large secondary leakso system thermal-hydraulic (TH) analyses of 12 variants of this event o mixing analyses of the 3 worst cases

2) Primary-to-secondary leak (PRISE)o system TH analyses of 17 variants o mixing analyses of 10 worst variants

3) Loss-of-coolant accident (LOCA) o system TH analyses of 22 variantso mixing analyses of 14 worst variants

4) Other events - inadvertent opening of pressurizer safety valve (PRZ SV), false initiation of safety injection (SI) or make-up, feed&bleed, flooding of reactor cavity etc.

o system TH analyses of 18 caseso mixing analyses of 33 worst cases

The original PTS assessments were performed for surface-breaking postulated cracks, finally 6 worst scenarios were recalculated for underclad cracks.

As the most severe PTS was identified the regime „LOCA DN 200 mm – break in hot leg“.

IE group System TH analyses

Mixing calculations

Structural analyses

MSLB 12 3 8PRISE 17 10 10LOCA 22 14 13

OTHER 18 3 7Total 69 30 38

Overview of PTS events analysed for NPP Temelínwithin the original PTS assessment

5

Overview of TH analyses performed in PTS evaluation:

As the most severe PTS was identified the regime „Inadvertent opening of PRZ SV with subsequent reclosure“.

IE group System TH analyses

Mixing calculations

Structural analyses

MSLB 13 4 4PRISE 15 2 2LOCA 18 5 5

OTHER 26 13 13Total 72 24 24

PTS re-evaluation project

� Within preparation of Dukovany NPP for long term operation, PTS re-evaluation project was started.

� It is obvious that since that time some changes in the NPP equipment, procedures etc. have been performed and also the applied methodologyfor PTS assessment has been improved. Moreover, significant improvement of software tools and hardware capabilities proceeded, which enabled improvement of applied models. This was one of the reasons for starting the PTS re-evaluation project. Due to these facts, PTS re-evaluation project was started also for Temelín NPP.

� The PTS re-evaluation project is beeing performed during 2016 - 2018.

� During the current PTS re-evaluation project, the most significant PTS scenarios are being recalculated.

6

Main new features of PTS analyses in the current PTS re-evaluation project

7

� System thermalhydraulic (TH) analyses – updated RELAP5 models considering current status of both Dukovany and Temelín NPP.

� Mixing TH analyses – using CFD code Ansys-FLUENT instead of regional mixing code REMIX/NEWMIX used in the original PTS assessment –significant improvement of modelling.

� Structural analyses – application of IAEA-NULIFE VERLIFE

� new postulated crack,

� new warm pre-stressing (WPS) approach (possible application for non-monotonic loading),

� new formulae for residual stresses in weld, heat affected zone and RPV cladding.

System thermal hydraulic (TH) analysismodel for Temelín NPP and example of results

Nodalization of VVER-1000 for RELAP5:

8

1 2 4 5 6 7 103 98 11

12

13

21 22 23 25 26

27

20 24

28 29

30

909

911 913

PSK2

14

18

17

16

15

19

025

1

2 3

4

916

026

1

2 3

4

918

027

1 8765432

9

10 11 12 13

15

14

19 20 21 22 23 24181716

25

26 27

28

917919

915

PSK1

1 2 3 4 5 6 7 8

9

10 11 12

182

186

TG4

990

989

900

904

901

902

022

1

2

3 4

5910

023

1

2

3 4

5

912

024

1 2 3 4 5 6 7 8 9

10

1211

382

386

928

488

487

489490 491

492

484

021

485 483

019

486

1 2

11109

8

76543

482

020

929

903

HPK

RCA+zpet. klapka

PG1

SV1 SV2

PSV

TG1

PG2

RCA+zpet. klapka

SV1 SV2 PSV

TG2

PG3

SV1 SV2 PSV

RCA+zpet. klapka

TG3

SV1 SV2 PSV

PG4

RCA+zpet. klapka

188

187

189190

192

011

184

177

1

2

3

4

5

6

012

185

178

1

2

3

4

5

6

183

2 3

7

4

65

8

1

9

179

010

191

926

286

288

287

289290 291

2921 2 3 4 5 6 7 8 9 10

11

12

282

014

284

015

285 283

013

927

388

387

389390 391

392

017

384

018

385 383

016

2

3

4

5

RZV

1

2

3

4

1413

5

1

RZV

1

914

2

3

4

5

2

3

13

908

4

5

RZV

RZV

1

Active ECCS (1/3)

Containment

Main steam system

Primary circuit (1/4)

Input model statistics:

� 1800 control volumes

� 2400 hydraulic junctions

� 1600 heat structures with8700 mesh points

� 2680 control variables

� 1110 trips

FW system

System TH analysis – model for Temelín NPP and example of results

9

Example of results of system TH analysis – scenario PSV73:“Inadvertent opening of PRZ SV with 3 operating high-pressure safety injection pumps”

000.0E+0

2.0E+6

4.0E+6

6.0E+6

8.0E+6

10.0E+6

12.0E+6

14.0E+6

16.0E+6

18.0E+6

-1000 0 1000 2000 3000 4000 5000

time [s]

pre

ssu

re [P

a]

primary pressure (PRZ) steam pressure (SG1) MSH pressure

Primary pressure

drop due to false

opening of PRZ

safety valve

Opening of

secondary

steam dump

Effect of operator

switch-off of safety

injection pumps

Variation of primary and secondary pressure

System TH analysis – model for Temelín NPP and example of results

10

Flow at pressurizer safety valve and its integral

0

20

40

60

80

100

120

140

160

-1000 0 1000 2000 3000 4000 5000

time [s]

flow

[kg

/s]

0

50000

100000

150000

200000

250000

300000

350000

400000

450000

500000

mas

s [k

g]

PRZ RV PRZ SV1 PRZ SV2 integral of total flow

Inadvertent

opening of

PRZ safety

valveTransfer from

steam to

two-phase

flow

System TH analysis – model for Temelín NPP and example of results

11

Injection of safety systems

0

50

100

150

200

250

-1000 0 1000 2000 3000 4000 5000

time [s]

flow

k [kg

/s]

0

50000

100000

150000

200000

250000

300000

350000

400000

450000

mass

[kg]

PTTQ (TQx4) HPSI (TQx3) HDA LPSI (TQx2) integral of total flow

Start of injection

of 3 high pressure

(HP) pumps

Operator reduction

of injecting HP pumps

Restart and

final stop of

1 HP pump

Intermittent

injection of

accumulators

Mixing calculation – model for Temelín NPP and example of results

12

� Coolant mixing is simulated by the CFD code Ansys FLUENT (for single-phase scenarios only).

� Computational domain covers cold legs with ECCS injections, downcomerand lower plenum including solid walls.

� An example of CFD model for mixing simulation in VVER-1000 reactor is shown below. Only a fluid domain is displayed. The model also contains solid walls. The computational mesh contains about 2 millions of cells.

Arrows denote inlets into the computational domain

Mixing calculation – model for Temelín NPP and example of results

13

� Initial and boundary conditions for the mixing simulation in FLUENT are used based on the system TH analysis performed with RELAP5 code.

� Boundary conditions for the CFD simulation are time variations of:

� mass flow rate and temperature of ECCS injections,

� mass flow rate and temperature of coolant at cold leg inlets and reactor inlets.

� The goal of the simulation is to calculate time evolution of temperature fieldat wetted surfaces of the reactor pressure vessel wall and cold legs.

� The results serve as an input for the subsequent structural analysis.

� CFD simulations of mixing for PTS evaluation are computationally expensive.

Mixing calculation – model for Temelín NPP and example of results

14

Example of calculated case:Scenario “Inadvertent opening of PSV with three operating high-pressure injection pumps” at NPP Temelín (PSV73).

Calculated temperatures [°C] of wetted surfaces, PSV73 scenario, time 550 s.

Thermal stratification in

cold leg with ECCS

injection.

Cold plumes in downcomer

Mixing calculation – model for Temelín NPP and example of results

15

Comparison of results from FLUENT and RELAP5 codesTime variation of wall surface temperatures at RPV weld 4, PSV73 scenario.

Mixing calculation – model for Dukovany NPP

16

� Another example of CFD model for mixing: VVER-440 reactor. Only a fluid domain is displayed. The model also contains solid walls. The computational mesh contains about 2 millions of cells.

Arrows denote inlets into the computational domain

Structural analysis – model

17

� Finite element model of cylindrical part of RPV created.

� Crack postulated in accordance with VERLIFE:� semielliptical underclad crack partially (1 mm) penetrating into cladding,� crack postulated in welds No. 3 and 4 (Temelín NPP) or in weld No. 4 (Dukovany NPP) in beltline zone,� axial and circumferential orientations of crack,� aspect ratios a/c = 0,3 and 0,7,� crack depth a = 13 + 1 mm (based on NDE qualification requirements and results).

� In total, 8 (4) models for 8 (4) postulated crack configurations created for Temelín (Dukovany) NPP.

� SYSTUS FEM code used for the analyses.

� Heat transfer transient analysis performed as the 1st step.

� Elastic-plastic analysis performed as the 2nd step for loading by residual stresses, dead weight, inner pressure and transient temperature field.

Structural analysis – model for Temelín NPP

18

FEM mesh – the whole model and detail of the crack (in section)

Structural analysis – fracture mechanics assessment

19

� In the post-processor of the SYSTUS code, the energy release rate G iscalculated using G-theta method for all nodes of the crack front and for all timesteps.

� From value of G, value of stress intensity factor KI is calculated using thefollowing formula:

� Allowable value of stress intensity factor [KIC]3 :

� critical temperature of brittleness approach

� Master curve approach

Size correction is performed within Master curve approach.

� The warm pre-stressing approach or the tangent approach is used fordetermination of maximum allowable reference temperature Tk

a or T0a.

21 ν−

⋅=

GE

IK

[ ] ( )[ ]{ } mMPa ;.expmin 21⋅−⋅⋅+= 20002036263 kIC

TTK

[ ] ( )[ ]{ } mMPa ;.exp..min 21⋅−⋅⋅+= 2000190636225 03TTK

IC

Structural analysis – example of results for Temelín NPP

20

PSV73 – circumferential stress at time 3750 s (model for weld 3, axial crack, a/c=0,7)

Structural analysis – example of results for Temelín NPP

21PSV73 – KI vs. T and [KIC]3 vs. T diagrams (weld 3, axial crack, a/c = 0,7)

Overview of PTS scenarios for Dukovany NPP analysed within the current PTS re-evaluation project until now

22

Abbreviation Name

PSV23n-r1800Inadvertent opening of PRZ SV with reclosure in 1800 s, maximum ECCS, full power

LOCA_H200_2016LOCA – break in hot leg with equivalent diameter

Dn 200 mm, maximum ECCS, full power

PRISE SGTR3a3 steam generator tubes rupture, maximum ECCS,

zero power, non-closure of main isolation valve

PRISE SGTR4a3 steam generator tubes rupture, maximum ECCS,

zero power, closure of main isolation valve

PRISE SGIMF6 SG cold collector head loosening, minimum ECCS,

Overview of planned analyses for Dukovany NPP within the current PTS re-evaluation project for 2017 - 2018

23

Abbreviation Name

SLIz3oBreak of main steam line in hermetic zone, stuck-open PRZ SV (single failure)

SLIz3rBreak of main steam line in hermetic zone, stuck-open PRZ SV and its later reclosure by operator

LOCA H30z.maxLOCA – break in hot leg with equivalent diameter Dn 30 mm, maximum ECCS, zero power

LOCA H90n.maxLOCA – break in hot leg with equivalent diameter Dn90 mm, maximum ECCS, full power

LOCA D500 H&C

LOCA – rupture of hot leg with equivalent diameter Dn 2x500 mm, ECCS condition - question of further discussion, full power

FW Flooding reactor cavity by rupture of feedwater pipe

Overview of PTS scenarios for Temelín NPP analysed within the current PTS re-evaluation project until now

24

Abbreviation Name

PSV73zrInadvertent opening of PRZ SV with reclosure

at 1500 s, maximum ECCS, full power

PSV73Inadvertent opening of PRZ SV, maximum

ECCS, full power

PSV71zraInadvertent opening of PRZ SV with reclosure at

1500 s, minimum ECCS, full power

PSV83zrInadvertent opening of PRZ SV with reclosure at

1500 s, minimum ECCS, zero power

LOCA 32min LOCA Dn 32 mm, minimum ECCS, zero power

Overview of planned analyses for Temelín NPP withinthe current PTS re-evaluation project for 2017 - 2018

25

Abbreviation Name

PRISE 3SGT3 steam generator tubes rupture (cold collector), minimum ECCS, zero power

FB1 feed and bleed

LOCA PP210min

LOCA - break of pipe between PRZ and PRZ SV Dn 210 mm, minimum ECCS, full power

PRISE SGH1Steam generator head lift (equivalent diameter Dn 40 mm) maximum ECCS, zero power

H300minLOCA - break of hot leg with equivalent diameter Dn 300 mm, minimum ECCS, full power

H850LOCA - break of hot leg with equivalent diameter Dn 2x850 mm, maximum ECCS, full power

MSLB SLB1BMain steam line break near SG 1, minimum ECCS (injection to loop 3), zero power

MSLB SLB1C Main steam line break near SG 1, maximum ECCS, zero power

MSLB SLB1AMain steam line break near SG 1, minimum ECCS (injection to loop 1), zero power

Conclusions

26

� Within the Dukovany (1996 - 2004) and Temelín (2001 - 2004)original PTS projects, a large number of PTS scenarios wereanalysed.

� Within the current PTS re-evaluation project (2016 – 2018)all significant PTS scenarios are being recalculated based oncurrent NPP status, current methodology and state of the artmodels.

� 5 PTS scenarios for NPP Dukovany and 5 PTS scenarios forNPP Temelín have been analysed until now.

Conclusions, cont.

27

� More precise TH mixing analyses using CFD codeAnsys-FLUENT are performed which leads to more realisticresults.

� In most cases, current results for re-evaluated PTS regimesshow less adverse results than results obtained within theoriginal PTS projects, which suggests good perspective for LTO.

� At least 6 new full PTS analyses for NPP Dukovany and 9 newfull PTS analyses for NPP Temelín are planned for 2017 – 2018.

� After finalising the planned PTS analyses, the worst cases willbe re-assessed with the aim to optimize the emergencyprocedures.

� PTS analyses for the RPV inlet nozzle region will be performedfor selected scenarios at the end of the project.

28

Thank you for your attention