*TATSUYA KUNISHI, HITOSHI MUTA, KEN MURAMATSU AND YUKI KAMEKO

TOKYO CITY UNIVERSITY GRADUATE SCHOOL

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Methodology of Treatment of Multiple Failure Initiating Events for Seismic PRA

(2)Success Criteria Analysis for Multiple Pipe Break Accidents of a PWR

Contents

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• Background & Purpose • Position of this study • Analysis conditions • Results & Consideration • Conclusion & Future task

Background

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• Multiple rupture events are not usually considered in PRAs of pressurized water reactors (PWRs) for internal events.

• Analyses assuming pipe break accidents in multiple location are not extensively performed.

• The possibility of Multiple Pipe Break Accidents occurrence by earthquake

is very small, but the release of radioactive material etc. may be considered .

• Understanding the thermal hydraulic behavior in the reactor vessel and success criteria for core cooling systems considering beyond DBA

Purpose

Position of this study

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1. Location (Location dependency) Effects of differences in location of the break points Confirm the influence by a combination of various break points.

2. Number of break points (Number of break points dependency) Relationship between fracture area and number of break points Confirm the difference in behavior due to the number of break points, assuming the Fracture area of the primary system piping is constant.

3. Fracture area (Fracture Area dependency) Relationship between the fracture area and the number of break points when changing the fracture area Confirm how the behavior differs depending on the size of the fracture area of the primary system piping from the viewpoint of 2.

In this study, we analyzed the following viewpoint and organized the findings.

Analysis Conditions

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• The target model plant is 3-loop PWR (Output Power 2660MWt)

• 3-loops were separately modeled • Detailed dimensions of the 3 loops PWR plant are not disclosed • Lack of detailed dimension data were filled with simplified models.

Model Plant

RELAP (Reactor Excursion Leak Analysis Program) series Thermal hydraulic analysis code RELAP5-3D It is a well known and widely used code for thermal-hydraulic analysis.

Analysis Code

Validity of the plant model was checked by comparison with published LOCA analyses for similar PWRs.

Noding diagram

6 Noding diagram used for analysis

Pipe rupture is simulated by valve

HPIS ACC LPIS

HPIS ACC LPIS

HPIS

ACC LPIS

: Volume

: Junction

: ECCS`s

Crossover(CR)

Hot leg(HL)

Cold Leg(CL)

Pressurizer surge pipe(Psu)

• Rupture of primary system piping • Loss of ECCS function Accident Scenarios

Analysis cases

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Number of points Location Fracture area ECCS operation

1 CR CL HL Psu Large~Small* area HPIS 2 line LPIS 2 line ACC 3 line

(Combination of ECCS actuation)

2~ Combination of CR CL HL Psu

Changing the fracture area (Combination of Large

, Medium, Small*)

*:Large(rupture area of pipe inner diameter), Medium (rupture area of pipe inner diameter ½),Small is 2 inches rupture area

Location Diameter*[m] Fracture Area＊[m2]

Large Medium Small Crossover pipe 0.79 0.490 0.1225 0.0081 Hot Leg pipe 0.74 0.430 0.1075 0.0081 Cold Leg pipe 0.70 0.385 0.0962 0.0081

Pressurizer surge pipe 0.28 0.062 0.0154 0.0081

Analysis Flow

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②Select the Scenario and Pipe Combination

③Analysis the selected conditions Definition of Success Criteria and Analysis

Based on analysis result

Change scenario and reset piping combination

①Construction of Input Data and

Benchmark

④ Clear the event progress and summarized the technical knowledge on Success Criteria of core cooling

Applied to seismic PRA, contributing to safety improvement

Definition of conditions for success of core cooling in progress scenario of accident

Understanding the thermal-hydraulic behavior &

Grouping the analysis results that is Similar

Assumption of multiple piping rupture

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1. Location (Location dependency) The fracture area are 100% rupture of the pressurizer surge pipe. Equal fracture is assumed for hot leg , crossover, cold leg .

2. Number of break points (Number of break points dependency) The fracture area are 100% rupture of the pressurizer surge pipe. In 2 break points, each points fracture area is 50% rupture of the pressurizer surge pipe.

3. Fracture area (Fracture area dependency) Fracture area twice as large as the pressurizer surge pipe in hot leg, crossover, cold leg.

Definition of core damage shall be when the surface temperature of cladding exceeds 1200 ͦC. (about 1500 K)

Evaluation

Analysis Cases

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Number of points Location Fracture area ECCS operation

1 CR CL HL Psu Large~Small* area HPI 2 line LPI 2 line

ACC 3 line (Combination of ECCS

actuation) 2 Combination of CR CL HL Psu

Changing the fracture area (Combination of Large

, Medium, Small*)

Analysis Combination[Location]

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Number of break points Piping Combination Case

1

Crossover pipe (CR) No ECCS operate HPI+LPI+ACCx3

HPIx2+LPIx2+ACCx3

Hot leg pipe (HL) No ECCS operate HPI+LPI+ACCx3

HPIx2+LPIx2+ACCx3

Cold leg pipe (CL) No ECCS operate HPI+LPI+ACCx3

HPIx2+LPIx2+ACCx3

Pressurizer Surge pipe (Psu) No ECCS operate HPI+LPI+ACCx3

HPIx2+LPIx2+ACCx3

Analysis Results (No ECCS operate)

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0

500

1000

1500

2000

2500

0 100 200 300 400 500

Tem

pera

ture

of f

uel c

ladd

ing

[K]

Time [sec]

Surface temperature of fuel cladding

CL CR

HL Psu

Timings of temperature rise are different, however, temperature rise does not differ greatly each other CL break could represent because of it’s the severest behavior, if simplification needed

Analysis Results (HPI+LPI+ACCx3)

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0

100

200

300

400

500

600

700

800

0 100 200 300 400 500

Tem

pera

ture

of f

uel c

ladd

ing

[K]

Time [sec]

Surface temperature of fuel cladding

CL CR

HL Psu

Cooling timing are different because of RV pressure behavior and cooling order is CL, CR , HL and Psu And the CL takes more time to cool down

Analysis Results (HPIx2+LPIx2+ACCx3)

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0

100

200

300

400

500

600

700

0 100 200 300 400 500

Tem

pera

ture

of f

uel c

ladd

ing

[K

Time [sec]

Surface temperature of fuel cladding

CL CR

HL Psu

Behavior is same as the previous case HPI+LPI+ACCx3. Cool down little bit faster because of the capacity of injection

Conclusion for the Location

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From the results, • Severer in the order of Cold leg > Crossover > Hot leg≧

Pressurizer surge pipe • Different behavior depending on the location and the severest one

is CL break

Location of break could affect plant behavior

Analysis Combination [break points]

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Number of break points Piping Combination Case

1 Cold leg pipe (CL) No ECCS operate

HPIx2+LPIx2+ACCx3

2

CL+HL(Same loop) CL+HL (Different loop)

No ECCS operate HPIx2+LPIx2+ACCx3

CL+CL No ECCS operate

HPIx2+LPIx2+ACCx3

Analysis Results (No ECCS operate)

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0

500

1000

1500

2000

2500

0 100 200 300 400 500Tem

pera

ture

of f

uel c

ladd

ing

[K]

Time [sec]

Surface temperature of fuel cladding

CL

CL+HL（Different loop)

0

500

1000

1500

2000

2500

0 100 200 300 400 500Tem

pera

ture

of f

uel c

ladd

ing

[K]

Time [sec]

Surface temperature of fuel cladding

CL

CL+HL(same loop)

0

500

1000

1500

2000

2500

0 100 200 300 400 500

Tem

pera

ture

of f

uel c

ladd

ing

[K]

Time [sec]

Surface temperature of fuel cladding

CL

CL+CL Same Behavior No dependency at breakpoints

There is Location

dependency influence

Location dependency exists in the case of different loops

Analysis Results (HPIx2+LPIx2+ACCx3)

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0

100

200

300

400

500

600

700

0 100 200 300 400 500Tem

pera

ture

of f

uel c

ladd

ing

[K]

Time {sec]

Surface temperature of fuel cladding

CL

CL+HL（Different loop)

0

100

200

300

400

500

600

700

0 100 200 300 400 500Tem

pera

ture

of f

uel c

ladd

ing

[K]

Time [sec]

Surface temperature of fuel cladding

CL

CL+HL(same loop)

0100200300400500600700

0 100 200 300 400 500

Tem

pera

ture

of f

uel c

ladd

ing

[K]

Time [sec]

Surface temperature of fuel cladding

CL

CL+CL

It take more time to cool for the 2 points break case

If all ECCS inject into RV, the core will cool faster

Conclusion for the Break points

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From the result, • 1 point break of CL and 2points break CL+CL are quite

similar behavior, so NO dependency could exist in number of break points

• However, 2 points break of the combination of different loops, location dependency could exist

There is no Dependency of Number of break points in the same position However,

Location dependency could exist in the different position

Analysis Combination[Fracture area]

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Fracture area Piping Combination Case

100% Pressurizer surge pipe

Cold leg pipe (CL)

No ECCS operate

HPI+LPI+ACCx3

HPIx2+LPIx2+ACCx3

200% Pressurizer surge pipe

Cold leg pipe (CL)

No ECCS operate

HPI+LPI+ACCx3

HPIx2+LPIx2+ACCx3

Analysis Results (No ECCS operate)

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0

500

1000

1500

2000

2500

0 50 100 150 200 250 300 350 400

Tem

pera

ture

of f

uel c

ladd

ing

[K]

Time [sec]

Surface temperature of fuel cladding

CL

CL200%

Timing of temperature rise are different However, In different of fracture area, temperature rise rate does not differ greatly

Analysis Results (HPI+LPI+ACCx3)

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0

100

200

300

400

500

600

700

800

0 50 100 150 200 250 300 350 400

Tem

pera

ture

of f

uel c

ladd

ing

[K]

Time [sec]

Surface temperature of fuel cladding

CL

CL200%

The bigger fracture area size, the faster the clad temperature rise However, ECCS could operate earlier that core could be cooled down faster than smaller size of fracture area

Analysis Results (HPIx2+LPIx2+ACCx3)

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0

100

200

300

400

500

600

700

800

0 50 100 150 200 250 300 350 400

Tem

pera

ture

of f

uel c

ladd

ing

[K]

Time [sec]

Surface temperature of fuel cladding

CL

CL200%

Same as ECCS injected case with single failure

The bigger fracture area size, the faster the clad temperature rise However, ECCS could operate earlier that core could be cooled down faster than smaller size of fracture area

Conclusion for the Fracture area

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From the results • The size of fracture area is different, the temperature rising

point is different • However, the temperature rise does not differ greatly

regarding to the fracture area • Even in all ECCS operate or the case considering single

failure of ECCS, cooling speeds are not different greatly

Rupture fracture area could affect plant behavior

Summary

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Considering to define Success Criteria, the followings need to be discussed • Location of break could affect plant behavior

Cold leg break is the severest case. • There could be no dependency of the number of break points in the same

position

• Fracture area dependency need to be noted in considering success criteria.

• In this presentation , primary piping ruptures are focused on, but consideration should also be given to the combinations with secondary piping such as steam generator tube.

• The technical knowledge on Success Criteria of core cooling should be summarized, and it should be applied to seismic PRA

Future task

Conclusion

Thank you for your attention.

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