core bypass

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1^st EUROPEAN MELCOR USERS’ GROUP Villigen, Switzerland

15-16 December 2008

LFW-SG ACCIDENT SEQUENCE IN A PWR 900:

CONSIDERATIONS CONCERNING RECENT MELCOR 1.8.5 / 1.8.6 CALCULATIONS

F. DE ROSA

ENEA FIS NUC - Bologna

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REFERENCE PLANT:

PWR 900 MWe

LFW – SG (H2)

LOSS OF NORMAL (AND AUXILIARY) SG FEEDWATER

ACCIDENT SEQUENCE:

INITIATOR:

F. DE ROSA - ENEA FIS NUC - Bologna 2/30

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Steam generator

to turbine

Main steam relief valve

Main feedwater control valve

Reactor coolant pump

Pressurizer

Reactor pressure vessel

Pressurizer relief tank

Accumulator

Pressurizer Safety relief valve valve

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DURING OPEN CALCULATIONS, WE HAVE THE

OPPORTUNITY TO COMPARE OUR RESULTS WITH AVAILABLE EXPERIMENTAL DATA.

DEALING WITH PLANT CALCULATIONS THERE IS NOT AN EXPLICIT REFERENCE: A PLANT CALCULATION IS

PRACTICALLY A BLIND CALCULATION.

WHY A CODE-TO-CODE COMPARISON IS IMPORTANT

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WP4 (SARNET 1 NoE)

GAVE A GOOD OPPORTUNITY

TO PERFORM A CODE-TO-CODE

COMPARISON USING ASTEC, MAAP

AND MELCOR.

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• THE LEVEL OF WATER IN SG GOES DOWN REACTOR SCRAM

• THE TEMPERATURE OF FLUID COMING OUT FROM THE CORE (T_RIC) GOES UP

DETAILS: ACCIDENT EVOLUTION

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cooling fluid outlet Steam Generator RPV and Core details

• WHEN TRIC > 603 K (330 C)

TOTAL OPENING OF RELIEF VALVES

1

2

3

Relief valve

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• WHEN ∆T_sat < 283 K (10 C) MAIN COOLANT PUMPS TRIP

DETAILS: ACCIDENT EVOLUTION

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• ACCUMULATORS INJECTION

• FIRST CORE UNCOVERY

4

5

6

ACCUMULATOR

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• FIRST FUEL CLADDING RUPTURE AND FISSION PRODUCT RELEASE

DETAILS: ACCIDENT EVOLUTION

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• ACCUMULATOR ISOLATION

7 ACCUMULATOR

8

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• FIRST MATERIAL SLUMP IN LOWER HEAD

DETAILS: ACCIDENT EVOLUTION

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9

Corium and metal pools in lower head of reactor pressure vessel

10

Corium slump from lower head to cavity

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COMPUTING DETAILS

COMPUTER (the same for MELCOR and ASTEC)

PC - PENTIUM 4 - CPU 2400 MHz - 1024 MB of RAM - WINDOWS XP Pro TRANSIENT LENGHT (CALCULATION TIME)

First Phase: 17 h (61200 s) Now : 24 h (86400 s)

CPU TIME

First Phase: 15 h (54000 s) for ASTEC V1.1p2 2 h ( 7200 s) for MELCOR 1.8.5 p3 Now : 13 h 25m (48300 s) for ASTEC V1.2 rev.1

2 h 50 m (10200 s) for MELCOR 1.8.6

(the same for MELCOR and ASTEC)

CALC RATIO (calc time / cpu time)

ξastec = 1.133 ξmelcor = 8.500

First Phase ξastec = 1.788

ξmelcor = 8.471 Now

Reason: To check CODE stability and capability to perform long calculations without computer hangs.

good news

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ASTEC

INPUT DECK

PROVIDED BY IRSN

ASTEC MODULES INVOLVED IN THE CALCULATION:

CPA

CORIUM

SOPHAEROS CESAR

DIVA

RUPUICUV MEDICIS IODE

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DIFFERENT INPUT APPROACH

IN MELCOR SOME DATA AND CONDITIONS MUST BE PROVIDED BY MEANS OF CONTROL (CFs) AND TABULAR FUNCTIONS (TFs), ESPECIALLY IN THE CASE UNDER EXAMINATION, IN WHICH THE MODELLING OF SEVERAL PHENOMENA AND COMPLICATED SYSTEMS IS REQUIRED.

IT MEANS THAT, IN GENERAL, ASTEC AND MELCOR INPUT DECKS ARE NOT EASILY COMPARABLE.

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PRIMARY CIRCUIT MODELLING (1)

Upper Plenum

of Down RPV

Comer of RPV

To containment volume CV501

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VOLUMES

CV300 : pressurizer

CV311, 321 and 331: cold leg plus pump 1, 2 and 3 CV312, 322 and 332: intermediate leg 1, 2 and 3 CV313, 323 and 333: hot leg 1, 2 and 3

CV314, 324 and 334: ascending part of steam generator 1, 2 and 3 (SG inlet)

CV315, 325 and 335: descending part of steam generator 1, 2 and 3 (SG outlet)

PRIMARY CIRCUIT MODELLING (2)

JUNCTIONS

FL214, 224 and 234: from upper-plenum of reactor vessel to hot leg 1, 2 and 3 FL300 : from hot leg1 to pressurizer

FL301, 302 and 303: from valve 1, 2 and 3 of pressurizer to containment

FL311, 321 and 331: from cold leg 1, 2 and 3 to downcomer of reactor vessel FL312, 322 and 332: from u-leg 1, 2 and 3 to pump/cold leg 1, 2 and 3

FL313, 323 and 333: from hot leg 1, 2 and 3 to SG1, 2 and 3 inlet FL314, 324 and 334: from SG1, 2 and 3 inlet to SG1, 2 and 3 outlet FL315, 325 and 335: from SG1, 2 and 3 outlet to u-leg1, 2 and 3

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HEAT STRUCTURES

HS30001 : pressurizer

HS30002 : expansion line of pressurizer HS31100, 32100 AND 33100 : cold leg plus pump 1, 2 and 3 HS31200, 32200 AND 33200 : intermediate leg 1, 2 and 3 HS31201, 32201 AND 33201 : pump1, 2 and 3

HS31300, 32300 AND 33300 : hot leg 1, 2 and 3

HS31400, 32400 AND 33400 : ascending part of SG 1, 2 and 3 HS31500, 32500 AND 33500 : descending part of SG 1,2 and 3

PRIMARY CIRCUIT MODELLING (3)

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SECONDARY CIRCUIT MODELLING (1)

VOLUMES

CV411, 421 and 431: cavity of SG1, 2 and 3

CV412, 422 and 432: downcomer of SG1, 2 and 3

CV414, 424 and 434: aux volumes to describe feedwater for SG1, 2 and 3

CV413: normal feedwater tank for SGs

CV415, 425 and 435: steam line for SG1, 2 and 3

CV416: barrel (end steam line) CV417: steam header

To containment volume CV502

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SECONDARY CIRCUIT MODELLING (2)

JUNCTIONS

FL401, 404 and 407: from the first group of safety valves to environment FL402, 405 and 408: from the second group of safety valves to environment FL403, 406 and 409: from the third group of safety valves to environment FL411, 421 and 431: from cavity 1, 2 and 3 to downcomer of SG1, 2 and 3 FL412, 422 and 432: from downcomer 1, 2 and 3 to cavity of SG1, 2 and 3 FL413, 423 and 433: from normal feedwater tank to aux volume 1, 2 and 3 FL414, 424 and 434: from aux volume 1, 2 and 3 to downcomer of SG1, 2, 3 FL415, 425 and 435: from cavity 1, 2 and 3 to steam line 1, 2 and 3

FL416, 426 and 436: from steam line 1, 2 and 3 to barrel FL417: from barrel to steam header

HS41100, 42100 and 43100: bottom internal of cavity side of SG 1, 2 and 3 HS41101, 42101 and 43101: top 1 internal of cavity side of SG 1, 2 and 3 HS41102, 42102 and 43102: top envelop of SG 1, 2 and 3

HS41103, 42103 and 43103: top 2 internal of cavity side of SG 1, 2 and 3

HS41200, 42200 and 43200: bottom envelop of downcomer side of SG 1, 2, 3 HS41201, 42201 and 43201: envelop of tube bundle of SG 1, 2 and 3

HEAT STRUCTURES

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VESSEL (1)

CV311

CV321

CV331 cold leg 1

cold leg 2

cold leg 3

downcomer

lower head (bottom vessel)

core core bypass upper plenum

upper head (vessel dome)

CV313

CV323

CV333 hot leg 1

hot leg 2

hot leg 3

FL311

FL331 FL321 FL224

PRIMARY CIRCUIT

STRONG LIKENESS BETWEEN ASTEC AND MELCOR RPV

MODELLING

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FL210: from downcomer to dome FL211: from downcomer to bottom FL212: from bottom to core

FL213: from bottom to core bypass FL214: from core to upper plenum

FL216: from core bypass to upper plenum FL215: from dome to upper plenum

VESSEL (2)

JUNCTIONS

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HS21001: downcomer, rectangular part HS21002: downcomer, lateral, bottom side HS21003: downcomer, lateral, upper side HS21100: bottom, hemispherical shape HS21101: bottom, cylindrical side 1 HS21102: bottom, cylindrical side 2 HS21103: bottom, cylindrical side 3

HS21201: core, partition plate 1, rectangular, vertical (located at level 1) HS21202: core, partition plate 2, rectangular, vertical (located at level 2) HS21203: core, partition plate 3, rectangular, vertical (located at level 2) HS21301: core bypass, cylindrical shape

HS21302: core bypass, partition plate, rectangular, horizontal HS21401: upper plenum, downcomer side, cylindrical shape HS21402: upper plenum, support wall of guide tubes

HS21403: upper plenum, horizontal core support plate, rectangular shape HS21404: upper plenum, vertical guide for tubes, rectangular shape

HS21405: upper plenum, vertical structure, rectangular shape HS21501: dome, lid, hemispherical side

HS21502: dome, lid, cylindrical side

HS21503: dome, internal structures, rectangular shape

HEAT STRUCTURES

VESSEL (3)

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ASTEC - MELCOR (Timing Comparison)

EVENT ASTEC v1.1

patch 2

MELCOR 1.8.5 patch3

Loss of SG Feedwater (initiator) 0 s 0 s

Scram 28 s 28 s

Total Opening of Relief Valves (T_ric> 330 C) 2209 s 2259 s Main Coolant Pumps trip (∆∆∆∆T_sat< 10 C) 2214 s 2266 s Accum injection (start of first accum

discharge) 3524 s 3603 s

Accum isolation (accum off) 6189 s 7184 s

Start of relocation to Lower Plenum (first

corium slump) 7487 s 8700 s

Vessel failure (Lower Head Vessel failure) 18254 s 18474 s

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0 500 1000 1500 2000 2500

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000 13000 14000 15000 16000 17000 18000 19000 20000

time (s)

Temp (C)

T_Melc T_Astec Threshold (330 C)

TOTAL OPENING OF THE RELIEF VALVES

VERTICAL AXIS: Core Outlet Temperature (T_RIC)

fairly good agreement

strong discrepancy

moderate discrepancy

time interval to be better investigated

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Accumulators isolation

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 1000 0

1100 0

1200 0

1300 0

1400 0

1500 0

1600 0

1700 0

1800 0

1900 0

2000 0 time (s)

Pprim (bar)

Melcor 1.8.5.

Astec v1.1p2 Threshold

reactor scram

start of 1^st accumulators’ discharge accumulators’ isolation (ASTEC) start of relocation (ASTEC)

start of relocation (MELCOR)

accumulators’ isolation (MELCOR)

Accum isolation when P_prim < 15 bar (1.5 MPa)

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0 50 100 150 200 250 300 350 400 450

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000 13000 14000 15000 16000 17000 18000 19000 20000

time (s)

Mass_H2 (kg)

Astec Melcor

HYDROGEN PRODUCED DURING THE IN-VESSEL PHASE

marked convexity

marked concavity

good agreement (8200-13500 s)

strong disagreement ^(*) (beyond 13500 s) strong

disagreement ^(*) (3500-8200 s)

Strong H2 production (less water in the core)

(*) time intervals to be better investigated using latest ASTEC and MELCOR versions

accum isolation (ASTEC) accum isolation (MELCOR)

Relocation phase core

uncovery

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CONCLUSIONS (1)

ASTEC AND MELCOR are able to calculate a whole LFW- SG (H2) sequence for a French PWR 900 MWe.

A new comparison made using MELCOR 1.8.6 and the latest version of ASTEC evidenced a general tendency of results to become closer for both codes, but...

EVEN IF THERE IS A FAIRLY GOOD CONSISTENCY BETWEEN ASTEC AND MELCOR, MAINLY IN THE “FRONT - END” PART OF THE LFW-SG SEQUENCE (PRIMARY SYSTEM BEHAVIOUR, CORE

HEAT-UP PHASE UNTIL STRONG OXIDATION), IN THE SECOND PART OF THE ACCIDENT EVOLUTION, WHEN SIGNIFICANT MATERIAL RELOCATION OCCURS, SOME DISCREPANCIES

STILL APPEAR.

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F. DE ROSA - ENEA FIS NUC - Bologna

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CONCLUSIONS (2)

IT IS VERY IMPORTANT TO CONTINUE A COMPARISON WORK TAKING INTO ACCOUNT ALSO THE SOURCE TERM.

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