decay heat (MW)

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Wir schaffen Wissen – heute für morgen

Paul Scherrer Institut

MELCOR and CORQUENCH MCCI analyses in a KONVOI containment

EMUG, Brusseles 3/2015

Adolf Rýdl, Bernd Jäckel

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Introduction: MELCOR/CORCON MCCI analyses Why we still need CORQUENCH

Base case SBO calculations by MELCOR and CORQUENCH Sensitivity CORQUENCH calculations

Conclusions

Content

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Old MELSIM input deck of KONVOI 1000MWe PWR was translated for use with MELCOR 1.8.6 by deleting and/or adapting all MELSIM functions and MELSIM related commands

An SBO sequence was chosen for the long-term accident progression simulation, including MCCI; mostly defaults used in MELCOR/CORCON analyses, also the homogeneous melt option

Basemat ablation was the main parameter of the study, radial ablation thought to be less important here; containment pressurization from MCCI mitigated by FCVS

A high pressure scenario used to get the starting conditions for MCCI, with water on top of the melt just after VF, recalculated also with M2.1

A "dry case" and a low pressure scenario were also calculated as sensitivity cases

Introduction: MELCOR for MCCI and selected scenarios

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Base case by MELCOR/CORCON

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Sensitivity on long-term downward erosion by MELCOR

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why we need CORQUENCH (for cases with water atop the melt)?

Introduction: CORQUENCH

traditional view of MCCI, also CORCON in MELCOR

(Farmer et.al, Nucl.Eng.Technol., 2009)

water ingression, melt eruptions,

and also unstable crust, all modeled in CORQUENCH

(plus heat conduction into concrete)

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• Base case with the same modeling options as in MELCOR/CORCON calculations, typically

– homogeneous melt layer

– melt-concrete interfacial heat transfer by gas_film model;

the slag_film model also used for base case and for most of sensitivity cases

– limestone/CS concrete with ~26%wt CaCO3

• Trying to mimic the decay heat exactly as given by MELCOR calculations (even though there might be some issues with

MELCOR), trying to have the right amount of water atop the melt for about the right time for the base case

• Sensitivity calculations mostly concerned with coolability models, different ways, for long-term water addition

• Main parameter for comparisons is the maximum ablation depth axially (downwards), potentially, of course, quenching of the melt

CORQUENCH modeling

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Base case (gas film model) CORQUENCH

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0 2 4 6 8 10 12 14 16 18

0 12 24 36 48 60 72

decay heat (MW)

time (hrs)

melt arrested (CORQUENCH) MELCOR/CORCON total decay heat

CORQUENCH total decay heat CORQUENCH decay heat in melt zone CORQUENCH decay heat in top crust

Base case: Decay heat

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0 2 4 6 8 10 12 14 16 18

0 12 24 36 48 60 72

heat loss up [to water or structures] (MW)

time (hrs)

melt arrested (CORQUENCH) CORQUENCH

MELCOR/CORCON

0 2 4 6 8 10 12 14 16

0 12 24 36 48 60 72

heat loss to concrete (MW)

time (hrs)

CORQUENCH MELCOR/CORCON

Base case: Heat losses "up" and heat losses to concrete

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0 5 10 15 20

0 10 20 30 40 50 60 70 80

time (hrs) 100

120 140 160 180 200 220 240

top crust thickness (cm)

CORQUENCH MELCOR/CORCON

Base case: Top crust and arrest of the melt

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Base case: Water in cavity and maximum ablation depth

0 100 200 300

0 10 20 30 40 50 60 70 80 90

maximum axial (downward) ablation (cm)

time (hrs) MELCOR/CORCON

CORQUENCH (gas_film) CORQUENCH (slag_film)

0 10 20 30 40 50 60 70 80 90

0 5 10 15 20

mass of water in cavity (tonnes)

time (hrs)

MELCOR/CORCON CORQUENCH (gas_film) CORQUENCH (slag_film)

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MELCOR/CORCON implementation issues

5 10 15 20

0 5 10 15 20 25 30

500 1000 1500 2000 2500 3000

decay heat (MW) melt temperatures (K)

time (hrs) CAV-T-HMX.10 CAV-T-HOX.10 CAV-T-LMX.10 CAV-T-LOX.10 CAV-T-MET.10

5 10 15 20

0 5 10 15 20 25 30

500 1000 1500 2000 2500 3000

decay heat (MW) melt temperatures (K)

time (hrs) CAV-T-HMX.10 CAV-T-HOX.10 CAV-T-LMX.10 CAV-T-LOX.10 CAV-T-MET.10

MELCOR 1.8.6 MELCOR 2.1

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CORQUENCH sensitivity analyses: time of water addition

0 5 10 15 20 25 30

0 20 40 60 80 100

heat loss up [to water or structures] (MW)

time (hrs)

base case water added 5hrs after VF water added 25hrs after VF

0 100 200

0 20 40 60 80 100

time (hrs)

base case water added 5hrs after VF water added 25hrs after VF

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Sensitivity calculations: modeling of melt eruptions

0 100 200

0 10 20 30 40 50 60 70 80 90

time (hrs)

eruptions: Ricou-Spalding model eruptions: Farmer’s model earlier water addition with Farmer’s model

0 0.02 0.04 0.06 0.08 0.10 0.12

0 10 20 30 40 50 60

average melt entrainment coefficient (%)

time (hrs)

eruptions: Ricou-Spalding model eruptions: Farmer’s model earlier water addition with Farmer’s model

˙

m

_me

= ( K

_e

· j

_g

) · ρ

_m

· A

_m

CORQUENCH melt entrainment:

different models for melt entrainment coeff can be used,

Ricou-Spalding, Farmer's (used in base_case), user defined const.

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Sensitivity calculations: worst case by CORQUENCH

ANCHORED CRUST

with late water addition (30hrs into the accident)

0 100 200 300 400

0 1 2 3 4 5 6 7 8 9 10

time (days)

MELCOR/CORCON CORQUENCH (anchored crust) CORQUENCH (standard floating crust)

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Conclusions

• MELCOR and CORQUENCH analyses of various long-term SBO scenarios were done in support of understanding the MCCI

progression in KONVOI PWR containment cavity

• basemat melt-through at this type of cavity doesn't seem to be a question of a short time: for the base case SBO scenario,

MELCOR/CORCON calculations show it would take at least 3 months (2000hrs)

• as inherent to MELCOR/CORCON, no "melt arrest" is predicted in any of the cases, not even with the water atop the melt

• in contrast, CORQUENCH calculations show quenching of the melt in cavity, i.e. stop of the MCCI progression, in most cases analyzed

– in base case scenario, melt arrest calculated at ~63hr, with less than a half of the total basemat thickness eroded

• sensitivity analyses with CORQUENCH support the assumed picture: putting water atop the melt (as a part of long-term AM) makes the quenching of the melt highly likely

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Thank you for your attention

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Proper choice of MELCOR/CORCON coordinate system

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Cavity model

Small case R=1.6 m A=8.0 m² Base case R=2.1 m A=13.9 m² Large case R=3.0 m A=28.3 m² 1 cylindrical cavity instead of 7 cavities no complicated flow paths

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Small case

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Large case

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Secret of CORCON (Origin of Rays)

Ray

Wall

calc. erosion

used radius

A high origin of the rays lead to reduction of the radius increase and therefore to an

increase of the melt level.

MELCOR manual: Origin of rays should be in the centre of the cavity PSI rule: Origin of rays should be at the expected melt surface

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Cavity size

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Influence of origin

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Cavity size

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Sensitivity with different sequence progression

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• The melt level does not support melt spreading to cavities outside the biological shield

• The increase in viscosity with increasing take up of concrete into the melt mixture would not allow significant spreading after the ventilation channels are reached (after melt-through of ~50cm of concrete above the channels), certainly not significant spreading upwards

• The estimate of the overall energy and mass balances from MCCI likely much more reasonable with the simpler model

• Chosen approach conservative anyway, that is, with respect to axial (downward) melt-through

Why to use the simpler model

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Example of corium-concrete mixture viscosities

• viscosities calculated by codes -also CORCON and

CORQUENCH- usually lower than experimental

• difficult to estimate VOLUMETRIC SOLID FRACTION in the melt which has crucial impact on viscosity

(typically for fractions >40%)

• for reliable estimates

specialized chemical equilibria calculations needed,

for multiphase, multicomponent system in question -example of a tool which can be used for such calculations is French code GEMINI

by GEMINI

(for SIL concrete)