Mid term Containment TH with Ext. Vessel cooling and

MELCOR 1.8.6 Thermal Hydraulic and Iodine
Release Calculations for a Small LOCA Initiated
Severe Accident with Accident Mitigation
Measures
Gábor L. Horváth,
NUBIKI,Budapest
HorvathLG@nubiki.hu
Nuclear Safety Research Institute, Budapest
1/25
Contents
• Full circuit model for VVER-440/213 with
external vessel cooling and filtered vent
• Full circuit model TH results
• Stand alone VVER-440/213 containment model
• Stand alone containment model iodine results
• Conclusions
• Problems in MELCOR 1.8.6 IPM
Nuclear Safety Research Institute, Budapest
2/25
Accident conditions for dominating PSA-2 case: PDS-05C
Initiating event
SBLOCA d=11mm
ECCS
No
Cont. Init. State
Intact
Spray
No
Sec. Side depressurisation
Yes
Sec. Side FW
No
Prim. Side depressurisation
Yes
Early cont failure
No
Ex-vessel cooling
Yes
Filtered vent
Yes
Late phase cont. Failure
No
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Plant solution: External Vessel Cooling
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Detailed Full Circuit model: External Vessel Cooling
Double
• Drain to cavity and
• Discharge from cavity
Resulted in
Thermo ERROR and DT
decrease below limit
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Simplified Full Circuit model: External Vessel Cooling
Single
• Cavity flooding and
• Steam discharge from cavity
worked poorly in Stand Alone Cont.
Cavity Drain-Discharge Balance was OK
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6/25
Cavity flooding: Draining the pressure suppression pool water
Drainage starts at:
•550C core exit temp+30 min
•Drain duration 80 min
•Drain to cavity starts immediately
•In reality cirtain level should form
before drain to cavity
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VVER-440 Simplified Stand Alone Containment:
Sources: Primary circ + Ex-vessel cooling + BC drain+Fvent
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Filtered vent model for VVER-440/213 Stand Alone
Containment
Filtered vent from SG box:
• starts: P=3.3 bar
• stops: P=2.5 bar
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External reactor vessel cooling: Heat balance details
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External reactor vessel cooling: Heat balance - Gross
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External Reactor Vessel Cooling: Heat balance - Conclusions
Heat removed from RPV:
• early stages: Vessel wall dominates
• late stages: Lower Head dominates
• Very late stages: Vessel wall and LH are similar
Unresolved:
•Presence of FOCUSING EFFECT of molten metallic layer on RPV wall
•Effect of crust separating the molten metallic layer from the RPV wall
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12/25
Mid term Containment TH with Ext. Vessel cooling and FVent
in Stand Alone Containment – Small LLOCA d=11mm
Limiting case:
– No Alkalising agent added
to Sump water
– Water limited to Bubbler
Condenser (BC) amount
Thermal hydraulic sources from
Full Circuit Model:
– Pipebreak blowdown d=11mm
– PRV relief valve discharge
– SGbox-Cavity balance aft. Cavity
flooding
Model verification:
Full circuit and Stand Alone Containment Model
Calculated Pressures and Temperatures were very similar
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Mid term Containment TH with Ext. Vessel cooling and FVent
in Stand Alone Containment – Small LLOCA d=11mm : Pressures
Only BC water supply!
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Mid term Containment TH with Ext. Vessel cooling and FVent
Stand Alone Containment – Small LLOCA d=11mm : SGBox Local levels
Only BC water supply!
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in
Mid term Containment TH with Ext. Vessel cooling and FVent
Stand Alone Containment – Small LLOCA d=11mm : H2, O2, Steam
Only BC water supply!
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Mid term Containment TH with Ext. Vessel cooling and FVent
in Stand Alone Containment – Small LLOCA d=11mm : Leak rates
Only BC water supply!
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Small LLOCA d=11mm
Stand Alone Containment
No Alkalizing
Only BC water supply!
Medium term I release:
Containment, Op.Rooms,
Environment, AftFVent
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Mid term Containment I-131 Release with External Vessel cooling and FVent
in Stand Alone Containment – Small LLOCA d=11mm
•Corrected I-131 distribution:
Decay=10days
MELCOR: 2 CVs only calculated = 2.8-times
MELCOR mass balance errors: 8% (only!)
Volatile I-131
Aerosol I-131 (in CsI)
Room
I-131 TBq
Phase
Room
I-131 TBq
Phase
Oper.Rooms
172
Vapor in Gas
Oper.Rooms
4301
Deposited
Environment
216
Vapor in Gas
Environment
909
Vapor in Gas
Rest of
containment
3530
Bound
Rest of
containment
56342
Liquid
Only BC
water supply!
Deposited in the pool I-131 (in CsI)
Room
I-131 TBq
Phase
Sump
472431
MI (Bound)
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Long term Containment TH with Ext. Vessel cooling and FVent
in Stand Alone Containment – Small LLOCA d=11mm : Pressures
External water supply!
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Long term Containment TH with Ext. Vessel cooling and FVent
in Stand Alone Containment – Small LLOCA d=11mm : Levels
External water supply!
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Long term Containment TH with Ext. Vessel cooling and FVent
in Stand Alone Containment – Small LLOCA d=11mm : Leaks
External water supply!
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Small LLOCA d=11mm, Stand Alone Containment, No Alkalizing
1 .E+ 0 0
A e r o s o l (C s I a s I) a n d V o la tile (I) Io d in e s u s p e n d e d in s ys te m
p a r ts
Cs I- En v ir o n me n t
Long term I release:
Containment, Op.Rooms,
Environment
I- Co n ta in me n t
1 .E- 0 1
I- O p Ro o ms
I- En v ir o n me n t
Mass,kg
1 .E- 0 2
1 .E- 0 3
1 .E- 0 4
External water supply!
T im e ,d a y s
1 .E- 0 5
0
5
10
15
20
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Small LLOCA d=11mm, Stand Alone Containment, No Alkalizing
1 .0 0 0 0 0
80
V o la t ile ( I) Io d in e r e le a s e r a t e t o e n v ir o n m e n t
70
I-131 rate, Orig.Inv%/day
60
External water supply!
50
0 .0 1 0 0 0
40
0 .0 0 1 0 0
30
20
0 .0 0 0 1 0
I-131 rate, decayed Inv. TBq/d
Long term I-131 release with decay: Environment
0 .1 0 0 0 0
O ri g . In v% /d
10
7
T im e ,d a y s
D e c a ye d In v.T B q /d
0 .0 0 0 0 1
0
0
5
10
15
20
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Long term Containment I-131 release with External Vessel Cooling and FVent
in Stand Alone Containment – Small LLOCA d=11mm:
Summary
Environment after 3-25 days
Aerosol I-131 ceases after 3 days
For relatively short lived isotopes a single release
number for volatile I-131 is not informative
Release can be described by a rate:
– 10-20 TBq/d
during 10-20 days
– 5-7 TBq/d after 23 days
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MELCOR 1.8.6 In Vessel Retention and IPM model: Problems
In-Vessel Retention
– Optimum No of axial levels in LP/LH for FOCUSING EFFECT
– Melting of LH nodes? Is it calculated? How to plot?
IPM
– Max 2 control volumes with IPM can be activated
– Mass balance error is between 8-100% (depends on sequence)
– NRC questions the validity of the IPM model
However
– IPM reproduced the Phebus FPT-1 test well
– The plant calculated results seems to be reasonable
(although there is nothing to compare with)
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