Severe Accident Analysis of KKNPP-India - iaea.org loop heat transport system Double Containment ......

Severe Accident Modelling and

Analysis of VVER 1000 (V-412)

for Kudankulam NPP, India

Abhishek K, Sil S, Gaurav K, Gokhale

O.S, Mithilesh K, Saxena S.M.,

Mukhopadhyay D, Rammohan H.P ,

Biswas G

Presented by

Abhishek Kumar, Engg-LWR, NPCIL,

India

Technical Meeting on the Status and Evaluation of Severe Accident

Simulation Codes for WCR, Vienna, 09-12 Oct , 2017

Outline

� Salient Features of KKNPP

� Safety Systems of KKNPP

� In-vessel Analysis

� Ex-vessel Core Catcher Analysis

� Identification of Modelling Issues

� Experimental Program on Degraded Core RE-flood

� Conclusion

� References

� Questions

IAEA Technical Meeting on Status of Severe Accident Simulation Codes , Vienna, 09-12 Oct, 2017 1

Salient Features of KKNPP


� Rated Nuclear power: 3000

MW

� Rated Electrical power: 1000

MW

� Water cooled water moderated

(VVER) reactor

� Fuel–Enriched uranium (up to

3.92%) as fuel

� Four loop heat transport system

� Double Containment

� Steel lined Pre-Stressed

inner containment

� Reinforced Outer

Containment

� Plant designed to withstand

Tsunami, Seismic event ,Aircraft

Crash, Shock Waves etc.

Salient Features of KKNPP (Continued)Reactor Coolant System


� Rated Pressure at the

core outlet : 15.7 MPa

� Coolant temperature

� inlet to the reactor: 291 оС

� outlet from the reactor: 321оС

� Flow rate of coolant through

the reactor: 86000 m3/h

� No. of FAs in the core:163

� Shell and Tube Type Horizontal

SGs

� Pressure of steam generated

at the rated capacity: 6.27 MPa

Safety Features of KKNPP

� Inherent Safety

� Negative power coefficient: Wherein any increase in reactor

power is self terminating

� Negative Void Coefficient: reactor will shut down, if there is

loss of water

� Multiple Safety Barriers

� Fuel Matrix, Fuel Cladding, RCS Circuit, Inner and outer

Containment

� Engineered features and administrative measures provided to

protect these barriers

� Active Safety Systems

� Passive Safety System

� Safety Culture


Safety Features of KKNPP (Active)

� State-of-art reactor protection

and safety systems to handle all

the Design Basis Accidents and

Design Extension Conditions.

� 4 train active Safety systems :

single-failure criteria,

maintenance, knocked off by

PIE

� Active safety systems backed

by emergency power supply

(4x100% Emergency DGs)

� Safety Batteries for 24 hrs are

available for monitoring

� Containment spray system

5IAEA Technical Meeting on Status of Severe Accident Simulation Codes , Vienna, 09-12 Oct, 2017


Safety Features of KKNPP (Passive)

� High pressure passive ECCS

system (HA-1)

� Low pressure passive ECCS

system (HA-2)

� Passive Heat Removal System

� Core Catcher

� Passive Catalytic Hydrogen Re-

combiners

� Quick Boron Injection System

Safety Features of KKNPP (Passive) (Contd..)High Pressure Passive ECCS

� Passive injection of water directly into the core when primary

pressure drops below 5.89 MPa

� Four hydro accumulators kept pressurized to 5.89 MPa with

Nitrogen, each having 50m3 of water,10m3 gas

� Injections from passive accumulator are from both top & bottom of

the core and inject directly to RPV (Combined Top and Bottom

Flooding)

� Quick closing isolation valves prevent entry of N2 into the reactor at

low tank levels

� 4*33% capacity for large LOCA (2A break size of largest pipe,

850NB)


Safety Features of KKNPP (Passive) (Contd..)Low Pressure Passive ECCS


� Safety system for handling

LOCA+SBO

� Supplies Borated water into the core

to prevent core meltdown

� Maintains the inventory in the core for

24 hrs with HP accumulators along

with PHRS

� 8 tanks of 120 m3, actuation

pressure: 1.5 MPa

� Out of the four groups, 3 groups are

enough for fulfilling the intended

design criteria (4 X 33%)

� Combined Top and Bottom Flooding

� Designed Flow Profiling done in such

a way to match decay heat removal

0 40000 80000 120000

Time, sec

0

10

20

30

40

50

Ma

ss

flo

w r

ate

, k

g/s

Atmospferic air

LOSS OF POWER

Drag shaft

Atmospferic air

Drag shaft

Atmospheric

airAtmospheric

air

System

ensures long-

term removal

of reactor

core decay

heat in

absence of all

power

supplies

Steam

generator

Reactor

PASSIVE HEAT REMOVAL SYSTEM

Safety Features of KKNPP (Passive) (Contd..)Passive Heat Removal System (PHRS, Contd..)

� System maintains the hot shutdown condition of the reactor,

during a blackout(System operates in passive manner)

� Four trains with 3 air cooled HXs in each train

� Three trains are adequate to remove 2% decay heat

� Steam from SG is continuously passing through PHRS Hx

� Only air dampers at inlet and outlet are opened on SBO

signal, Regulatory dampers at outlet does the pressure

control

� Together with 2nd stage hydro accumulators remove decay

heat under LOCA+ SBO


Safety Features of KKNPP (Passive)(Contd..)Core Catcher


Fuel pondRVI

well

Core Catcher

� LOCA+SBO water from

primary loop, HA-1 and

HA-2 collects in sump

� Borated water of fuel

pond is drained in to core

catcher sump

� After 24 hrs HA-1 and

HA-2 gets exhausted

� Molten corium relocates

in core catcher after RPV

failure

� Melt is flooded from top

after 30 min

Analysis with ICARE Module of ASTEC

LOCA, 850mm, 2A + SBO Scenario


� No active system available, only

HA1 and HA2 available

� 5 radial mesh, 15 axial mesh

along with radial and axial power

profile

� One additional ring for bypass

and one for down-comer

� The lower head of RPV is

modelled as hemispherical

rather than elliptical(Conservative)

� Decay heat power profile

� 2 -D gas flow and 0-D liquid

channels

� Boundary condition: � Flow and Convective heat

transfer on Outer RPV wall

� Break flow as function of

pressure (from RELAP)

Nodalization




Physical Process Components Involved Correlation

Chemical Kinetics

Zr oxidation by steam Clad, Control rod guide

tube, grid spacers, debris

Bed

BEST-FIT (specific for

VVER)

UO2 & Zirconia

dissolution by molten Zr

Fuel element KIM-CONV

Steel oxidation by steam Control rod cladding,core

barrel, core baffle

MATPRO (J.F. White)

Magma oxidation by

Steam

U-Zr-O mixture UZOXMAG

Physical processes in reactor core & corresponding modelling




Physical Process Components Involved Correlation

Loss of integrity rules

Fuel rod cladding Zircaloy clad covered by

ZrO2 layer

Based on Tclad, δ –Cladding

temp, thickness of ZrO2

layer

Creep of clad Fuel rod cladding CREE model specific for Zr

1%Nb clad, Allows higher hoop

strain compared to Zr-4

Material movement Grid spacers, control

rod, clad, fuel, debris,

steel structures

DECA (radial movement);

MOVEMAG (2-D movement and

relocation of molten corium)

SLUMP (Material relocation in lower

head)

Vessel bottom head

failure

Lower head T-1500K

P-150MPa,

Melting fraction-0.7

Physical processes in reactor core & corresponding modelling


LOCA, 850mm, 2A + SBO Scenario (Results)

� RPV fail time, H2 generation

� Due to natural circulation

currents inside molten pool,

Non uniformity in heat flux

(Temperature) distribution on

RPV wall

� Vessel failure near highest

Temperature point


Full plant modelling of KKNPP in ASTECLOCA, 850mm, 2A + SBO Scenario


AC

CU

1,2

AC

CU

3,4

UPPLE2

UPPLE1

CORE

LOWER PLENUM

BY

PA

SS

DO

WN

CO

ME

RD

OW

CO

1

HA2

FLOW

PROFILE

AS BC

Reactor Nodalization

X X

Full plant modelling of KKNPP in ASTECLOCA, 850mm, 2A + SBO Scenario


MCP

PR

ES

SU

RIZ

ER

SURGE LINE

From UPPLE1

HOT LEG

COLD LEG

To DOWNCO1

Feed water

STEAM HEADER

MSH

BRU-A

SG

-CC

OL

SG

-HC

OL

SG-SEC

Nodalization of loop

Full plant modelling of KKNPP in ASTECLOCA, 850mm, 2A + SBO Scenario(Contd..)

� One broken loop modelled and balance three loops clubbed as one

� HA-2 flow profile as Boundary condition (Absence of dedicated model)

� Import of ICARE Reactor Core Model

� Early fuel heat-up observed in combined top and bottom

flooding of HA-2 water

� Stage -2 Accumulators water falling down in the core due to gravity head not getting simulated as there is no boundary condition available in ASTEC which allows the flow and pressure both implemented together with time

� Water was not able to penetrate till bottom of core for adequate cooling due to steam pressure

� Liquid water mass flow rate is not constant and even changes the direction at the interface between core channels and upper plenum bottom portion (Counter Current Flow Limitation)


RELSIM based VVER-1000 KKNPP Plant Analyser Development

� Analytical assessment of

SAMG

� Prediction of reactor

system behaviour during

normal and accident

conditions

� User is capable of

changing course of the

calculation

� Continuous display of plant

conditions


Extended SBO Simulation for KKNPP

� 7 days ESBO scenario with

6.6 t/hr leak from RCS

� Natural circulation assisted

by PHRS

� Air as the ultimate heat sink

� Maximum clad temperature :

below 600 K

� 200 kg/s natural circulation

flow for 168 hrs


Ex-vessel Core Catcher Analysis


1 – RPV 2 – concrete vault

3 – lower plate 4 – support

5 – Core catcher vessel 6 – Sacrificial

material

Sacrificial Material

(SM)

Oxidic SM

(OSM)

� Mass:65 tons

� Purpose:Preconditioning

of melt

� Composition:70 % Fe2O3,30 %

Al2O3, 0.02 %

Gd2O3

Steel SM

(SSM)

� Mass:100 tons

� Purpose:lowering the

incoming Melt

temperature

Ex-vessel Core Catcher Analysis(Contd..)


Molten corium relocates in core catcher

Melt stratifies into layers

Melt starts interacting With SM

Density of metallic layer decreases

Inversion of metallic layer takes place

Melt is flooded from top

54 tons of steel and 6 tons of Zr

80 tons of UO2 and 11tons of

ZrO2 and 8 tons of Zr

Bottom layer is oxidic layer and

top layer is metallic layer

Due to mixing of Al2o3

Zr+Fe2O3 = ZrO2+Fe+heat

Phenomena

Ex-vessel Core Catcher Analysis(Contd..)

Salient Features:

� 2-D Axi-symmetric model

with FVM methodology

� Homogeneous melt pool is

considered

� Homogeneous mixing of

SM after ablation

� 1-D modelling of top crust

� Modelling of melt-SM

interaction

� Interaction of melt with

OSM is only considered

1. Vessel

2. Basket

3. Thermal

protection

screen

4. SM

5. Concrete

6. Structure


Melt

SM

Basket

concrete

vessel

Ex-vessel Core Catcher Analysis (Contd..)

Governing Equations

� Melt

� Sacrificial material

� Crust


��

�= � �� + �� − � �� −�� − ��

1

�

�

��

�

��+

�

��

��+ �� =

�!

�

�

��

��+ �� = ��

�

�

Boundary condition

� Top surface of core

catcher:

� First 30 min: Radiative

� Afterwards:

Convective

� Side and bottom (outer

surface) of core catcher:

� Convective boundary

Ex-vessel Core Catcher Analysis (Contd..)


Steam and Zirconium reaction

Zr + 2H2O ZrO2 + 2H2 + Q

Q = 586.9 kJ/gm-mole of Zr

Zirconium reaction with Fe2O3

Fe2O3 + 1.5Zr 1.5ZrO2 + 2Fe + Q

Q = 914.611 KJ/gm-mole of Fe2O3

Governing Equations (contd..)

Kinetics of oxidation :

Parabolic law:

∅ ∅

�=

$∅

%

Arrhenius Equation: �∅ = &'()*+,

-.

Ex-vessel Core Catcher Analysis (Results)

� Benchmarking of code

against ACEL-5

experiment

� Melt front tracking in

VVER core catcher

� Cumulative hydrogen

generation from core

catcher is 16 kg


After 100 secs After 2500 secs

Modelling Issues Related to

In-vessel Phenomena

� Difficulty in modelling of CCFL phenomena in combined top

and bottom flooding

� More bypass through break and top flooding falling water in

the core driven away by steam

� To generate data validation of ASTEC model in case of top

flooding, experimental programme with both top and bottom

flooding into VVER type fuel bundle has been taken up.

� Entire HA-2 Circuit needs to be modelled in such a way to

capture the flow profile such that gravity head driven flow

takes places till core bottom


Modelling Issues Related to Ex-vessel Phenomena

� Detailed modelling of heat transfer behavior of stratified molten corium pool or suitable modelling improvements to address this

� Due to natural convection currents inside molten pool, Non uniformity in heat flux, local hot spots can be generated, therefore modelling improvements of NC

� Body Fitted Co-ordinate System to capture the exact core catcher geometry need to be adopted (CHF Variation along the curvature)

� Scarcity of experimental measurements of oxidation kinetics for a liquid U-Zr-O mixture , therefore Suitable numerical model of molten pool oxidation considering steam ingress instead of clad oxidation model considering unlimited steam

� Relocation of Molten Corium from RPV to core catcher assumed instantaneous whereas it is over a period,

� Numerical model for mixing of SM into molten pool considering mass diffusivity

� Physical properties of Corium (Solidus and Liquidus Temperatures, effective viscosity etc. especially at High temp


Experimental program on Re-flood of

degraded Core


� Work on Re-flood related thermal-

hydraulic process in core with large

axial ballooning- Scarce

� System codes for SA analysis have

re-flood models based on mostly DBA

experiments, thus 2 D 2 phase thermal

hydraulics in degraded central

ballooned region and intact bypass with

low flow an area of research

� May/May not be suitable for largely

ballooned geometry and low mass flow

rate in SAMG as compared to ECCS

mass flow rate

� To study the re-flood phenomena

with low mass flow rate (much lower

than SAMG recommended value) in

first state of degraded geometry with

60% axial ballooning and 45 % radial

blockage ratio of the effective fuel pin

simulator (FPS) length.

Experimental program on Re-floodDegraded Core Re-flood Experiment Test (DCRET) Facility at

Bhabha Atomic Research Centre, India


� A bundle of 57 FPS arranged in square pitch have been used

� Three different types of fuel pin simulators

�Straight fuel pins- 20 Nos

�Ballooned fuel pins – 25 Nos, Prefabricated

�Dummy fuel pins – 12 Nos

� Straight and ballooned fuel pins are electrically heated by central tungsten

rods

� SS clad for all three types of fuel pin simulators

Schematic

� SS pipe of 6 inch. Diameter Closed at either

ends by sealing flange free pin assembly

� 4 inlet ports for water or steam entry

� 2 outlet ports for steam exit

� 4 inch thick layer of Glass- wool to

minimize the heat loss

� 11 KW electric power, 25g/s flow at t=0

Modelling details and results


� Length of axial computational domain=1.48m

� No. of axial grid = 80 , Grid independence

� 2D Axi-symmetric model radial, axial meshing

� Four radially concentric rings• 1st ring – 9 no. of FPS with larger dia.

• 2nd ring – 16 no. of FPS with larger dia

• 3rd ring -20 no. of FPS with smaller dia.

• 4th ring -12 no. of dummy FPS

� Satisfactory predictions, peripheral clad temp

predictions slightly higher due to may be larger

bypass, More the ballooning/low flow more is the

chances of 2 D flow

300

400

500

600

700

800

900

-100 400 900Tem

pe

ratu

re (

K)

Quench Time (Sec)

Level 600

T1_600mm

T2_600mm

T3_600mm

EXP_T1

EXP_T2

EXP_T3

Experimental program for VVER Triangular

pitch fuel Simulator

� Efforts are on to have VVER scaled down triangular pitch

bundle configuration fuel pin simulator for

� Normal Bundle Configuration

� Pre-fabricated ballooned condition for low temperature

(upto 1000 deg C) and high temperature (upto 2200 deg.

C) with bottom and top re-flood conditions with different

flow rates

� To understand complex thermal-hydraulics and quenching

pattern with combined top and bottom flooding

� The minimum flow rates may be identified which would be

able to quench the bundle through both and individual

mode of flooding.


Conclusion

� Core is adequately cooled even for 7 days in case of Extended SBO

� In case of LOCA+ESBO Core Heat up till 24 hrs is avoided as core is

adequately cooled

� By introducing Core catcher the Hydrogen generation is adequately

suppressed thereby lowering load on Containment

� Core can be confined in Core Catcher

� Sufficient understanding developed by analytical and experimental

works for severe accident and modelling with state of art

computational tools as available internationally

� Modelling issues identified and dedicated efforts are ongoing to

address the issues

� Experimental programs are on-going to understand the complex re-

flood phenomena for bottom and top re-flooding modes


References

1. A. D. Vasiliev, “Modeling of Thermal Hydraulics Aspects of Combined Top and Bottom Water

Reflood Experiment PARAMETER-SF2 Using SOCRAT 2.1 Code”, Proceedings of International

Mechanical Engineering Congress and Exposition, Boston, Massachusetts, USA, October 31–

November 6, pp. 1915-1922 (200

2. Yu Zvonarev, A. Volchek, V. Kobzar, P. Chatelard, J.P. Van Dorsselaere, “ASTEC and

ICARE/CATHARE modelling improvement for VVERs”, Nuclear Engineering and Design, Volume

241(Issue 4), 2011, pp. 1055-1062 (2011)

3. Khabensky, V.B. , Granovsky, V.S., Bechta, Sevostian, Gusarov, V.V., “Severe accident

management concept of the VVER-1000 and the justification of corium retention in a crucible-type

core catcher”, Nuclear Engineering and Technology, Volume 41, pp 561-574 (2009)

4. B. Spindler, B. Tourniaire, J. Seiler, “Simulation of MCCI with the TOLBIAC-ICB code based on

the phase segregation model”, Nuclear Engineering and Design, Volume 236 (Issue 19), pp.

2264-2270 (2006)


NPCIL: Working towards green Future


Thank You

Questions?


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