Development and verification of molten corium–concrete interaction code

Progress in Nuclear Energy 85 (2015) 701e706

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Progress in Nuclear Energy journal homepage: www.elsevier.com/locate/pnucene

Development and verification of molten coriumeconcrete interaction code Bo Lin, S.Z. Qiu*, G.H. Su, W.X. Tian, Y.P. Zhang Department of Nuclear Science and Technology, State Key Laboratory of Multiphase Flowing Power Engineering, Xi'an Jiaotong University, Xi'an 710049, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 18 June 2014 Received in revised form 14 May 2015 Accepted 13 July 2015 Available online xxx

During a severe accident of Pressurized Water Reactor(PWR), the core materials was heated, melt located on the lower head of Reactor Pressure Vessel(RPV). With the temperature rise, the corium will melt through the lower head and discharge into the reactor cavity. Those corium will interact with the concrete and damage the integrity of the containment, so some coolability method should used to quench the corium. In order to investigate the progress of MCCI, a MCCI analysis code, that is MOCO, was developed. The MOCO includes the heat transfer behavior in axial and radial directions from the molten corium to the basemat and sidewall concrete, crust generation and growth, and coolability mechanisms reveal the concrete erosion and gas release, which are important for the interaction process. Cavity ablation depth, melt temperature, and gas release are the key parameters in the interaction research. The physical-chemistry reaction is also involved in MOCO code. In the present paper, the related MCCI experiment data were used to verify the models of the MOCO and the calculation results of MOCO were also compared with other MCCI analysis codes. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Severe accident MCCI Code Corium Top flooding

1. Introduction In a severe accident event in a Light Water Reactor, corium, a mixture of molten materials issued from the fuel, cladding and structural elements, appears in the reactor core. In some series of severe accidents, corium is assumed to penetrate through the reactor pressure vessel and spread over the concrete basemat. Molten Corium Concrete Interaction (MCCI) then occurs, characterized by concrete ablation. During the interaction between molten corium and concrete, the molten materials are maintained at high temperature by decay heat from fission products retained in the melt and the chemical heat from the exothermal reaction. The temperature and heat fluxes involved are sufficient to decompose and ablate concrete (Alarcon-Ruizet et al., 2005; Khoury, 2000). The containment failure would occur if the basemat concrete is penetrated. Besides, a large amount of water vapor and carbon dioxide produced with the decomposition of concrete, which can react with metals and produce hydrogen and carbon monoxide, can lead to the overpressurization of containment. Hydrogen and carbon monoxide are also combustible

* Corresponding author. E-mail addresses: [email protected] (B. Lin), [email protected] (S.Z. Qiu). http://dx.doi.org/10.1016/j.pnucene.2015.07.003 0149-1970/© 2015 Elsevier Ltd. All rights reserved.

gases, giving an additional risk of sudden overpressurization if they are ignited. Radioactive aerosols that evolve during core debris interactions can enhance radiological consequences of containment failure. Therefore a study on MCCI, which considers heat transfer mechanisms between molten corium and concrete determining the concrete ablation rate, and melt pool temperature, cooling from top flooding that determining the heat removal from the coolant (Fig. 1.), and chemical reactions that determining the noncondensible/ combustible gas generation and aerosol release rate, is needed for safety assessment under severe accidents. Various experimental and theoretical investigations have been conducted to understand the phenomena about MCCI. Large-scale or small-scale experiments were performed in many laboratories, and detailed computer codes were developed to simulate the MCCI phenomena. Many experiments were conducted in 1970e1990 at Sandia Nat. Lab.-SS, SWISS, SURC, HOT SOLID. (Powers and Arellano, 1981; Cronager et al., 1986) These tests mainly analyzed the behavior of concrete during the ablation process, the release of fission products, and also the ablation kinetics. Recent experiments on MCCI mainly address two subjects: the 2D aspects of the ablation and crust formation and melt segregation. Such as CCI (Farmer et al., 2005, 2006), VULCANO (Journeau et al., 2007), COMET-L (Alsmeyer et al., 2005). Several numerical methods have been

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Nomenclature C m j r x h e k d g E Ll P Q

specific heat (J/kg) mass (kg) velocity (m/s) reaction energy (kJ/mole) distance (m) heat transfer coefficient (W/m2 K) specific enthalpy (J/kg) thermal conductivity (W/(m K)) thickness (cm) gravity constant (m/s2) energy (J) Laplace constant system pressure (Pa) heat energy (J)

developed to simulate molten core concrete interaction and are used in safety studies to calculate the reactor cavity behavior during severe accident: CORCON (Bradley et al., 1993), COSACO (Nie et al., 2002), WECHSL (Reimann, 1987),TOLBIAC-ICB (Spindler et al., 2006) and ASTEC/MEDICIS (Cranga et al., 2005). It is well known that the calculation results of one-dimensional concrete ablation model predict well with the experimental results of SWISS (Blose et al.) and SURC (Copus et al., 1992) although the model cannot calculate the local ablation differences. And melt temperature history with time has been predicted exactly. But it is observed that large differences exist among the various experimental data and computer code calculations. Many efforts have attempted to increase the basic understanding of the unique heat and mass transfer processes that occur at the interface between corium and water when a coriumeconcrete interaction is flooded. A simple parametric core-concrete interaction code was developed for direct comparison with experimental results to provide a framework for assessing the ability of these models to adequately capture the corium cooling behavior. The purpose of this work is to develop an analysis code, MOCO, to understand the physical phenomena of MCCI during severe accidents. Compared with experimental data, the calculation results showed a reasonable agreement with the melt temperature, concrete ablation depth, and gas generation rate. The MOCO code considers the heat transfer behavior in axial and radial directions from the molten pool to the basemat and sidewall concrete, crust generation and growth, thermal stresses built-in the crust, disintegration of crust into debris by brittle fracture, natural convection heat transfer in debris, water ingression into the debris bed, and the gas release from the decomposed concrete. Calculation results are compared with corresponding experimental reports of MACE and CCI test series.

Coolant

Molten Corium

Concrete Fig. 1. Top flooding for achieving coolability.

T q kent G F N t

temperature (K) heat flux (J/m2 s) melt entrainment coefficient weight (N) mechanical strength (N) hole site density time (s)

Greek letters s surface tension (kg/m2) m viscosity (kg/m s) 4 enhancement factor h ablation depth (cm) r density (kg/m3) n gas velocity (cm/s) ε crust permeability

2. Physical and chemical models 2.1. Conservation equations The basic mass and energy conservation equations are solved to calculate the Molten Corium Concrete Interaction process. The mass conservation equation in the molten corium area can be written as:

vm vmside vmbasemat vmcore ¼ þ þ þ madd vt vt vt vt

(1)

where mside and mbasemat denote the mass from sidewall and basemat concrete to the corium area, respectively. mcore is the mass discharged from the lower head. madd is the mass as the volume change for the concrete ablation. Six heat source/sinks are considered in the energy conservation equation would consider in the code. The main heat source is the decay heat from the fission products. The others are heat from melt discharged from the failure RPV, chemical reactions between metallic melt constituents and concrete decomposition gases H2O, CO2, condensed phase chemical reactions between Zr and SiO2, heat transfer to concrete, including slag heat sink, and heat transfer to overlying atmosphere. The energy conservation equation in the molten corium area can be written as:

vE ¼ Qm þ Qdec þ Qht þ Eox þ Ede  Qc  Qent þ Qrpv þ Qadd vt (2) Where Qm is the energy due to the change of corium mass, Qdec is the decay heat, Qht is the heat transfer between corium and other parts.,Eox is the oxidation heat in the corium, Ede is the decomposition heat from the concrete, Qc is the heat transferred to the crust.,Qent is the entrainment heat from the corium to the upper crust, Qrpv is the heat from the mass from RPV, Qadd is the additional energy due to error from the mass calculation. In real reactor conditions, the melt composition can range from fully metallic to fully oxidic while those two phases are assumed to be well mixed in this code. The mass conservation considers most core, concrete metals and their corresponding oxides. Besides, the conservation of mass equations for the crusts and the particle bed are solved in separate ways, and then the material properties can be evaluated on the basis of the actual compositions in different layers.

B. Lin et al. / Progress in Nuclear Energy 85 (2015) 701e706

703

CaCO3 /CaO þ CO2  177:82 kJ=mole

(3)

there are two cases, porous or impervious crust, for the contact models of melt and concrete. The heat transfer model equations in MOCO adopted the gas film models that implemented in CORCON Mod3. The gases released from concrete are envisioned to produce a gas film that blankets the concrete surface. Heat transfer across the film is by convection and radiation. For the axial situation, the energy balance across the filmemelt interface is:

CaðOHÞ2 /CaO þ H2 O  109:45 kJ=mole

(4)

qaxial ¼ hmelt ðTmelt  TI Þ ¼ hrad;c ðTI  Tdec Þ þ hgas;c ðTI  Tdec Þ

MgCaðCO3 Þ2 /MgO þ CaO þ 2CO2  697:26 kJ=mole

(5)

2.2. Chemical reaction models During the interaction between the corium and concrete, the concrete would decompose as the high temperature of the corium, and the gas will release from the concrete. The main decompose reaction can be written as follow:

(15) Water vapor and carbon dioxide generated from the concrete decompose reaction could react with the metals in the molten corium, which will produce the hydrogen and carbon monoxide. Those reaction equations can be written as:

Zr þ 2H2 O/ZrO2 þ 2H2 þ 6:4 MJ=kg

where hrad,c and hgas are the radiation and convection heat transfer coefficients across the gas film, respectively, and hmelt is the melt convection coefficient. The melt heat transfer coefficienthmelt is evaluated with the KutateladzeeMalenkov correlation:

8 2=3  > kmelt > 3 cmelt Pjb > ; 1:5$10 > < k g Ll

(6)

2Cr þ 3H2 O/Cr2 O3 þ 3H2 þ 2:4 MJ=kg

(7)

Fe þ H2 O/FeO þ H2 þ 5:1 MJ=kg

(8)

Zr þ 2CO2 /ZrO2 þ 2CO þ 6:0 MJ=kg

(9)

2Cr þ 3CO2 /Cr2 O3 þ 3CO þ 2:0 MJ=kg

(10)

Fe þ CO2 /FeO þ CO  0:4 MJ=kg

(11)

Condensed phase chemical reactions between metallic Zr and SiO2 will be considered:

Zr þ 2SiO2 /ZrO2 þ SiðlÞ þ 1:9 MJ=kg

(12)

Zr þ 2SiO2 /ZrO2 þ SiOðgÞ  5:0 MJ=kg

(13)

melt

hmelt ¼

    > > c Pj 2=3 jtr 1=2 kmelt > > : 1:5$103 melt b ; kmelt g jb Ll

jb < jtr (16) jb  jtr

The radiation heat transfer coefficienthrad,c across the gas film is:


 2 ðTI þ Tdec Þ hrad;c ¼ Urad;c TI2 þ Tdec

(17)

Where Urad ¼ sstef/1/εmel tþ 1/εcon  1. Finally, the convection heat transfer coefficient is evaluated based on Taylor instability theory:

hg ¼ Co

kg 1=3 Re Lm

(18)

With those equation, the effective heat transfer coefficient from melt to the concrete is:

hmelt ðTmelt  TI Þ Tmelt  Tdec

where the reaction in the Eq. (12) happened below 2784 K. The other reaction only react above 2784 K.

haxial ¼

2.3. Concrete ablation models

2.4. Molten corium cooling models

Several different models are proposed to calculate the concrete ablation depth when the decomposable concrete was heated by the corium. A crust could form between the corium and concrete during the ablation process. The models in this code account for initial concrete surface heat-up phase, conduction into the concrete, even the growth and eventual failure of an interfacial corium crust. This is a complete model developed for calculating the transient crusting behavior in order to interpret the test results. The concrete ablation depth can be solved by the energy balance between the concrete and crust:

In the top flooding condition, this code considers the crust formation and evolution at the melt upper surface, and also considers additional cooling mechanisms, i.e., bulk cooling, water ingression, melt eruption, and crust failure. During the initial phase with surface flooded, the upper surface only considers film boiling. The heat transfer rate from the corium pool to the coolant can be written as (Farmer et al., 1990):

rDe

 vh dcrust vQ vT
DT ¼ þ kconcrete x ¼ xinterface þ kcrust vt vx dcrust 2 vt (14)

where, kconcrete and kcrust denote the thermal conductivity coefficient of concrete and corium crust, respectively. dcrust denotes the thickness of the crust. h denotes the concrete ablation depth. MOCO is able to perform a 1-D or 2-D ablation calculation. In the calculation, the surface area on the top of the melt pool is simplified as same as the area of the basemat for all the time. The phase stratification in melt pool is considered. It should be noted that

qcoolant

kfilm DT ¼4 þ qrad;w dfilm

(19)

! (20)

4 can explain the area enhancement as the gas sparging. The first term at the right denotes the heat removal across the gas film from corium to the coolant. qrad denotes the radiant heat flux. kfilm denotes the effective thermal conductivity of gas mixture in the film. The area enhancement factor is:

4 ¼ 1 þ 4:5

j uter

(21)

where j is the sparging rate. uter is the bubble terminal rise velocity. The radiant heat transfer coefficient is:

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  2 2 hrad;w ¼ sstef εmelt Tt;I þ Tsat Tt;I þ Tsat

(22)

Where sstef is StefaneBoltzman constant,and εmelt is melt radiation emissivity. The dynamic incipient crust growing progression can be calculated before a stable crust formation. As crust formation with the contact between the coolant and corium, the film boiling will be terminated after a stable crust is achieved. The other heat transfer mechanisms will play an important role in the following phase. The melt eruption through the crust has been observed in the reactor material experiments. Melt entrainment flowed through the top crust by the sparging gases can greatly augment the debris cooling rate. The melt entrained into the water is assumed to be quenched to form a particle bed on the crust. Generally, melt entrainment rate is assumed to be proportional to the gas volumetric flowrate. In this code, the models developed by Tourniaire and Seiler (2004) which consider the melt solidification and gas flow effects is adopted to calculate the average melt entrainment coefficient through the crust. The melt entrainment coefficient is correlated with the hole site density and mass entrainment rate:

kent ¼

dm N dt v

(23)

where, N denotes the hole site density:

00

Nh ¼

1 0 ! ðrt;c rmelt Þgk j C B 4 C Brffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi mg A @ 2 pDhole 2ðrt;c rmelt Þgdt ðrt;c rmelt Þgk  m KL r g

(24)

g

It is explicit that the heat transfer resistance from melt to water is proportional to the thickness of the crust which has to be passed through. So the heat removed from the coolant is limited by the conduction in the crust. The water ingression cooling can augment the heat transfer process which should be conduction-limited otherwise if the crust is porous. The criterion for onset of water ingression into the crust is that the total heat flux at the crust/water surface must fall below the crust dryout limit:

qdryout  kcrust

DT dcrust dQdecay þ rv hlv jT¼Tsat þ dcrust 2 dt

(25)

The dryout heat flux took into account the counter-current flow of noncondensable gases from core-concrete interaction and the decay heat within the crust. And the gas volumetric flowrate is evaluated at the coolant saturation temperature, since the debris bed is assumed at the saturation temperature during all times. The value of the dryout heat flux can be evaluated using the Jones et al. (1984) model with the specified crust permeability, which can be the form of:

qdryout ¼

εrv Delv ðrl  rv Þg 2mv

(26)

where ε is the crust permeability. Crust anchoring occurred in the prototypic experiments under the top flooding (Farmer et al., 2009) although it is expected impossible for the real plant scale. So crust anchoring has to be considered in this case rather than a simple floating crust treated in the dry cavity. Firstly, the criterion for crust anchoring criterion can be treated simply by the mechanical stability. According to the formula given by Roark and Young (2011), the equation to determine the minimum crust thickness required for mechanical stability can be expressed as:

Gapplied

load

 2:53scrust d2crust;min

(27)

where Gapplied load consists of three parts, the weights of overlying water, particle bed, and crust itself. From the above equation, the minimum crust thickness can be solved. Once the crust thickness meet the requirement, the crust upper surface is considered being fixed and the elevations of the bottom surface of the crust and the top surface of the melt will be saved. With the progress of MCCI, the melt-crust separation occurs when the two elevations are different. If the crust anchoring is initiated, it remains that way unless the crust thickness falls below the minimum value by remelting process. Then the crust is declared to return to floating case, correspondingly heat transfer equations and floating crust boundary conditions are applied again. 3. Verification and analysis of MOCO code The verification of MOCO includes the comparison between the MOCO calculations and experimental results, and the comparison with the other code results. The experiments which were used to validate the effectiveness of the MOCO include the tests with prototypic materials in a 1D geometry: MACE-M3b, and tests with prototypic materials in a 2D geometry: CCI-2. This set of experiments enables the simulation of oxidic corium concrete interaction and oxidic/metallic corium concrete interaction (CCI). The simulation of the MCCI experiments with prototypic materials is not easy since the initial conditions are not always known due to unexpected phenomena occurring in part of these tests. Melt ejection to the upper sidewalls of the apparatus sometimes occurs which lead to uncertainties in the corium mass participating in the MCCI. The consequence is that an interpretation of the real test conditions is often necessary and that several initial and boundary conditions can be defined for the simulation of one MCCI test. The purpose of an MCCI code is not to simulate the peculiar transient of a MCCI test, but to simulate long term MCCI in a reactor case, where corium at high temperature is supposed to be poured in the reactor cavity. Some results of validation are presented here, with short descriptions of the hypotheses used for the simulation. In the MACE M3b test, the basemat size was 1.2 m*1.2 m. Water was added at 52 min. The initial corium composed o f SiO2, CaO, MgO, Al2O3, Cr, Zr, ZrO2, UO2, which the mass were 63.80 kg, 49.10 kg, 28.60 kg, 7.8 kg, 100.00 kg, 0.0 kg, 513.00 kg, 1000 kg, respectively. The initial melt temperature was 2150 K. The calculations and the experimental data of the melt temperature for the M3b test describe in the Fig. 2. From the Fig. 2, the calculation with MEDICIS as well as WEX, which used the solidus temperature at the solid interface, underestimate the melt temperature in the actual experimental results. The MOCO, which used the liquidus temperature, is closed to the M3b test. There was a rapid decline at about 50 min, which was added with water at this moment. Fig. 3 shows the axial concrete ablation depth change with time. The results from those three codes are in the appropriate range compared with the test, which implied the heat transfer from the corium to the concrete in the code are assumed reasonable. The CCI-2 test addressed the long term 2-D corium concrete interaction with top flooding. The CCI-2 was conducted with limestone/common sand concrete. Water was poured into the cavity after 300 min. The initial melt temperature is 2150 K and the structure temperature is 750 K. The radial and axial ablation depths are compared with the test data in Fig. 4. The comparison indicates that the trend and magnitude of the ablation depths are reasonably agreement. After flooding, the ablation rate begins decreasing. The MOCO adopted the Bradley slag film heat transfer model (Bradley,

B. Lin et al. / Progress in Nuclear Energy 85 (2015) 701e706 2400

2500

MOCO MEDICIS WEX M3b

2200

2400

CCI-2 experimental data MOCO calculation

2300 2200

2000

temperature(K)

temperature(K)

705

1800

1600

2100 2000 1900 1800 1700

1400

1600 0

100

200

300

400

1500

500

0

40

80

120

160

t(min) Fig. 2. Melt temperature vs time (M3b experimental results and MOCO, WEX, MEDICIS calculation comparisons).

35

MOCO MEDICIS WEX M3b

ablation depth(cm)

30

25

20

15

10

5

0 0

100

200

200

240

280

320

360

400

time(min)

300

400

500

t(min) Fig. 3. The ablation depth vs time (M3b experimental results and MOCO, WEX, MEDICIS calculation comparisons).

Fig. 5. CCI-2 melt temperature vs. time.

1988) for the both lateral and axial conditions, so the both evolution of axial and radial ablation depth are identical. The melt temperature is compared with the CCI-2 test data in Fig. 5. It can be seen that, before 300 min when the cavity was dry, the MOCO calculation is in good agreement with the experimental data. But the calculation result is under-predicted after flooding. The CCI-2 test was conducted with the limestone/common sand concrete while CCI-3 used siliceous concrete. The biggest different between those two concrete types is the content of the decomposable component and SiO2. The limestone/common sand concrete, which be full of the CaCO3 and MgCa(CO3)2, will release more CO2 and H2O(Fig. 6.). From the picture, it is obvious that there weren't any gas release before about 50 min as the concrete temperature didn't reach the decomposable temperature. The water release of CCI-2 is almost the same as CCI-3 test, while the carbon dioxide is much higher than the siliceous concrete. The gas released from concrete will contact and react with the metals in the corium while the gas flows into the molten corium. Carbon monoxide and hydrogen produced from the reaction between the gas and metals (Fig. 7.). The large amount of carbon monoxide produced from the CCI-2 test because of the more release gas carbon dioxide from the concrete.

2000

H2O(CCI3) CO2(CCI3) H2O(CCI2) CO2(CCI2)

1600

gas realease(mole)

30

ablation depth(cm)

1800

CCI2 experimental data(axial) MOCO calculation(radial) MOCO calculation(axial) CCI2 experimental data(radial)

20

10

1400 1200 1000 800 600 400 200 0

0 0

50

100

150

200

250

300

time(min) Fig. 4. CCI-2 2D ablation depth vs. time.

350

400

0

20

40

60

80

100

120

140

160

time(min) Fig. 6. Gas realease from concrete decomposion.

180

200

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References

gas realease(mole)

600

CO(CCI2) H2(CCI2) CO(CCI3) H2(CCI3)

400

200

0 0

40

80

120

160

200

240

280

time(min) Fig. 7. Gas production from the oxide reaction.

4. Conclusions The MOCO code calculations for molten core concrete interaction under severe accidents are performed in order to understand the physical phenomena of MCCI. While the calculation results are compared with selected three experimental data, several areas are identified where future model development is required, and MOCO improvement needs have been assessed. Several peculiar models, e.g., ablation with shape of the cavity, radiation, melt ejection, mixing and stratification, enable a complete simulation of the different phenomena that occur during MCCI. MOCO has been validated through comparisons with data obtained in full oxide corium as well as in oxide/metal stratified melt experiments. The results of the simulations of MACE and CCI experiments with MOCO are satisfactory. Concrete ablation is fairly well-predicted by the model in all cases. This indicates a good balance between upward losses and heat transfer to the walls and floors contacting the debris. Transient heat conduction into concrete and crust are found to be important both during the very early and very late interaction phases.

Acknowledgments The authors wish to thank the colleagues in the NuThel laboratory for helpful and fruitful discussions.

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