Research on the Mechanism of Debris Bed Stratification. in Vessel Lower Plenum

Transcription

1 Research on the Mechanism of Debris Bed Stratification in Vessel Lower Plenum PEIWEN GU, KEMEI CAO, JIAYUN WANG 1 1 Shanghai Nuclear Research and Design Engineering Institute, SNERDI (China) ABSTRACT The 3-layer corium pool produced by MASCA Russian past experiments has posed a great threat to the success of In-Vessel Retention (IVR) strategy. Based on the study of core heat-up and corium relocation to the lower plenum, the integrity of the crust in the quasi-steady state of natural convection becomes an important factor to determine the configuration of corium pool. In the present paper, a novel analysis method is developed to study the debris bed stratification and predict the configuration of corium pool in lower plenum of AP1000 plant by using a Computational Fluid Dynamics (CFD) code. The results show that the validated Large Eddy Simulation (LES) model can well predict thermal-hydraulics in oxide and metal phases, thus producing a correct boundary condition for the crust integrity analysis. The stress in the crust is compared with experimental data to find that the crust has the possibility to maintain its integrity and keep oxide and metal phase separated, which prevents the reaction among molten corium materials and the formation of heavy metal layer. However, a more accurate and prototypical experiment is needed to narrow the range and reduce the uncertainty for the determination of corium pool configuration in the lower head.. 1 INTRODUCTION In-vessel retention (IVR) of core debris is a primary strategy for managing severe accidents in advanced passive plant design AP1000. The accident management strategy is to fully depressurize the reactor coolant system, flood the reactor cavity and submerge the reactor vessel by cooling water [1]. If the heat flux transferred from corium pool does not exceed the critical heat flux at every vessel location, the IVR becomes established. The IVR analyses are based on 2-layer configuration of corium pool, in which metal phase is formed above the oxide phase, as shown on Fig.1 [2]. However, the MASCA experiments at NRC Kurchatov (Russia) had shown that some amount of metallic uranium and zirconium may migrate from oxide phase to metal phase when corium pool is sub-oxidized, thus leading to the change of density and inversion of corium pool configuration [3]. If additional structural material melts after occurrence of layer inversion, a possible 3-layer corium pool is formed, as shown in Fig.2. The 3-layer corium pool has a thinner upper metal layer Session "Severe Accident Scenarios and Codes" 1/11

2 than the 2-layer pool, resulting in the increased heat flux on its contact location with vessel wall and a great threat to IVR success. In the present paper, a new analysis method is developed to study debris bed stratification and predict the configuration of corium pool by using CFD code in AP1000 NPP. The chapter 2 describes the relocation process of in-core debris to lower plenum which determines the initial corium pool state. The chapter 3 presents the selection of an appropriate turbulent model through a benchmark on the BALI experiment. The thermal-hydraulic study of oxide and metal phases has been performed in the chapters 4 and 5 to provide the boundary conditions for crust integrity analysis. The chapter 6 describes the analysis of crust integrity and possibility of contact between oxidic and metallic materials. The conclusions of the analyses are provided in the chapter 7. Vessel Wall Radiation Molten metal phase Molten oxide phase Crust Heat transfer Cooling water Fig.1: 2-layer corium pool Fig.2: 3-layer corium pool 2 RELOCATION OF IN-CORE DEBRIS TO LOWER PLENUM The phenomena associated with melting of the core and relocation of the molten debris to the lower plenum play an important role in the configuration of corium pool in AP1000. In large LOCA accident, the core is uncovered and heats up with the decreasing of water level. Portion of cladding is oxidized by steam, resulting in the rapid temperature rise in the core. The non-oxidized cladding and other materials (such as control rod), which has lower melting points, melt first and drain downward to the cooler regions of core and refreeze at the top of the support plate, in the fuel assembly bottom nozzles and bottom of the fuel rods, which are unfueled. The uranium dioxide fuel and oxidized cladding, which has a much higher melting point, melts afterwards and forms a molten corium pool in the core region. As the in-core corium pool develops towards the core barrel with the melting of more oxidic material, the relocation of molten debris to the lower plenum occurs due to the Session "Severe Accident Scenarios and Codes" 2/11

3 melt-through of core barrel. The lower core support plate, which supports lower internal structures and refrozen zirconium cladding, is superheated and melted by the ever increasing debris in lower plenum [1]. In the process of corium relocation, metallic and oxidic material have little chance to contact with each other and a 2-layer corium will be formed just after the melting of metallic material. An oxide crust, induced by temperature gradient, will be formed between metal and oxide phases and keep them separated. If the crust integrity is maintained in the quasi-steady state of natural convection in lower plenum, the 2-layer corium pool will be unchanged; otherwise a 3-layer configuration may occur due to the material interaction. Based on the conclusion of relocation process, the following analysis is carried out to determine the integrity of crust and possibility of chemical reaction. 3 SELECTION OF TURBULENT MODEL The oxide phase is a volumetrically heated liquid pool, with internal Rayleigh number (Ra ) up to No turbulent model has been validated under such high Ra. The CEA BALI experiment has been selected to demonstrate the validity of two promising models (k-ε model and LES model) [4] [5] for the modeling of oxide phase. The BALI experiment adopted simulant fluid and 2D slice geometry at scale 1:1 of constant thickness (15cm) [6]. The simulant fluid is volumetrically heated by Joule effect and cooled at boundary by organic liquid to reproduce the prototypical condition with Ra up to The mesh and boundary condition of BALI test modeling is shown in Fig.3. The ratio between power extracted from the top of oxide phase and the total dissipated power is an important parameter (P up /P tot ) in the IVR analysis. Table 1 shows the parameters obtained in the BALI test and calculated by k-ε and LES model respectively. The result calculated by LES model has a good agreement with the BALI test value while k-ε model cannot correctly predict P up /P tot with the error up to 21%. Fig.4 shows the comparison of dimensional temperature at the centerline obtained by two turbulent models with BALI test results. The LES model well predicts a stratified temperature profile in the bottom (H/R=1) and a uniform temperature profile at the top (H/R=0). The temperature profile estimated by k-ε model is totally uniform from H/R=0 to H/R=0.9, which shows an obvious discrepancy with the experiment results. Therefore, the LES model has been proven to be more appropriate for internally heated flow with high Ra and selected in the future modeling of oxide and metal phase. Table 1: Results calculated by k-ε and LES model Turbulent model P up /P tot BALI test result Error LES model 44.1% 44% 2% k-ε model 53.3% 44% 21% Session "Severe Accident Scenarios and Codes" 3/11

4 1.2 Dimensionless Temperature LES model BALI Test k-ε model Height/(Radus) Fig.3: Mesh and boundary condition in the modeling of BALI test Fig.4: Comparison of dimensionless temperature obtained by k-ε and LES model 4 MODELING OF OXIDE PHASE The oxide phase is volumetrically heated by the residual decay power and cooled at boundary by the surrounding crust with the uniform temperature of 2973 K. A 30 slice piece of the oxide phase with a height of 1.9 m in AP1000 lower plenum is simulated by CFD code due to computational capability. Fig.5 and Fig.6 show the boundary condition and the mesh of 1.03 million cells. Fig.5: Mesh of oxide phase in front view Fig.6: Mesh of oxide phase in top view The flow in oxide phase is a highly turbulent, quasi-steady, buoyancy-driven natural convection. The parameters such as temperature, pressure, heat flux, flow rate will fluctuate randomly within a range. The heat balance has been regarded as the convergence criterion. An unsteady calculation is conducted until heat imbalance is below 5%, as shown in Fig.7. The results obtained at the time of 1850 s and 2100 s have been selected as boundary condition in the following crust integrity analysis. Session "Severe Accident Scenarios and Codes" 4/11

5 Heat flow rate(w) Heat flow rate at boundary +5% error -5% error Heat balance Case1 Case Time(s) Fig.7: Heat balance of oxidic phase analysis Fig.8 and Fig.9 show the temperature and velocity magnitude distribution of oxide phase. The upper portion of oxide phase is a well-mixed pool volume dominated by the highly-turbulent flow with no observed temperature difference. The lower portion of oxide phase is an almost still fluid volume with clearly stratified temperature pattern. A steep boundary layer region adjacent to the isothermal boundary, which is indicated in Fig.9, is produced as a result of downward flow along the curved boundary layer from upper volume. All these flow features have a good agreement with interferograms of Jahn and Reineke in a slice (semi-circular) geometry [7]. Fig.8: Temperature distribution of oxide phase (K) Fig.9: Velocity distribution of oxide phase (m/s) Testing and evaluation of the uncertainties associated with the thermal loading produced by the molten debris pool in AP1000 were performed in the ACOPO experiment [8]. Based on its results, the Angelini-Theofanous correlations were developed to describe the heat transfer from the oxide phase containing the decay heat to lower head wall of reactor vessel. The correlations are valid up to a Ra of for oxide phase [8] [9]. The average heat flux on the top of oxide phase is calculated to be MW/m 2 and 1.05 MW/m 2 by using Angelini-Theofanous correlations and CFD code respectively with an error of 6.11%. Fig.10 shows the Session "Severe Accident Scenarios and Codes" 5/11

6 comparison of local heat flux along the vessel wall obtained by two different methods. The shapes and trends of both heat flux profiles are very similar. Only small deviations are observed in some local positions of vessel wall. Therefore, it seems that the results obtained by CFD code can reproduce the actual thermal-hydraulic phenomena of oxide phase in the lower plenum of AP calculated by Angelini-Theofanous correlations calculated by CFD code Heat flux(w/m2) Angle Fig.10: Comparison of heat flux profile along the vessel wall 5 MODELING OF METAL PHASE The metal phase is flat-plane shaped with a height of 0.63 m. The metallic phase contains no heat source, but is heated from bottom by molten oxide phase and cooled from sidewall and top by external cooing water and radiation respectively, which induces the natural convection in the metal phase. The temperature of sidewall is fixed at 1600 K due to the melting of vessel wall. The top wall of metal phase transfers heat to internal reactor structures with the emissivity of 0.4 and internal reactor s temperature of 1000 K. The heat flux at the bottom of metal phase can be obtained by the results of oxide phase modeling. A 30 slice piece of metal phase is simulated by CFD code as shown in Fig.11. An unsteady calculation is conducted to simulate the thermal-hydraulics in the metal phase. The heat imbalance is reduced below 5% at time of 1000s and fluctuates around the zero point up and down without exceeding 5% error, as shown in Fig.12, which means that a quasi-steady state is reached. The results obtained at the time of 1850 s and 2100 s have been selected as boundary condition in the following crust integrity analysis. Session "Severe Accident Scenarios and Codes" 6/11

7 80000 Heat flowrat (W) Heat flowrate at boundary +5% error -5% error Case 2 Case Time(s) Fig.11: Boundary condition of metal phase Fig.12: Heat balance of metal phase The metal phase is of a large aspect ratio (thin compared to its diameter) and with its sidewalls almost vertical. The flow regime in the metal phase belongs to Rayleigh-Benard natural convection, with the fluid well-mixed [10]. Fig.13 and Fig.14 show the temperature and velocity profile at the metal phase mid-plane. The velocity at the boundary of metal phase is large enough to take away the heat transferred from bottom to the cooler fluid at sidewall and top. Therefore, no stratified temperature layers are observed and the metallic fluid is well-mixed. Fig.13: Temperature profile of metal phase (K) Fig.14: Velocity profile of metal phase (m/s) The heat transfer from the metal layer to the vessel wall is calculated from Churchill-Chu and Globe-Dropkin correlations [11] [12] in AP1000 Probabilistic Risk Assessment (PRA) report. Table 2 shows the comparison of the results obtained by CFD code and correlations. The CFD code has reproduced the thermal-hydraulic phenomena with the maximum error of only 7%. Session "Severe Accident Scenarios and Codes" 7/11

8 Table 2: Comparison of metal phase results obtained by CFD code and correlations Parameter Average temperature at the bottom of metal phase(k) Average temperature at the top of metal phase(k) Obtained by correlation Obtained by CFD Error % % Average temperature of bulk fluid(k) % Average heat flux transferred from sidewall (MW/m 2 ) % 6 ANALYSIS OF CRUST INTEGRITY The crust is formed due to the temperature difference between the oxide phase and metal phase. In the quasi-steady state of natural convection at lower plenum, the flow pattern in oxide and metal phase applies forces on the crust bottom and top respectively, which may threaten the integrity of crust and create the opportunity of corium material mixing. A structural analysis of crust is conducted with the mechanical ANSYS code [[13]] to predict the maximum stress at different pressure boundary conditions. Two pressure distributions at crust top and bottom are obtained by the modeling results of metal and oxide phase, as shown from Fig.15 to Fig.18. Four stress calculations are performed through the combination of different pressure distribution. The crust, which is the product of solidified oxidic and metallic materials, is formed between the oxidic and metallic phase. The thickness of the crust is deduced to be 2.9 mm through Fourier s law of heat conduction. Table 3 shows the maximum stress (Von Mises) under four different boundary conditions. The second case shows the maximum stress of 4.97 MPa among four cases. The MACE experiments (performed in ANL in the past) were intended to provide the strength of corium which shares similar composition and configuration with the crust. The results indicate that the sample strength varies over a wide range, from a minimum of 2 MPa to a maximum of nearly 20 MPa, but the samples with the lowest content of concrete, which are very close to the composition of the crust, have a lowest strength of 5 MPa and a highest strength of 17 MPa [[14]]. Although the MACE experiments have some uncertainties associated with measurement techniques and non-prototypical condition, the result is still a valuable reference to assess the integrity of the crust in the quasi-steady state of Session "Severe Accident Scenarios and Codes" 8/11

9 natural convection. The samples conducted in MACE experiments contain some pores and cracks, which largely decreases the strength of crust. However, the in-vessel condition is different from the ex-vessel condition. The cracks and pores, caused by flashing of cooling water, will not appear in the in-vessel crust. Therefore it is conservative to determine the crust integrity based on MACE experiments. The lowest strength value of the sample (5 MPa) exceeds the possible maximum stress of the crust (4.97 MPa), which means the crust may remain intact and prevent the mixing of oxidic and metallic materials. Fig.15: Pressure distribution 1 at crust bottom (Pa) Fig.16: Pressure distribution 2 at crust bottom (Pa) Fig.17: Pressure distribution 1 at crust top (Pa) Fig.18: Pressure distribution 2 at crust top (Pa) Table 3: Maximum stress of the crust Serial number Top pressure distribution Bottom pressure distribution Max stress (MPa) Session "Severe Accident Scenarios and Codes" 9/11

10 7 CONCLUSION 6 th European Review meeting on Severe Accident Research (ERMSAR-2013) The 3-layer corium pool produced by MASCA Russian experiments has posed a great threat to the success of IVR strategy in AP1000 NPP. A novel method is established to predict the corium pool configuration in the lower plenum by using CFD code. Some assumptions have been made, such as the absence of debris formation by corium fragmentation when slumping in water present in the lower plenum and such as the absence of several successive corium flows from the core to the lower head. They are consistent with the AP1000 PRA results. The major results are summarized below. 1) Based on the progression of core heat-up and corium relocation to the lower plenum, oxidic and metallic materials have little chance to contact and react with each other before all the corium has been melted and relocated into the lower plenum and a quasi-steady state of nature convection is reached. A standard 2-layer corium pool is formed with the crust to keep the oxidic and metal phase separated. Therefore, the integrity of the crust in the quasi-steady state of natural convection becomes an important factor to determine the configuration of corium pool. 2) The analyses of the BALI CEA experiments with various turbulent models show that the k-ε model overestimates the heat flow rate at the top of oxidic pool and the temperature profile along the centerline while the LES model shows a good match with experimental results. The LES model is regarded as appropriate for the following modeling of oxide and metal phase. 3) The simulation of oxide and metal phase is performed by CFD code to reproduce the actual thermal-hydraulic phenomena in the lower head. The results have shown a good agreement with that obtained by correlations derived from MASCA experiments. Therefore a correct boundary condition can be provided for the integrity analyses of crust. 4) The theoretical analyses of crust integrity indicate that the maximum stress induced by the natural convection in oxide and metal phases is 4.97 MPa, which is below the lowest measured crust strength of 5 MPa. The crust has the possibility to maintain its integrity and keep oxide and metal phases separated, thus preventing the reaction between molten corium materials and the formation of heavy metal layer. 5) The experimental data about crust strength vary over a wide range, which produces difficulties for the assessment of crust integrity. A more accurate and prototypical experiment is needed to narrow the range and reduce the uncertainty for the determination of corium pool configuration in the lower head. REFERENCES [1] APP-GW-GL-700, AP1000 Design Control Document. [2] T. G. Theofanous, C. Liu, S. Additon el al., In-Vessel coolability and Retention Session "Severe Accident Scenarios and Codes" 10/11

11 of a Core Melt, DOE/ID (1994). [3] V. G. Asmolov, S. V. Bechta, V. F. Strizhov et al., Main Results of the MASCA1 and 2 Projects, OECD MASCA Integrated Report (2007). [4] R. R. Nourgaliev, Modeling and Analysis of Heat and Mass Transfer Processes during in-vessel Melt Progression Stage of Light Water Reactor (LWR) Severe Accidents, Doctoral Thesis, Royal Institute of Technology Stockholm, Sweden (1998). [5] T. N. Dinh, Y. Z. Yang et al., Rayleigh-Benard natural convection heat transfer: Pattern formation, complexity and predictability Proc. ICAPP 2004, Pittsburgh, USA, June 13-17, 2004, Paper 4241 (2004). [6] J.M. Bonnet, Thermal hydraulic phenomena in corium pools: the BALI experiment, Proc. Workshop on severe accident Research (SARJ-98), Tokyo, Japan, Nov. 4-6, 1998, (1999). [7] Jahn M. and Reineke H. H., Free Convection Heat Transfer with Internal Heat Sources Calculations and Measurements, Proceedings of the 5 th International Heat Transfer Conference, NC2.8, Tokyo, Japan, Sept. 3-7, (1974). [8] Theofanous, T. G. and S. Angelini, Natural Convection for In-Vessel Retention at Prototypic Rayleigh Numbers, Nuclear Engineering and Design, 200, 1-9 (2000). [9] Theofanous, T. G., In-Vessel Retention as a Severe Accident Management Strategy, Plenary Lecture, Proceedings of the Eighth International Topical Meeting on Nuclear Reactor Thermal Hydraulics, NURETH-8, Kyoto, Japan (1997). [10] A. S. Filippov, Numerical Simulation of Experiments on Turbulent Natural Convection of Heat Generating Liquid in Cylindrical Pool, Journal of Engineering Thermophysics, Vol.20, No.1, p.64 76, [11] Churchill, S. W., and H. S. Chu, Correlating Equations for Laminar and Turbulent Free Convection from a Vertical Plate, International Journal Heat Mass Transfer 18, (1975). [12] Globe, S. and D. Dropkin, Natural Convection Heat Transfer in Liquids Confined by Two Horizontal Plates and Heated from Below, Journal Heat Transfer 81, 24 (1959). [13] [14] S. Lomperski, M. T. Farmer, Corium Crust Strength Measurements, Nuclear Engineering and Design, 239, (2009). Session "Severe Accident Scenarios and Codes" 11/11