Corium Retention Strategy on VVER under Severe Accident Conditions

Transcription

1 NATIONAL RESEARCH CENTRE «KURCHATOV INSTITUTE» Corium Retention Strategy on VVER under Severe Accident Conditions Yu. Zvonarev, I. Melnikov National Research Center «Kurchatov Institute», Russia, Moscow Technical Meeting IAEA on Phenomenology and Technologies Relevant to In-Vessel Melt Retention and Ex-Vessel Corium Cooling China, Shanghai, October 17-21, 2016

2 Safety Concept Defense in Depth for NPPs with VVER Level of DiD Situation Objective Essential means Level 1 Normal operation Prevention of abnormal operation and failures Conservative design and high quality in construction and operation Level 2 Operational occurrences Control of abnormal operation and failures Control, limiting and protection systems and other surveillance features Level 3 Design Basis Accidents Control of accidents within the design basis Reactor protection system, safety systems, accident procedures Level 4 Beyond Design Basis Accidents Control of accident with core melt to limit off-site releases Complementary measures and accident management Level 5 Emergency planning Mitigation of radiological consequences of significant releases Off-site emergency response and accident management NPP safety is ensured by consistent implementation of defense in depth, based on the usage of physical barriers systems. 2

3 Physical barriers in the path of ionization radiation propagation on NPPs with VVER 1. Fuel matrix 4. Containment last barrier 2. Fuel rod cladding 3. Primary circuit boundary The question is: How to save the last barrier under SA conditions? 3

4 Two corium retention concepts In-vessel retention Corium localization into water cooled RPV Ex-vessel retention Corium localization into special device (core catcher) RPV RPV Corium pool Corium RPV failure elevation Core catcher Reactor cavity filled by water Both concepts require justification 4

5 Technical Background of Severe Accident Measures Procedures Development: The knowledge base obtained in as a result of realization in NRC «Kurchatov Institute» of the international experimental research RASPLAV and RASPLAV-2 projects The knowledge base obtained in as a result of realization in NRC «Kurchatov Institute» of the international experimental research MASCA and MASCA-2 projects Development in NRC «Kurchatov Institute» specialized HEFEST-ULR code for modeling of processes take place in the core catcher or reactor lower head 5

6 Main Findings of RASPLAV and MASCA Projects Molten steel extracts some metallic uranium and zirconium from sub-oxidized corium Low iron to corium ratio Low corium oxidation degree Low iron to corium ratio Medium corium oxidation degree High iron to corium ratio High corium oxidation degree Oxide Metal body Oxide Metal body Metal body Oxide ρ met > ρ oxide ρ met ~ ρ oxide ρ met < ρ oxide 6

7 Mesh Domain HEFEST-ULR code HEFEST-ULR code based on results of performed experiments in common with available thermodynamic and thermochemistry knowledge was developed in NRC Kurchatov Institute. HEFEST-ULR simulates processes in lower plenum and core catcher during SA. Modeled thermal physics and physical chemical processes: 2-D axial symmetric conductivity Volumetric heat decay Typical lower plenum numerical domain Typical core catcher numerical domain Melting of the sacrificial material and mixing with the corium Thermal ablation of the concrete Chemical reactions between the sacrificial materials and the corium Molten pool formation and stratification Convective heat transfer between the layers of the molten materials Crust formation Radiation heat transfer from the upper surface of the molten pull External water cooling of the core catcher vessel 7

8 In-Vessel Melt Retention Strategy of Severe Accident Management for VVER IVR Process Modeling - Using SOCRAT/HEFEST and ASTEC Codes Russian SOCRAT/HEFEST code: simplified model of heat transfer to the water, detailed modeling of the melt. West European ASTEC code: model of 2-phase hydraulics for external cooling, simplified (point) simulation of melt structure. Uncertainty and Sensitivity studies: variation of code uncertain parameters (initial melt temperature, mass of the melt, melt composition, decay heat decrease due to FP release, etc..) 8

9 SCENARIO OF CALCULATIONS The Large Break LOCA Scenario with Simultaneous Loss of the off-site Power Supply Type of reactors: VVER-600, VVER-1000, VVER-1200 (Project AES-2006), VVER (Project VVER-TOI) Break location: A double-ended guillotine break of the cold leg near the reactor inlet Water supply into the reactor vessel: From SITs only Measure on severe accident management: In-vessel melt retention strategy (Flooding the reactor cavity and transferring the decay heat from corium through the wall to external water. Cooling mode is a pool boiling) 9

10 CALCULATION RESULTS for VVER-600 Expected corium pool configuration and temperature distribution Heat flux profile at 7500 s (on external RPV wall) Data on CHF - J.Yang, F.B. Cheung et al, Correlations Of Nucleate Boiling Heat Transfer And CHF For External Reactor Vessel Cooling ASME Summer Heat Transfer Conference, 2005 For VVER-600: Margin to critical heat flux ~ 10%

11 CALCULATION RESULTS for VVER-1000 Quasi-steady state SOCRAT/HEFEST code ASTEC code Metal layer Oxide layer Temperature field distribution, K 11

12 CALCULATION RESULTS for VVER-1000 Heat flux distribution along the RPV height SOCRAT/HEFEST calculation results Time = 9030 s ASTEC calculation results Time = 9728 s 12

13 SOCRAT/HEFEST RESULTS OF VARIANT CALCULATIONS Chronology of main events of simulated accident Event Preliminary calculation #1 G = 4 kg/s Time, s #2 FP release #3 G = 4 kg/s FP release + Deflector Accident initiation Injection of water from SITs Core heatup onset Fuel cladding burst Hydrogen generation onset Fuel cladding melting onset Melt transfer onset Full core dryout Beginning of corium transfer to the lower plenum Core barrel melting-through Reactor vessel failure * the margin to CHF - 25% not predicted* 13

14 Relative HFD, Q/CHF SENSITIVITY STUDY with SOCRAT/HEFEST Calculation # 3 was chosen as basic calculation 14 sensitivity study calculations were performed with variation of: 1. Corium oxidation degree Mass of steel in the molten pool 3. Initial temperature of corium 4. Degree of power reduction due to volatile fission products 1 Critical value 5. Heat transfer coefficient on upper surface of the molten pool 6. Vessel steel heat conductivity 7. Molten pool chemical composition 8. Power distribution between molten pool layers Eutectic temperature of (Fe, U, Zr) SS interaction Time, h Relative heat flux density on the reactor vessel surface 14

15 Conclusion to IVR strategy Preliminary estimations show the possibility of core melt retention in the RPV during severe accident for low powered VVER. Meanwhile, success of IVR strategy for VVER-1000 without any measures (such as additional water supply and intensification of heat transfer on external RPV wall) was not proved. The result is based on calculation results by SOCRAT/HEFEST and ASTEC codes. Justification of IVR strategy for VVER-1000 requires additional research, uncertainty decreasing and revised estimations. Performed calculations predict impossibility of IVR strategy usage during severe accidents for high powered VVER (VVER 1200 and higher). It is necessary to consider ex-vessel melt retention strategy to prevent last safety barrier destruction. 15

16 Ex-vessel retention strategy Main Technical Decisions for Core Catcher Development The choice of "crucible" type of core catcher design for melt localization and cooling by water; The application of double-layer wall for core catcher vessel to prevent its destruction due to thermal stress; The use of sacrificial materials from iron oxide and aluminum oxide to reduce the molten corium temperature and the volume density of decay heat release; Adding to the sacrificial materials a gadolinium oxide to provide subcritical state of the corium. Sacrificial Material Selection. Main Stages of Work Stage 1: Preliminary selection of oxidic material Stage 2: Theoretical and experimental examinations of corium and sacrificial materials phase properties. Stage 2 : Experimental study of corium Interactions with sacrificial materials 16

17 Sacrificial Material Requirements to sacrificial materials Intensive chemical interaction with corium oxide phase, resulting in decreasing of liquidus temperature. Intensive chemical interaction with corium metallic phase with oxidation of the strongest reducing agents, which are able to hydrogen generation at interaction with steam. Dilution of heat-generating corium with corresponding decreasing of density of energy flux and assurance of the system nuclear sub-criticality. Decreasing of both initial peak temperature and long-term temperature of corium due to proper cooling capability of sacrificial material. Minimizing of gases, vapors and aerosols generation, including radioactive ones. High degree of construction stability under dynamic mechanical loads and thermal shock. Absence of influence on normal operation during NPP lifetime. 17

18 Sacrificial Material Material composition Theoretical and experimental examinations allowed to recommend the following composition for sacrificial material: Fe2O3 (65-70 %), Al2O3 (28-30 %), Gd2O3 ( 0.15 %), SiO2 7 % Experimental research. SACR project Experimental investigation of corium with SM interaction was performed by NITI Test facility (NITI) Photo from experiment SACR-2 18

19 Design of the Core Catcher for VVER-1200 Core Catcher. Filler blocks installed into CC vessel 1 - reactor vessel ; 2 - bottom plate; 3 - console truss; 4 - technological corridor; 5 - core catcher vessel; 6 - reactor cavity; cassettes with sacrificial materials; 12 - thermal protection; 13 service platform; 14 - ventilating corridor. 19

20 50 Core Catcher filler block with Sacrificial Materials (4-th Layer) Sacrificial material composition: Fe2O3 (65-70%), Al2O3 (28-30%), Gd2O3 ( 0.15%), SiO2 7% Manufacturing technology: dosage; mixing; pressing; sintering. 213 Sacrificial material brick in the form of a triangular prism 20

21 Finite element model for HEFEST-ULR code 2-D axial geometry Mass of the sacrificial materials in the core catcher Mesh Material Mass Iron oxide, ton 66 Aluminum oxide, ton 28 Concrete, ton 8 Steel, ton 64 Free volume, m

22 Modeled chemical reactions in HEFEST-ULR code On the melt front: Zr oxidation: Zr + 2H 2 O = ZrO 2 + 2H 2 + Q Fe 2 O Zr = 2Fe + 1.5ZrO 2 + Q Cr and Ni oxidation: Сr + 1.5H 2 O = 0.5Сr 2 O H 2 + Q Ni + H 2 O = NiO + H 2 + Q Fe 2 O 3 + 2Cr = 2Fe + Cr 2 O 3 + Q Fe 2 O 3 + Ni = 2FeO + NiO + Q Hematite restoration: Fe 2 O 3 = 2FeO + 0.5O 2 Q Fe oxidation: Fe + 0.5O2 = FeO + Q Fe + H 2 O = FeO + H 2 + Q In the molten pool volume: Zr oxidation: FeO Zr = Fe + 0.5ZrO2 +Q Zr + O2 = ZrO2 + Q Cr oxidation: Cr + O2 = Cr2O3 + Q Ni oxidation: Ni + 0,5 O2 = NiO + Q Chemical heat (Q) is taken into account in the total energy balance 22

23 HEFEST-ULR models verification Basis for verification The analytical decision of a problem of Stefana Verified models Propagation of the melting front Salt experiments of RASPLAV project (NRC KI) Convective heat exchange in the conditions of crust formation on a cooled wall Experiments of series AW-200 of RASPLAV project (NRC KI) Experiments of series SACR project (NITI) Dynamics of the molten pool formation in a natural corium Corium interaction with sacrificial materials 23

24 Mass, t Temperature, K Numerical Analysis of corium localization in core catcher for VVER-1200 Severe accident scenario: Large Break LOCA with simultaneous station blackout. Main assumptions: Double-ended rupture of a RCS DN = 850 mm; No operator actions. Corium parameters coming from RPV UO 2 ZrO 2 Zr U Steel Time, s Mass of corium, ton Time, s Corium temperature, K 24

25 Calculation results: Corium distribution First corium portion slumps into CC Stratified corium pool configuration: direct stratification 25

26 Core Catcher cooling Stratified corium pool configuration: inverse stratification Heat flux profile at 5h Scale Oxide layer Metal layer Temperature field at quasi-steady state, K Minimal margin to critical heatflux is about 4 26

27 Hydrogen mass, kg Calculation results: Hydrogen generation Hydrogen generation from core catcher Total hydrogen generation during severe accident on NPP with and without of the core catcher NPP project In-vessel phase (kg) Ex-vessel phase (kg) Total (kg) Without CC Time, h With CC ~ 780 Maximal hydrogen amount from core catcher is ~ 100 kg in case of complete concrete melting Core catcher application for severe accident management removes a sharpness of a hydrogen hazard during ex-vessel stage of the accident 27

28 Conclusions Obtained results allow to draw the following conclusions: Justification of In-vessel* and Ex-vessel melt retention strategy was performed using HEFEST-ULR code. The IVMR strategy due to external RPV cooling is possible for low powered VVER. Success of IVMR strategy for VVER-1000 without any additional measures was not proved. Justification of IVR strategy for VVER-1000 requires additional research, uncertainty decreasing and revised estimations. R&D requires. Ex-vessel melt retention strategy should be used for high powered VVER power units. Based on ex-vessel melt retention strategy core catcher was designed for AES-2006 and VVER- TOI projects with VVER *melt on RPV bottom 28

29 Core Catcher montage on NPP with VVER

30 Thank you for your attention! 30