Corium Retention Strategy on VVER Under Severe Accident Conditions

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 Safety Concept Defense in Depth for NPPs with VVER

Level Situation Objective Essential means of DiD

Prevention of abnormal Conservative design and high quality in Normal operation Level 1 operation and failures construction and operation

Control, limiting and protection Operational Control of abnormal operation systems and other surveillance Level 2 occurrences and failures features

Design Basis Control of accidents within the Reactor protection system, safety Level 3 Accidents design basis systems, accident procedures

Beyond Design Control of accident with core Complementary measures and accident Level 4 Basis Accidents melt to limit off-site releases management

Mitigation of radiological Emergency Off-site emergency response and consequences of significant Level 5 planning accident management releases

NPP safety is ensured by consistent implementation of defense in depth, based on the usage of physical barriers systems. 2 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 Two corium retention concepts

In-vessel retention Ex-vessel retention Corium localization into water cooled RPV Corium localization into special device (core catcher)

RPV RPV

RPV failure elevation Corium pool Corium

Core catcher

Reactor cavity filled by water

Both concepts require justification… 4 Technical Background of Severe Accident Measures Procedures Development:

• The knowledge base obtained in 1994-2000 as a result of realization in NRC «Kurchatov Institute» of the international experimental research RASPLAV and RASPLAV-2 projects

• The knowledge base obtained in 2000-2006 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 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 iron to corium ratio High iron to corium ratio Low corium oxidation degree Medium corium oxidation degree High corium oxidation degree

Metal body Metal body Oxide Oxide Oxide

Metal body

ρmet> ρoxide ρmet~ ρoxide ρmet< ρoxide 6 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 Typical lower plenum Typical core catcher physical chemical processes: numerical domain numerical domain  2-D axial symmetric conductivity

 Volumetric heat decay

 Melting of the sacrificial material and mixing with the corium

 Thermal ablation of the concrete Domain

 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 Mesh surface of the molten pull

 External water cooling of the core catcher vessel 7 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 SCENARIO OF CALCULATIONS

The Large Break LOCA Scenario with Simultaneous Loss of the off-site Power Supply

VVER-600, VVER-1000, VVER-1200 (Project AES-2006), Type of reactors: VVER-1200+ (Project VVER-TOI)

A double-ended guillotine break of the cold leg near the Break location: reactor inlet

Water supply into the From SITs only reactor vessel:

In-vessel melt retention strategy Measure on severe (Flooding the reactor cavity and transferring the decay accident management: heat from corium through the wall to external water. Cooling mode is a pool boiling) 9 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% CALCULATION RESULTS for VVER-1000 Quasi-steady state

SOCRAT/HEFEST code ASTEC code

Metal layer

Oxide layer

Temperature field distribution, K 11 CALCULATION RESULTS for VVER-1000

Heat flux distribution along the RPV height

SOCRAT/HEFEST calculation results ASTEC calculation results Time = 9030 s Time = 9728 s

12 SOCRAT/HEFEST RESULTS OF VARIANT CALCULATIONS Chronology of main events of simulated accident

Time, s Preliminary #1 #2 #3 Event calculation G = 4 kg/s FP release G = 4 kg/s FP release + Deflector Accident initiation 0 0 0 0 Injection of water from SITs 5.5-54 5.5-54 5.5-54 5.5-54 Core heatup onset 700 980 700 980 Fuel cladding burst 1050-1280 1380-1610 1050-1280 1380-1620 Hydrogen generation onset 1160 1480 1160 1480 Fuel cladding melting onset 1250 1580 1250 1590 Melt transfer onset 1350 1680 1360 1680 Full core dryout 2290 3110 2340 3020 Beginning of corium transfer 3250 3620 3450 3860 to the lower plenum Core barrel melting-through 4340 4690 4910 5440 Reactor vessel failure not predic- 8900 11100 14000 ted* * the margin to CHF - 25% 13 SENSITIVITY STUDY with SOCRAT/HEFEST Calculation # 3 was chosen as basic calculation

14 sensitivity study calculations were performed with variation of:

F 1.5

1. Corium oxidation degree

H C

2. Mass of steel in the molten pool /

3. Initial temperature of corium , Critical value D 1

4. Degree of power reduction due to F H

volatile fission products

e v

5. Heat transfer coefficient on upper i

t a

surface of the molten pool l 0 7 e 0.5 1 8

6. Vessel steel heat conductivity R 2 9 3 10 7. Molten pool chemical composition 4 11 5 12 8. Power distribution between molten 6 13 pool layers 0 9. Eutectic temperature of (Fe, U, Zr) – 0 4 8 12 16 SS interaction Time, h

Relative heat flux density on the reactor vessel surface

14 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 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 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 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.

4 5

1 6 2

Test facility (NITI) Photo from experiment SACR-2 18 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; 7-11 - cassettes with sacrificial materials; 12 - thermal protection; 13 – service platform; 14 - ventilating corridor. 19 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. 50

213 Sacrificial material brick in the form of a triangular prism 20 Finite element model for HEFEST-ULR code

2-D axial geometry

Mesh Mass of the sacrificial materials in the core catcher Material Mass Iron oxide, ton 66 Aluminum oxide, ton 28 Concrete, ton 8 Steel, ton 64 Free volume, m3 35 21 Modeled chemical reactions in HEFEST-ULR code On the melt front: In the molten pool volume: Zr oxidation: Zr + 2H O = ZrO + 2H + Q 2 2 2 Zr oxidation: Fe2O3 + 1.5Zr = 2Fe + 1.5ZrO2 + Q FeO + 0.5 Zr = Fe + 0.5ZrO2 +Q Cr and Ni oxidation: Zr + O2 = ZrO2 + Q Сr + 1.5H O = 0.5Сr O + 1.5H + Q 2 2 3 2 Ni + H O = NiO + H + Q 2 2 Cr oxidation: Fe O + 2Cr = 2Fe + Cr O + Q 2 3 2 3 Cr + O2 = Cr2O3 + Q Fe2O3 + Ni = 2FeO + NiO + Q

Hematite restoration: Ni oxidation: Fe O = 2FeO + 0.5O – Q 2 3 2 Ni + 0,5 O2 = NiO + Q

Fe oxidation:

Fe + 0.5O2 = FeO + Q

Fe + H2O = FeO + H2 + Q

Chemical heat (Q) is taken into account in the total energy balance 22 HEFEST-ULR models verification

Basis for verification Verified models

The analytical decision of a problem of Propagation of the melting front Stefana

Salt experiments of RASPLAV project Convective heat exchange in the (NRC KI) conditions of crust formation on a cooled wall

Experiments of series AW-200 of Dynamics of the molten pool RASPLAV project formation in a natural corium (NRC KI)

Experiments of series SACR project Corium interaction with sacrificial (NITI) materials

23 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

80 3000

UO2 70 ZrO 2 2800 Zr 60 U Steel 2600 50 2400

40 Mass, t Mass, 2200

30 Temperature, K Temperature,

20 2000

10 1800

0 5000 6000 7000 8000 9000 10000 11000 12000 1600 5000 6000 7000 8000 9000 10000 11000 12000 Time, s Time, s Mass of corium, ton Corium temperature, K 24 Calculation results: Corium distribution

First corium portion Stratified corium pool configuration: slumps into CC direct stratification

25 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 Calculation results: Hydrogen generation

Hydrogen generation from Total hydrogen generation during core catcher severe accident on NPP with and without of the core catcher

100 In-vessel Ex-vessel 90 Total NPP project phase phase 80 (kg) (kg) (kg) 70

60 Without CC 700 1600 2300 50

40 Hydrogen mass, Hydrogen kg mass, 30 With CC 700 80 ~ 780 20

0 Maximal hydrogen amount from 0 2 4 6 8 10 12 14 16 18 20 22 24 core catcher is ~ 100 kg in case of Time, h complete concrete melting

Core catcher application for severe accident management removes a sharpness of a hydrogen hazard during ex-vessel stage of the accident

2727 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-1200.

*melt on RPV bottom 28 Core Catcher montage on NPP with VVER-1200

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