A.Borovoi, S.Bogatov, V.Chudanov, V.Strizhov RRC “KI” The Chernobyl corium: generation, interaction with concrete and progression A.Borovoi, S.Bogatov, V.Chudanov, V.Strizhov

ERMSAR 2008 September 23 –25, Nesseber (Bulgaria) Presentation outline What do we know about melt behavior during late phase of Chernobyl accident? Accident initiation Formation of Lava-like Fuel Containing Masses (LFCM) Spreading of LFCM and cooling ERMSAR 2008 September 23 –25, Nesseber (Bulgaria)

Events before accident Low level of operation (below 200 MW) High coolant flow rate through the core (56-58)·103 m3/hr instead of 42∙103 m3/hr Low subcooling (5oC instead of 21oC) Low cavitations margin (0.5 MPa instead of 2.15 MPa) Low level of operational reactivity margin (7.2 control rods) while allowed minimal reactivity margin was 15 control rods Some safety systems were turned off Design features: Large positive void reactivity Positive reactivity induced by emergency protection rods Start of experiment on slow down of turbogenerator ERMSAR 2008 September 23 –25, Nesseber (Bulgaria)

RBMK reactor main structures Core Reactor vessel (shroud) Lower support structure (scheme “OR”) Upper reactor vessel head (scheme “E”) Annual water tank (scheme “L”) Steel graphite base plate Channel shield plugs Sand filling Concrete reactor cavity walls ERMSAR 2008 September 23 –25, Nesseber (Bulgaria)

Materials of the RBMK reactor Graphite 2000 t Fuel (U) 190 t (2% enrichment) Zr-1%Nb cladding 43 t Zr-2.5%Nb central pipe 6 t Zr-2.5%Nb channel pipes 126 t Supporting scheme “OR” 1422 t Serpentine 489 t Black steel 233 t X18H10 202 t X10H1M 498 t ERMSAR 2008 September 23 –25, Nesseber (Bulgaria)

ERMSAR 2008 September 23 –25, Nesseber (Bulgaria) 1 - Serpentinite of the “ОR” component and the inter-compensatory gap 2 - Crushed “С” component (“Cross”) 3 - Fuel, fuel assemblies, fuel elements, process channels, graphite blocks, fragmented concrete 4 - ¾ ОR 5 - BWC tubes 6 - Additional support 7 - Reflector (channels and graphite blocks) 8 - Reinforced-concrete plate (fragments of wall of separator box) 9 - “L” tank 10 - Heat shielding lining of separator box’s wall 11 - “D” tank 12 - ¼ ОR 13 - Damaged wall 14 - Vault’s filling-up-origin sand 15 - Debris of reinforced-concrete constructions 16 - Fragment of reinforced-concrete construction LAVA GENERATION MODEL LAYOUT OF CONSTRUCTIONS Time: 30 minutes after the explosion ERMSAR 2008 September 23 –25, Nesseber (Bulgaria)

ERMSAR 2008 September 23 –25, Nesseber (Bulgaria) Melt progression Upper part of core was thrown out by explosion. South-Eastern upper part of the reactor basement plate (“OR”) was damaged by explosion; Fuel-cladding-channel tubes interactions, graphite burning out, formation of liquids; Compaction of molten materials, start of interaction with the OR plate; Formation of LFCM as a result of interactions, OR melting, start of interactions with concrete. ERMSAR 2008 September 23 –25, Nesseber (Bulgaria)

Computational model (Pancake model) Basemate concrete Under reactor structures (Steel, sand) “OR” Scheme (steel, serpentine) Fuel containing masses (zirconium, steel, graphite, etc.) Materials from upper structures (concrete, materials dropped into the reactor wreck) ERMSAR 2008 September 23 –25, Nesseber (Bulgaria)

Conditions: 10 days after accident The conclusion was that graphite burning and melting through of OR may last between 7 to 10 days ERMSAR 2008 September 23 –25, Nesseber (Bulgaria)

Location of LFCM in the room 305/2 ERMSAR 2008 September 23 –25, Nesseber (Bulgaria)

Core and structural materials in the LFCM ERMSAR 2008 September 23 –25, Nesseber (Bulgaria)

Molten core concrete interaction (1) Bore holes data obtained in 1988 – 1992 level of about 9m: 25 holes level of about 10 m: 10 holes level of about 11 m: 8 holes ERMSAR 2008 September 23 –25, Nesseber (Bulgaria)

ERMSAR 2008 September 23 –25, Nesseber (Bulgaria) Section K-3000 ERMSAR 2008 September 23 –25, Nesseber (Bulgaria)

ERMSAR 2008 September 23 –25, Nesseber (Bulgaria) Section K ERMSAR 2008 September 23 –25, Nesseber (Bulgaria)

ERMSAR 2008 September 23 –25, Nesseber (Bulgaria) Section K+2400 ERMSAR 2008 September 23 –25, Nesseber (Bulgaria)

Concrete erosion depth (Room 305/2) ERMSAR 2008 September 23 –25, Nesseber (Bulgaria)

ERMSAR 2008 September 23 –25, Nesseber (Bulgaria) Melt progression Initial melt was formed in the south-eastern part of the reactor after interaction with the serpentine filling of the “OR” scheme Spreading of the melt was in horizontal (through the breach through the wall between rooms 305/2 and 304/3) Spreading in the vertical directions (through the steam outlet valves of the accident localization system) Interaction with the concrete ERMSAR 2008 September 23 –25, Nesseber (Bulgaria)

ERMSAR 2008 September 23 –25, Nesseber (Bulgaria) VERTICAL LAVA FLOW ERMSAR 2008 September 23 –25, Nesseber (Bulgaria)

ERMSAR 2008 September 23 –25, Nesseber (Bulgaria) HORIZONTAL LAVA FLOW ERMSAR 2008 September 23 –25, Nesseber (Bulgaria)

Simulation of LFCM progression Initial data: 3D geometry of rooms Varied temperature (Base case 1400 K) Assume two layers: black ceramics atop Models included Advection of the melt Radiation from melt top surface Heat conductivity Temperature dependence of viscosity Melt source in the room 305/2 ERMSAR 2008 September 23 –25, Nesseber (Bulgaria)

Heat generation (W/kg U) 24 hours: 89 W/kg 72 hours: 68 W/kg 120 hours: 59 W/kg 240 hours:46 W/kg ERMSAR 2008 September 23 –25, Nesseber (Bulgaria)

Model of long term behavior of LFCM The objective is to analyze the following processes: Concrete erosion depth Concrete decomposition depth LFCM cooling rates to assess characteristic times Length of spreading Criteria of success Correspondence to observations Assessment of spreading ERMSAR 2008 September 23 –25, Nesseber (Bulgaria)

ERMSAR 2008 September 23 –25, Nesseber (Bulgaria) Melt progression Competitive processes: Spreading of molten materials Cooling of the melt Parametric studies were performed to assess Characteristic cooling time Characteristic spreading time ERMSAR 2008 September 23 –25, Nesseber (Bulgaria)

Results of calculations (4%) Characteristic upper boundary cooling time about 5 hr Characteristic LFCM cooling time about 20 hr ERMSAR 2008 September 23 –25, Nesseber (Bulgaria)

Spreading model: Parametric study ERMSAR 2008 September 23 –25, Nesseber (Bulgaria)

ERMSAR 2008 September 23 –25, Nesseber (Bulgaria) Results of analysis Spreading time is much shorter then cooling time if temperature exceeds 1100oC Most probable temperature at the start of spreading was between 1000 and 1100oC ERMSAR 2008 September 23 –25, Nesseber (Bulgaria)

Initial data and results of MCCI simulation ERMSAR 2008 September 23 –25, Nesseber (Bulgaria)

Example of LFCM spreading Through wall break between 305 and 304 rooms (0.5 m) Melt flow rate through the wall break Total volume of LFCM: 170 – 200 m3 (Mass of 460 – 540 tons) Mass source varied: 25 – 80 kg/s Duration varied: 6000 – 20000 s Temperature: 1400 K 305/2 302/3 303/3 304/3 318/3 301/5 301/6 ERMSAR 2008 September 23 –25, Nesseber (Bulgaria)

ERMSAR 2008 September 23 –25, Nesseber (Bulgaria) Summary Models developed allowed assessments of significant phenomena during late phase of accident and timing of events Significant MCCI was observed in the room 305/2 (up to 1.4 m or about 2/3 of the thickness) No significant interactions with concrete were predicted in the rooms 304/3, 301/5 and 303/3 Thickness of LFCM is about 0.5 m, sustainable temperature about 1300 – 1400 K. ERMSAR 2008 September 23 –25, Nesseber (Bulgaria)