1. 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

2. 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 September 23 –25, Nesseber (Bulgaria)

3. 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 September 23 –25, Nesseber (Bulgaria)

4. 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 September 23 –25, Nesseber (Bulgaria)

5. Materials of the RBMK reactor
Graphite t
Fuel (U) t (2% enrichment)
Zr-1%Nb cladding 43 t
Zr-2.5%Nb central pipe 6 t
Zr-2.5%Nb channel pipes t
Supporting scheme “OR” 1422 t
Serpentine t
Black steel t
X18H t
X10H1M t
ERMSAR September 23 –25, Nesseber (Bulgaria)

6. 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 September 23 –25, Nesseber (Bulgaria)

7. 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 September 23 –25, Nesseber (Bulgaria)

8. 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 September 23 –25, Nesseber (Bulgaria)

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

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

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

12. 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 September 23 –25, Nesseber (Bulgaria)

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

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

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

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

17. 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 September 23 –25, Nesseber (Bulgaria)

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

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

20. 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 September 23 –25, Nesseber (Bulgaria)

21. 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 September 23 –25, Nesseber (Bulgaria)

22. 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 September 23 –25, Nesseber (Bulgaria)

23. 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 September 23 –25, Nesseber (Bulgaria)

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

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

26. 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 September 23 –25, Nesseber (Bulgaria)

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

28. 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 – s
Temperature: 1400 K
305/2
302/3
303/3
304/3
318/3
301/5
301/6
ERMSAR September 23 –25, Nesseber (Bulgaria)

29. 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 September 23 –25, Nesseber (Bulgaria)