РОССИЙСКАЯ АКАДЕМИЯ НАУКИнститут проблем безопасного развития атомной энергетики

RUSSIAN ACADEMY OF SCIENCES
Nuclear Safety Institute (IBRAE)

Corium debris
configurations in courseof accident

Valery Strizhov

Presentation outline
 Results of BSAF Project on corium location in thecontainment
 Configurations of molten materials in the reactor
pressure vessel based on the results of OECD/NEARASPLAV-MASCA Project Nuclear fuel behavior modeling during active phase ofthe Chernobyl accident (Results of ISTC-2916
Project)
 Results of investigations lava-like Fuel Containing
Masses (LFCM)
 Modeling of formation, spreading and cooling of LFCM

Corium debris stabilization in course of accident
In-vessel:• TMI-2• Fukushima unit 2 (?)

Ex-vessel:• Chernobyl-4• Fukushima units 1 and 3“Reactor core conditions of unit 1 – 3 of Fukushima Daiichi Nuclear Power Station” (Nov.30, 2011)“Evaluation of the situation of cores and containment vessels of Fukushima Daiichi Nuclear Power
Station Units-1 to 3 and examination into unsolved issues in the accident progression” (Aug 6, 2014)
BSAF Project Summary Report (June 2015)

Significant issues
In-vessel:
 Debris composition:
UO2-Zr-ZrO2-SS
 Melt configurations:
Depends uponcomposition
 Chemical Interactions:OECD RASPLAV-MASCA
Project Fission products
partitioning between
phasesEx-vessel:
 Debris composition:
UO2-ZrO2-FeO-CC
 Melt configurations:
Usually metal phase below
oxides MCCI: Extensive
experimental database(USA, Germany) Fission products release Spreading of moltenmaterials (France,Chernobyl accident)Possible melt configurations in the reactor pressure
vesselMetal phase

Oxide phase
Test MA-3

Test MA-2 (reconstruction)

Three possible configurations ofmolten materials in the lower head
• Top left – Low zirconium oxidation degree,small amount of steel (30 – 40%)
•Top right – High zirconium oxidation
degree (>70%)• Left down – Large amount of steel, high Zroxidation degree

Test MA-6

Melt in the reactor pressure vessel Goals of OECD RASPLAV-MASCA Project Material interactions at high temperatures (U-Zr-O-Fe)
 Conditions for pool stratifications (U/Zr ratio, degree ofoxidation)
 U-Zr-O-Fe(SS)+Oxidation atmosphere (steam/air) Assessments of corium debris for Fukushima Daiichi
Unit 1 in the RPV:
 Zirconium oxidation degree about 50%
 U to Zr ratio 0,8
 Mass ratio of steel in the melt: 0,3

 This parameters indicate that most probably theclassic configuration of phases (metal layer atop ofoxides) will be observed

Accident initiation
 April 26, 1986 reactor shut down
was planned for maintanancepurposes The test of was planned onelectric power supply due toturbine rundown Some safety systems wereturned off Due to different reasons reactor
operated with the violation ofrequirements for save operation
 Operation at small power andreactor shut down by emergency
protection rods lead tointroduction of positive reactivity
 All these reasons lead to thepositive reactivity and reactor
explosion

Stages of Fuel Investigations
 1986 – 87: Study of contaminated areas
 Study of fallouts
 More than 95% of fuel was located inside the Shelter 1988 – 92: Investigations in the Shelter Observations of lava-like fuel containing masses(LFCM)
 Drilling of boreholes and data accumulation of 1991 – 95: Extensive analysis of samples
 Methods for LFCM mass assessments
 Chemical analysis and generalization of data
 2005 – 2007: ISTC-2916 Project
 Systematic data analyses Development of the model for molten fuel behavior andinteractions

Molten core concrete interaction

 Sources of data: Visual and remoteobservations
 Bore holes dataobtained in 1988 –1992• level of about 9m: 25holes• level of about 10 m: 10holes• level of about 11 m: 8holes

Main streams of LFCM
Horizontal flow






Vertical flow

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 theaccident localization system)
Interaction with the concrete

Visual observations of LFCM

11

LFCM Source – under reactor room



UZrMg
SiCaAlNaBlackceramics
4.7±1.1
3.2±1.2
2.4±0.8
29.8 ±4.8
5.5 ±2.0
4.8 ±1.3
4.2 ±0.7

1 – Dominantly black ceramic
2 – Dominantly brown ceramic
3 – LFCM with high fuelconcentration

Brownceramics
8.4±0.2
4.8±1.1
4.0±0.930.9±3.6
4.7 ±0.8
3.5 ±0.7
4.0 ±0.4

Slag like from “Pumice”
PSP8.3±0.2
8.3±1.0
4.5±1.4
3.3±0.5
6.2±2.2
4.6±0.432.3±2.836.6±0.5
4.0 ±1.1
4.8 ±0.6
3.4 ±1.4
2.8 ±0.4
1.5 ±0.5
1.4 ±0.2

Reconstruction of initial data for LFCM generation
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
concrete4 - ¾ ОR
5 - BWC tubes
6 - Additional support
7 - Reflector (channels and graphite blocks)
8 - Reinforced-concrete plate (fragments ofwall of separator box)
9 - “L” tank
10 - Heat shielding lining of separator box’swall
11 - “D” tank
12 - ¼ ОR
13 - Damaged wall
14 - Vault’s filling-up-origin sand15 - Debris of reinforced-concrete
constructions
16 - Fragment of reinforced-concrete
constructionComputational model (Pancake model)
1. Basemate concrete

2. Under reactor structures(Steel, sand)3. “OR” Scheme (steel,serpentine)
4. Fuel containing masses(zirconium, steel, graphite,etc.)
5. Materials from upperstructures (concrete, materials
dropped into the reactorwreck)

Initial data:
3D geometry of rooms
Varied temperature (Base case 1400 K)
Assume two layers: black ceramics atop
Model includes
Advection of the melt
Radiation from melt top surface
Heat conductivity
Temperature dependence of viscosity
Melt source in the room 305/2
Characteristic time for graphite burning and melting throughof reactor basemate was assessed (between 7 to 10 days)

Spreading Through wall break
between 305 and 304
rooms (0.5 m)
 Melt flow rate throughthe wall break Total volume ofLFCM: 170 – 200 m3(Mass of 460 – 540tons)
 Mass source varied:
25 – 80 kg/s
 Duration varied: 6000
– 20000 s

 Temperature: 1400 K

Summary OECD RASPLAV-MASCA Project results
demonstrate possible melt configuration in thereactor pressure vessel Chernobyl lava location demonstrates highcorium flow-ability and long distances for
spreading even for small uranium content
 There is significant differences in the geometry
and configuration of debris and its locations
between Chernobyl and Fukushima
 Fukushima has more difficulties in terms of theaccessibility
 Urania content of Fukushima corium seems to behigher

 Molten materials may spread up to PCV walls due
to high corium flow-ability

References
Detailed information on characteristics of Chernobyl
fuel containing materials such as physical andchemical properties, structure, and other issues canbe found in references:E.Anderson, B.Burakov,
E.Pazukhin, Secondary variationsof fuel containing masses (FCM) of
4-th Chernobyl NPP unit,
Radiochemistry, 34, pp. 135-138,
1992 (In Russian).

Object “Shelter” – 10 years. Main
results of studies (In Russian)Chernobyl, 1996

R. V. Arutyunyan, L. A. Bolshov,A. A. Borovoi, E. P. Velikhov, A. A.
Klyuchnikov. Nuclear Fuel in the«Shelter» encasement of theChernobyl NPP, 2010. Moscow,Nauka.