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First results and next step of experimental investigation on melt spreading on top of the shield plate penetrated by a forest of CRGTs and IGTs�

Di Fang

Royal Institute of Technology (KTH)

Division of Nuclear Science and Engineering (NSE)

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Half-time webinar for APRI-12, June 4, 2025

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Ex-vessel melt behavior

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  • In a SA scenario, the core melt (corium) is expected to discharge into the reactor cavity upon the failure of RPV.

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  • The corium will undergo fuel-coolant-interactions (FCI), and the ex-vessel debris coolability is of vital importance to corium stabilization and accident termination.

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  • Previous assumption for all studies on ex-vessel steam explosion and debris coolability: a coherent melt jet falls from the lower head of the RPV into a deep-water pool in the BWR.

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Illustration of corium injection into the drywell

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Lessons from Fukushima accident

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[1] Farmer, M. T. (2018). The MELTSPREAD Code for Modeling of Ex-Vessel Core Debris Spreading Behavior (ANL--18/30, 1483992; p. ANL--18/30, 1483992)

[2] Yamashita, T., Sato, T., Madokoro, H., & Nagae, Y. (2022). BWR lower head penetration failure test focusing on eutectic melting. Annals of Nuclear Energy, 173, 109129.

Reactor vessel head

Horizontal Beams

Control rod drive housings (185)

Hanger rods

Horizontal support bars, grid plates and grid clamp

Instrument tube TYP. (55)

Railings

Platform I-beams (4)

Wheels

Doorway

Sump

Water level

CRD flanges

Illustration of Below Vessel Structure in a Mark I containment [1]

Illustration of Below Vessel Structure in a Mark I System [2]

Pedestal wall

  • Out of this image: reactor vessel insulation panel (~10 cm thick) between the vessel head and horizontal beams

Vessel support skirt

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  • BWR has a forest of structures and supporting plates below the lower head of the RPV

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  • The structures should have impacts on the melt relocation process before entering the water pool, as revealed by the preliminary investigation of a remotely-operated robot in FDNPP.

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Depiction of corium interaction with

below vessel structure in a Mark I containment

CRD housing brackets now (Unit 3)

2017.7.24 Unit 3 PCV internal investigation (Preliminary report of July 22 investigation). 2017. [video] TEPCO: TEPCO

Lessons from Fukushima accident (contd.)

CRD housing brackets�before the accident (Unit 3)

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Concerned problems in the Nordic BWR

Simplified into

  • Will the structures be able to survive the thermal attack? If not, what will be the process/pattern of the structure failure?

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  • How will the jet flow pattern be influenced by the presence of surrounding structures (tube, beam, board)?

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  • How does the melt freezing on structures progress?

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BWR 75 – reactor containment [1]

[1] Nilsson, L. (2006). Development of an Input Model to  MELCOR 1.8.5 for Oskarshamn 3 BWR.

  • Jet behavior in air
  • Single jet or multiple jets
  • Jet mode into rain mode
  • Will a sufficient amount of melt freeze on the structure
  • Will that block the jet relocation path

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Approach for melt-structure interaction investigation

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Nordic BWR

DEFOR-S

Simulation

MPS or MELTSPREAD

  • Jet diameter
  • Material
  • Structures
  • Free fall height
  • Jet diameter
  • Substrate
  • Free fall height
  • Structures

Experiment

MELCOR

FDNPP

COSIN

  • Jet diameter
  • Material
  • Structures

Melt spreading

Structure melting

Melt distribution

Dynamic characteristics

Debris final configuration

Heat transfer characteristics

Extended prediction, guideline

Boundary condition, database

Scaling analysis

focused phenomenon

Simplified structures design

Prototypical prediction

Test separate effect

Test integral effect

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Simplified scaling analysis

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Jet releasing

Melt spreading

Jet impingement

Jet along structures

BWR

Bernoulli's equation

Dinh’s spreading model

Model of liquid metal flow in bundles

Jet impingement model

Vj , Hfree

DS, G, Hs, Dr

T, L

Hs, Lpicth, Dd

Radiation shield

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Simplified scaling analysis

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Parameters

Prototypical

Scaling ratios

Test

Selection

Pressure (bar)

2 - 5

-

1

1

Jet diameter from outlet (mm)

70 -223

1:6

13 - 37

20

Melt mass (kg)

64600

1:529

122

20

Releasing time of metallic melt (s)

181

1:6

30

22

Velocity at horizontal substrate (m/s)

4.26 -8.31

1:2.4

1.74 - 3.39

2.5

Structure height (m)

0.475

1:6

0.08

0.08

Structure diameter or CRD housing (mm）

190

1:6

33

33

Distance between Radiation protection and damper (m)

4.5

1:6

0.75

0.75

Damper height (mm)

122

1:6

20

20

Damper Diameter (mm)

265

1:6

44

44

Pitch diameter ratio of structures

1.58

1:1

1.58

1.58

Jet and structure diameter ratios

0.37 – 1.23

1:1

0.37 – 1.23

0.6,1

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Test facility - COSIN

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Induction heating furnace for melt preparation

Funnel

Spreading plate (quartz)

1000 x 1000 mm

Structures

Nozzle

TCs

Purpose

Thermocouples (1.5 mm above substrate)

Boundary layer temperature on the top surface of substrate

Thermocouples (5 mm above substrate)

Melt top surface temperature

Thermocouples insides structures (1.5 mm above substrate)

Heat transfer through structures

Thermocouples at different height

Melt temperature at different height

Instrumentation details

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Test matrix-melt spreading with structures (COSIN)

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Experiment

Simulant

Spreading plate

Melt mass (kg)

Melt superheat (℃)

Velocity

(m/s)

Free fall height (mm)

Jet diameter (mm)

With structures

Structure diameter

(mm)

Structure

material

COSIN-S1

Zn

Quartz

20

102

2

200

10

No

-

-

COSIN-S2

Zn

76

2

200

20

No

-

-

COSIN-S3

Zn

Quartz

80

2

200

20

Yes

33

SS

COSIN-S4

Zn

SS

20

80

2

200

20

Yes

33

SS

COSIN-S5

Zn

Quartz

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80

2.6

400

20

Yes

33

SS

COSIN-S6

Zn

80

2

200

30

Yes

33

SS

COSIN-S7

Bi2O3-WO3

80

2

200

20

Yes

33

SS

COSIN-S8

Zn

80

2

200

20

Yes

33

Zn or Sn

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The new test: COSIN-S3

  • Test S3 was performed to investigate the melt spreading with penetration structures on radiation shield beneath RPV.

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COSIN-S3: Visualization (compared with COSIN-S2)

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COSIN-S2

COSIN-S3

Tsup=76 ℃; Hfree=200 mm

Without structures

Tsup=80 ℃; Hfree=200 mm

With structures

0.2X

0.2X

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COSIN-S3: Final configuration (compared with COSIN-S2)

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Melt spreading profile

COSIN-S2

Melt thickness

Cavity formation

COSIN-S3

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Conclusion

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  • The first test of melt spreading on a plate with a forest of structures (COSIN-S3) has been successfully carried out;

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  • In comparison to plain-surface case, the structures in COSIN-S3 facilitates a heterogenous melt spreading pattern, leading to approximately 50% of the melt mass spreading away from the periphery of the substrate.

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Future work

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  • Conduction of more tests with a parametric study on the influences of melt superheat, jet diameter, melt free-falling height, etc.

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  • Develop CFD models to predict the ex-vessel melt behavior in prototypical Nordic BWR, with the presence of below-vessel structures.