Studies on melt pool convection, vessel failure, melt-structure interactions and steam explosion��Weimin Ma, Boshen Bian, Hongdi Wang�Yucheng Deng, Disen Liang��Nuclear Science and Engineering (NSE)�Kungliga Tekniska högskolan (KTH)

APRI 11 slutseminarium

Bergendal, 6-7 februari 2024

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Outline

• DNS of melt pool convection (PhD thesis)
• Analyses of vessel failure modes (PhD thesis)
• Ex-vessel melt-structure interactions
• Steam explosion in chemical solutions (PhD thesis)

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DNS of melt pool convection

• Simulation of internally heated (IH) melt pool convection
• It is important to estimation of thermal load on the lower head
• The biggest challenge is the modelling of the intensive turbulence flow
• Many studies have been done by RANS models

• RANS: Reynolds Averaged Navier Stokes
• LES: Large Eddy Simulation
• DNS: Direct Numerical Simulation

Simulation results using different turbulence models

DNS

RANS

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DNS of melt pool convection (cont’d)

• DNS study on IH melt pool convection in a hemispherical domain with a Rayleigh number of 1.6×10¹¹ and a Prandtl number of 0.5
• Results show velocity field has three distinct zones:
• Upper mixing zone
• Stagnation zone in the middle of the domain
• Descending flow along the curved boundary

Temperature

Normalized temperature

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DNS of melt pool convection (cont’d)

• DNS study on IH melt pool convection in a hemispherical domain − Averaged field:
• According to the averaged temperature distribution, there is a hot plane in the middle part of the domain
• Above the hot plane, the flow is propelled by the buoyancy force
• Below the hot plane, the flow is suppressed

Averaged distribution on the XZ-middle planar

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DNS of melt pool convection (cont’d)

• DNS study on bottom-heated melt pool convection
• Simulation of the BALI-Metal 8U experiment (metallic layer)
• Comparison of DNS with three RANS models: SST k-ω, Standard k-ε, RSM

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Computational Domain

Boundary conditions

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DNS of melt pool convection (cont’d)

• DNS study on bottom-heated melt pool convection
• Velocity field

Mean velocity distribution) in 4 cases

• The flow pattern can be divided into two distinct regions
• On the left side, the descending flow near the left lateral cooling wall and a ‘cold tongue’ extending towards the right side
• On the right side, Rayleigh-Bénard (RB) convection cell is observed.

Instantaneous velocity distribution on the XZ-middle plane (y=6.5cm) in DNS case

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DNS of melt pool convection (cont’d)

• DNS study on bottom-heated melt pool convection
• Heat flux

Mean heat flux distribution on lateral cooling wall

Heat flux profile along the lateral cooling wall

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Analyses of vessel failure modes

• Computational framework

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Analyses of vessel failure modes (cont’d)

• Prediction of vessel failure without penetrations on the lower head
• Failure of lower head wall

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Analyses of vessel failure modes (cont’d)

• Prediction of vessel failure without penetrations on the lower head
• Failure of lower head wall

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Analyses of vessel failure modes (cont’d)

• Prediction of vessel failure with penetrations on the lower head
• Failure of IGTs

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Analyses of vessel failure mode (cont’d)

• Prediction of vessel failure with penetrations on the lower head
• Failure of IGTs

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Analyses of vessel failure modes (cont’d)

• Prediction of vessel failure with penetrations on the lower head
• Failure of IGTs

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Ex-vessel melt-structure interactions

Depiction of corium interaction with

below vessel structure in a Mark I system

CRD housing brackets�before the accident (Unit 3)

CRD housing brackets now (Unit 3)

• Motivation
• Previous assumption for studies on ex-vessel steam explosion and debris bed coolability: a coherent melt jet falls from the lower head of the RPV into a deep water pool in a SA scenario of a reference (Nordic) BWR.
• However, BWR has a forest of structures and supporting plates below the lower head of the RPV.
• 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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Ex-vessel melt-structure interactions (cont’d)

• Geometry and simplification of the below-vessel structures of a Nordic BWR

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Ex-vessel melt-structure interactions (cont’d)

• A scenario of ex-vessel melt-structure interactions
• Falling of instrumentation housing: most probably
• Falling of CRD housing: less likely
• A circular opening through which the melt stream can pass initially
• As the melt stream diameter increases due to enlargement of the hole through the lower head, the stream will contact the horizontal structures, and on the structures first
• Before the horizontal structures are totally melted through, the melt stream will spread horizontally or splatter onto vertical structures ‒ a complicated process itself.
• A success of depicting such a process will enhance the realism of ex-vessel accident progression

CRD housing

Instrumentation housing

Top view of lower head

Failure location

Top view of lower head

Penetration failure

Wall failure

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Ex-vessel melt-structure interactions (cont’d)

• Research plan
• Analytical work
• Development of models
• Validation of models
• Experimental work
• Design test sections in the DEFOR and CoSMUS facilities
• Perform separate-effect and integral-effect tests to investigate the influences of below-vessel structures on melt relocation

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Steam explosion in chemical solutions

• MISTEE experiment
• MISTEE has been developed to produce experimental data in well-controlled test conditions for characterizing steam explosion of a single molten droplet

Crucible

Induction coil

TC

Plug

Pressure sensor

High-speed

Camera

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Steam explosion in chemical solutions (cont’d)

• Steam explosion in chemical solutions
• Test matrix

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Steam explosion in chemical solutions (cont’d)

• Steam explosion in chemical solutions
• Test matrix for seawater

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Steam explosion in chemical solutions (cont’d)

• Effect of chemical additives on single-droplet steam explosion probability
• T_m=800 ℃, T_c=20 ℃, the concentrations of H₃BO₃ change from 0~3.2 wt.%, each test is repeated 20 times.

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Steam explosion in chemical solutions (cont’d)

• Effect of chemical additives on single-droplet steam explosion probability
• T_m=800 ℃, T_c=20 ℃, the coolant salinity changes from 0~35 g/kg, each test is repeated 20 times.

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Concluding remarks

• Direction numerical simulation (DNS) of melt pool convection revealed details of turbulent natural convection which have not discovered by RANS models
• Analyses of vessel failure modes revealed the timing difference between vessel wall failure and penetration failure
• Literature study of ex-vessel melt-structure interactions revealed the complexity of corium melt discharge due to impacts of below-vessel structures
• Experiments of single Sn droplet steam explosion in chemical solutions revealed that seawater, H₃BO₃ solution and its neutral solutions by adding NaOH or Na₃PO₄ may significantly change the probability of spontaneous steam explosions.

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