Recent Developments of AC 2 for Spent Fuel Pool Simulations - - PowerPoint PPT Presentation

Recent Developments of AC2 for Spent Fuel Pool Simulations

Thorsten Hollands, Liviusz Lovasz

Gesellschaft für Anlagen- und Reaktorsicherheit (GRS) gGmbH, Germany

Technical Meeting on the Phenomenology, Simulation and Modelling of Accidents in Spent Fuel Pools IAEA Headquarters, Vienna, Austria 2-5 September 2019

Introduction of AC2

▪ AC2= ATHLET + ATLHET-CD + COCOSYS ▪ Covers the whole spectrum of fault sequences in nuclear reactors

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ATHLET ATHLET-CD COCOSYS

Containment mass and energy pressure, temperature, sump complete thermohydraulics

• core geometry (degradation)
• energy
• hydrogen generation
• fission products
• energy
• hydrogen mass
• melt discharge

Nodalization in core (state of the art)

▪ Fuel rods inside a radially/axially defined node behave identically ▪ Good applicability for symmetrical cases

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Reactor core Reactor core divided radially into rings Reactor core divided axially into segments

Severe accident sequences with strongly asymmetrical characteristics

▪ Examples of scenarios:

• Control rod ejection
• Asymmetrical top-flooding
• Uneven residual power distribution
• Small deviations from an ideally

symmetrical case can lead to asymmetrical behaviour due to non-linear effects

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Local phenomenon

Severe accident scenarios with strongly asymmetrical characteristics

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▪ Standard method homogenizes asymmetric effects over the whole ring ▪ Nodalization change is necessary to take local effects into account

Technical Meeting on the Phenomenology, Simulation and Modelling of Accidents in SFP, IAEA, 2019

Local phenomenon Standard method Averaged “local“ phenomenon Reactor core divided radially into rings

Severe accident scenarios with strongly asymmetrical characteristics

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Source: Bernd Jäckel, Federicho Rocchi, et al.: D6.8.4 Report on the benchmark (including criticality risk assessment), Spent Fuel Pool behaviour in loss of cooling

• r loss of coolant accidents

(AIR-SFP), NUGENIA-PLUS

Severe accident scenarios with strongly asymmetrical characteristics

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Source: Bernd Jäckel, Federicho Rocchi, et al.: D6.8.4 Report on the benchmark (including criticality risk assessment), Spent Fuel Pool behaviour in loss of cooling

• r loss of coolant accidents

(AIR-SFP), NUGENIA-PLUS

New nodalization

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▪ Most of the simulated phenomena are calculated for a single fuel pin:

• Heat generation, conduction, convection, oxidation, cladding failure, fission

product release, axial melt relocation

• Results multiplied by the number of rods in a node
• No model changes needed ✓

▪ Thermohydraulics (ATHLET) are flexibly definable

• No model changes needed ✓

▪ Radial relocation of melt

• Model changes needed

▪ Heat radiation between nodes

• Model changes needed

Technical Meeting on the Phenomenology, Simulation and Modelling of Accidents in SFP, IAEA, 2019

Heat radiation

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If middle ring molten: Heat radiation from inner ring to the outer ring

• Complex

configuration

• Intact sides can

block the heat radiation 3-D heat radiation model

Heat radiation

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▪ View factor: which portion of the radiation reaches surface ”Y” from surface ”X ”

Technical Meeting on the Phenomenology, Simulation and Modelling of Accidents in SFP, IAEA, 2019

VFX−Y = 1 π ∗ AX ∗ ෍

i=1 N

෍

j=1 N cos(θX) ∗ cos θY ∗ block

S2 ∗ dAj ∗ dAi 3+ rows of intact fuel rods are treated as a blocking, continuous wall Pairing every subdivisions with each other

Heat radiation

▪ Algorithm checks: Sum of view factors from one side is 1

• If not, more detailed subdivision is required

▪ View factors are re-calculated after a new node melts ▪ Emissivities of structures are user input ▪ Medium (both gas and fluid phase) is assumed transparent ▪ Radiation heat transfer is added to the energy balance equation ▪ Calculation time depends on complexity (order of seconds)

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Spent fuel pool model specifications

▪ Non cylindrical geometry

• Additional input required

− geometry has to be input explicitly (in reactor case nodalization is deducted from radius and height) ▪ Empty spaces/nodes ▪ Rack walls ▪ Heat radiation to the environment

• Radially: ATHLET objects
• Axially: user defined temperatures

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Severe accident simulations in spent fuel pools

▪ Three simulations conducted to:

• Check for plausibility of the new model
• Show the capabilities of the new model

▪ A LOCA scenario in a generic spent fuel pool was investigated ▪ Similar to Fukushima Daiichi unit 4 SFP ▪ Side ratio 1:2 ▪ Total power: 2.345 MW ▪ 2/3 full ▪ Leak: 10 kg/s ▪ Fuel assembly and power distribution: (red = high power grey = fresh fuel blue = empty spaces)

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Analyzed cases (I)

▪ SFP was divided into 24 equal sized nodes ▪ Three configurations investigated:

• Configuration 1: power and fuel assembly distribution as shown previously
• Configuration 2: equal power distribution over the nodes filled with fuel

assemblies but fuel assembly locations are same as previously

• Configuration 3: equal power distribution over the nodes filled with fuel

assemblies but fuel assemblies distributed uniformly in the spent fuel pool

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Analyzed cases (II)

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Simulation results (I)

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Simulation results (II)

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Simulation results (III)

▪ Location of first melt:

• Configuration 1:

Node 6

• Configuration 2:

Node 6

• Configuration 3:

Node 9 and Node 16, almost at the same time (less than 1 s difference)

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Main findings

▪ The developed new model and nodalization deliver plausible results ▪ Simulations took about 2.5 days on a normal PC, each ▪ Effects of fuel assembly distribution on the accident scenario was shown

• Importance of nodalization shown

▪ Impact of heat radiation is big

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Shortcomings and future tasks

▪ Relocation of melt to the spent fuel pool bottom is not yet possible

• Models applicable only in reactor configurations

▪ Further verification and validation of the developed models for the flexible nodalization ▪ Finding best practice with the flexible nodalization ▪ Analyzing further accident scenarios with strongly asymmetrical characteristics ▪ Implementation of transient inventory calculation, fission product release and transport

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Thank you for your attention!

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