spent fuel pool safety and performance; chan young paik

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Westinghouse Proprietary Class 2 © 2012 Westinghouse Electric Company LLC. All Rights Reserved.

Spent Fuel Pool safety and performance

Development and application of MAAP5 Spent Fuel

Model for enhancement of EOPs and SAMGs in light

of the accident at the Fukushima Daiichi Nuclear

Power Plant and stress test evaluations.

Chan Young Paik, Quan Zhou

Fauske and Associates, LLC., USA

Oleg Solovjanov , Robert Prior

Westinghouse Electric Company, LLC, Belgium

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Outline

1. Motivation

2. Features needed

3. Modeling

4. Testing

5. Status and Future Plans

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Motivation - Spent Fuel Pool Model

– Model development started before Fukushima

– Motivated by the need to evaluate Accident Management

strategies and provide (not unnecessarily conservative) inputs to

PSA level 2:

• Pool heatup times;

• Effects of boiling:

• Building thermal-hydraulics.

• Hydrogen from radiolysis.

• Source term analysis.

• Accident management and severe accident management strategy

evaluation

– In line with MAAP philosophy, aim was to produce best estimate

predictions.

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Features Required

• Fuel rack design.

• Heat loads and inventories.

• Heat sinks and gas space behavior including hydrogen.

• Fuel damage and melt progression:

– Cladding oxidation in air.

– Ru release.

• The model uses a special MAAP containment control

volume with additional models.

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The MAAP Code

• Modular Accident Analysis Program

• Integrated code for severe accident analysis.

• Applications include:

– SAM strategy verification

– PSA2 sequence analysis and source term analysis

– Design of mitigation systems

• Increasingly used for „pre-core damage‟ scenarios.

• EPRI owns MAAP and manages development.

• FAI is main developer.

• MAAP5 includes major model upgrades, including

improved primary system modeling, neutron kinetics.

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Key Features of MAAP5 Spent Fuel Pool Model

• The storage pool is represented by a water pool model in the

MAAP containment (reactor building) model.

• Current model does not calculate water natural circulation

within fuel racks.

• MAAP calculates a gas natural circulation once the water level

decrease below the bottom of the fuel rack or fuel rack melted

away.

• Current model allows up to 40 channels.

• Each channel can have filled or empty cells, and hot and cold

fuel assemblies, but the decay power within a channel is

homogenized.

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Key Features of MAAP5 Spent Fuel Pool Model (Continued)

• The checker board configuration can be modeled by

dividing channels based on assemblies (i.e., individually

modeling “hot” assembly as one channel and “cold”

assemblies as another channel).

• Each channel is homogenized. Thus, in-homogeneity and

non-symmetrical nature of the SFP pool must be modeled

at the hierarchical level of channel. Users are allowed to

use up to 40 channels and each rack can be modeled as

multiple channels.

• The storage pool wall is represented by distributed heat

sink model.

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Key Features of MAAP5 Spent Fuel Pool Model (Continued)

• Spent fuel rack and fuel assemblies are modeled similar to the

MAAP5 core model (uses identical models).

– Spent fuel racks and assemblies are modeled as 2-D (channels and

axial nodes) objects. There can be one or more channels for each rack.

– Up to 20 different types of fuel assemblies can be specified in each

channel.

– The rack wall (steel with boraflex plate) is modeled as one of the core

components surrounding each fuel assembly, such as control blades in

BWR.

– Boiled-up level for each channel is calculated based on the steam

generation rate within the channel.

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Defining Inputs for Representative Types of Spent Fuel Assembly

• Maximum of 20 different types of fuel

assemblies (power groups) can be defined

– Cycle and Burn-up History

– How long for each cycle and how much

accumulated burn-up at the end of the cycle

– Cooling time

– Time the fuel assembly is cooled in the

spent fuel pool (including time from

shutdown to transfer into pool)

– Initial Enrichment

– Masses of Materials

– UO2, Zr, Ag-In-Cd, B4C, Stainless Steel, etc.

– Number of Fuel Pins and Non-Fuel Rods

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Spent Fuel Rack Designs

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Spent Fuel Channel (Rack) Modeling

• Currently, only the High Density Rack (HDR) and square type racks are modeled.

• Spent fuel pool can be divided into 2-D model:

– # of channels (racks) * # of axial nodes

(up to 40) (up to 100)

• User can specify types and number of fuel assemblies for each channel (rack).

• For a given channel, there can be filled or empty cells. Averaged fuel assembly properties are used to represent filled cells

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Defining Inputs for Spent Fuel Channel (Rack)

• Geometric Information

– Number of storage cells for each channel (rack)

– Number of fuel assemblies (filled cells) for each channel (rack)

– Center-to-center distance between two adjacent cells

– Thickness of rack wall

– Thickness of Boraflex sheet in the rack wall

– Total height of the spent fuel rack

– Height of the top and bottom non-fuel regions in the spent fuel rack

• Assemblies in the Rack

– Number of specific type of spent fuel assemblies in the rack

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Zr Oxidation in Air

• Zr + O2 → ZrO2 + ∆HZr

∆HZr : Heat of reaction per mole of Zr

(=1.1 x 109 J/kg–mole [1])

[1] D.A. Powers, “Technical Issues Associated with Air Ingression During

Core Degradation,” SAND2000-1935C, Sandia National Laboratories,

2000

• MAAP5 uses NUREG Correlations (NUREG/CR-6218)

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Relocation Model

• Relocation model is similar to core relocation model except:

– Sideward relocation between channels is not allowed.

– Molten mass at the bottom node is allowed to relocate into the floor

without forming a crust.

• Molten steel from steel rack is relocated one axial node at a

time.

• Fuel rod collapse criteria is based on the model used in the

core based on the Larson-Miller approach (time at

temperature).

• Fuel rack collapse is modeled based on the bottom node

temperature using the Larson-Miller approach.

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Example Cases

• 4-Loop PWR Spent Fuel Pool is modeled as one reactor building node

• Spent fuel pool fuel assemblies are modeled as 30 channels (racks)

with 22 axial nodes

• Total decay heat ~ 7.73 MW with 840 fuel assemblies

• Pool floor area = 178 m2

• Limited gas circulation between upper space and fuel region

• Condensate in the walls and ceiling return to the water pool

• For a normal water level, it will take a more than a week to uncover the

top of fuel assemblies for a loss of pool cooling event

• Three Cases:

– 1) Normal Initial water level (12.0 m) with a loss of pool cooling

– 2) Initial water level is set to 5.0 m with a loss of pool cooling

– 3) No water in the spent fuel pool (just to test the models)

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Example: PWR Spent Fuel Pool

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Channel Nodalization (30 channels)

1 3 2 5 4 6

7 9 8 11 10 12

13 15 14 17 16 18

19 21 20 23 22 24

25 27 26 29 28 30

Spen

t

Fuel

Chan

nel

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Case 1 (Normal Water Level)

• It will take about one week to uncover the top of fuel

assemblies.

Top of fuel

assembly

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Case 2 (Water level and Pressure )

Corium

Level

Hydroge

n burn

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Case 2 (Gas Temperature and Hydrogen Mole Fraction)

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Case 2( Fuel Node Temperatures)

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Case 2 (Mass of Hydrogen and Molten Corium Mass )

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Status and Future Development

• MAAP 5.01 (December 2011), first version – includes Spent

Fuel Pool model.

Planned developments:

• Radiolysis.

• Tests for other types of spent fuel pool designs.

• Improvements to radiation and natural circulation models.

• Validate the model against available data and other detailed

codes.

spent fuel pool safety and performance; chan young paik

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