Improvements on FRAPCON3 and FRAPTRAN Mechanical Modelling

Arttu Knuutila,

Seppo Kelppe

SAFIR PUOLIVÄLISEMINAARI 20.-21.1.2005

VTT TECHNICAL RESEARCH CENTRE OF FINLAND

Introduction and Contents  Description of elaborations on fuel mechanical modelling made under a one-year attachment to PNNL Laboratory in the US  Contents – Introduction to requirements of modelling the mechanical behaviour in a fuel rod – – Summary description of the refined FEM approach Examples of verification 2

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Basic construction of a fuel rod 3  TYPICALLY:  Cladding material: various zirconium alloys  Diameter ~ 9mm  Length 2500-3500 mm  Fill gas: Helium to 0.6-1.5 MPa

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Geometry of a Cracked Fuel Pellet 4

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Pellet-Cladding Mechanical Interaction (PCMI) 5

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Modes of Axial PCMI 6

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Pellet-Ciad Interaction (PCI) Failure 7

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Deformed pellet schematically 1-D vs.2-D Desrciptions 8

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Generic Example of FEM Applied to Rod Structural Analysis 9

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Sequence of events in a LOCA 10

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Large Clad Deformations in a LOCA test  a) Säteilytetty sauva – polttoainemuruja pullistumassa  b) Tuore suojakuoriputki – säteilytetystä poikkeava halkeaman muoto  Muodonmuutokset samankaltaiset säteilyvauriot pääosin hehkuttuneet pois (?) 11

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Reactivity Transient (RIA)(1) 90 H (cal/g) 80 70 60 50 40 30 20 10 0 0 0.25

0.5

0.75

1 1.25

Pcore (MW) 4500 4000 3500 Enthalpy (cal/g) TIME-LAW 1.5

1.75

3000 2500 2000 1500 1000 500 2 0 t(s) 12

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Reactivity Transient (RIA)(2) 0.2

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Power Fuel Centerline Temperature Fuel Surface Temperature Total Hoop Strain 0.3

Time, s 0.4

0.5

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Different Clad Performance Scenaria in a RIA Transient 14

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Failure model Ballooning model Dispersed Fuel to Water Interaction

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Clad Failure Modes in a RIA  Competing Mechanisms  Early PCMI Failure  DNB - High Temperature Ballooning and Burst - Oxidation and Embrittlement

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USNRC steady-state and transient codes FRAPCON3  Steady-state fuel performance code  Capable of modelling fuel thermal mechanical behaviour of and fission gas release in a LWR fuel rod during normal operations  Validated up to 65 MWd/kgU burnup FRAPTRAN  Transient fuel performance code  Capable of analysing thermal mechanical behaviour of a LWR fuel rod in reactivity accidents, loss-of-coolant accidents, or anticipated transients without scram  Validated up to 65 MWd/kgU burnup 16

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FRAPCON3/FRAPTRAN mechanical modelling  Both codes employ a rather simple stress-strain modelling for the cladding called FRACAS I that originates from the development work done in the 70s  FRACAS I uses a 1D thin shell model for the cladding stress-strain analysys, where the fuel rod is divided into axial slices and each slice has its own separate 1D mechanical solution  FRACAS I can model pressure loaded cladding (open gap) or PCMI loaded cladding (closed gap) with solid contact, i.e. it does not allow slippage between the fuel pellet stack and the cladding if the gap is closed  FRACAS I does not include stress-strain analysis for the fuel pellet stack

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New mechanical modelling   a stress-strain analysis option with finite element model has been implemented in FRAPCON3/FRAPTRAN codes 1½D, 2D, and 3D analysis capability  capability of modelling large strains and displacements, e.g. localized deformations can be modelled  Modelled deformation mechanisms – Elasticity with nonlinear hyperelastic model – Large strain plasticity with J 2 hardening flow theory (von Mises) and isotropic – – Creep, a time dependent extension to the J 2 flow theory Thermal dilation and irradiation growth  – Dilation of ideal gas 1½D and 2D contact with friction (Coulomb friction model)

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1½D, 2D, and 3D elements    1½D axisymmetric linear, 2D axisymmetric bilinear, and 3D trilinear solid elements with mean dilation formulation 1½D and 2D contact interface with Coulomb friction with penalty method 1½D axisymmetric, 2D axisymmetric, and 3D gas cavities with ideal gas 19

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Efficient sparse matrix solver  Efficient sparse matrix solution by using matrix reordering to reduce the matrix profile and direct solution methods, LU and LDL T factorisations  An example of a stiffness matrix where the memory space needed for the matrix factorisation is reduced by 70% by reordering 20

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LARGE STRAIN MODELLING  Capability to model localized deformations. For example ballooning of the cladding during loss-of-coolant accident 21

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Pellet cladding contact with Coulomb friction  Coulomb friction t <  p  t =  p  solid contact sliding with friction Where t is tangential traction, p is contact pressure, and  is friction coefficient 22

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Verification of FE modelling  Patch tests to verify the element performance and correctness even in badly distorted element mesh  Extensive verification for large strain elasto-plasticity with verification cases that have an analytical solution or reference solution in the literature  An example of large strain verification case, a compressed billet 23

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FRAPCON3 validation case: IFA-525 24

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FRAPCON3 validation case: IFA-585 26

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Conclusion  Modelling elaborations to the USNRC fuel performance codes FRAPCON and FRAPTRAN that significantly improve provisions of more detailed analyses of rod mechanical behaviour were introduced and taken into use – 2-D and 3-D descriptions were made available through advanced FEM formulation – Bases for covering crucial detail as frictional pellet-to-clad contact, pellet and clad creep-plastic deformation, and large-deformation ballooning were laid  Modularity and flexibility were among the goals - Results will be readily applicable in other analytical environments  Verification of formulation confirms robust numerical performance and improved representativity to real-life behaviour  Validation and applications now (early 2005) in progress

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