Contact Author:
Mr Armando Carlos Marino
marino@cab.cnea.gov.ar
P:54 2944 445256
F:54 2944 445178
Centro Atómico Bariloche
Bariloche, Rio Negro 8400
Argentina
A 3D Behavior Modelling for Design and Performance Analysis of LWR Fuels
A. C. Marino, G. L. Demarco, D. O. Brasnarof, P. C. Florido (CNEA - Argentina)
The present trend for no PCI failure of the nuclear fuel rod under operation conditions requires a good understanding of the pellet shape evolution in order to determine an optimized geometry.
The geometry of a nuclear fuel rod with UO2 pellets is a compromise among the intention to maximize UO2 content and to minimize the operation temperature taking into account the thermo-mechanical behavior, the economy and the safety of the fuel management during and after irradiation. The thermo-mechanical behavior of a nuclear fuel rod under irradiation is a complex process where too many coupled physics and chemical phenomena are present.
That optimization means an improvement of the pellet geometry parameters as the dimension of shoulder, chamfer, central hole and dishing among others as the l/d relation (length/diameter of the pellet).
The optimization of these parameters leads to a deep reduction of the ridging at the fuel cladding and/or a decrement in the radial deformation of the fuel cladding and a reduction in the mechanical solicitations at the pellet-cladding interface.
The set of tools developed for the study of that phenomena and the improvement of the fuel rod design are the BaCo code and the software package “MeCom”.
The BaCo code is used for the simulation of the behavior of a nuclear fuel rod under irradiation. We enhanced the BaCo performance with a complete set of “ad hoc” software package named “MeCom”. The coupling of BaCo, a quasi 2D code based on a finite differences scheme, and the 3D MECOM tools, based on the method of finite elements, constitutes a complete system for 3D analysis of the stress strain state of a nuclear fuel under irradiation. We calculate the 3D stress-strain state and the deformations of the UO2 pellet at each time step of the BaCo code calculation. We find the stresses and the radial profiles of a fuel rod and the shape of the cracked pellet under irradiation. We define an “ad hoc” 3D pattern of cracks based on BaCo calculation and experimental data. By using an appropriate set of boundary conditions, based on BaCo calculations and data, and that pattern we find a good agreement between experiments of irradiation and calculations particularly for the pellet radial profile after irradiation.
The BaCo code includes time dependent phenomena as creep and the opening, closure and healing of cracks in the fuel pellet during the irradiation among others. The creep of UO2 and the dynamics of the cracks are the main mechanisms in BaCo to release stresses at the fuel pellet. The MECOM tools include the same laws for elasticity and thermal expansion than the BaCo code.
A first approximation of the fuel rod behavior is made using the BaCo code. The treatment is quasi bidimensional at this stage but using the complete set of models and options of BaCo. We generate the input data for the MeCom tools, in particular the geometry of the pellets and the boundary conditions for a particular time of the irradiation. The geometry of the pellet includes the dishing evolution, the shoulders, the deformations and the crack pattern calculated by BaCo. The boundary conditions are: 1) the pressure of the free inner gases in the fuel rod calculated with BaCo (for the dishing of the pellet, the central hole and the inner surface of the cracks), 2) the coolant pressure, a datum for the lateral surface of the cylindrical pellet, and 3) the axial stresses calculated by BaCo for the pellet shoulders.
The temperature field is an input data. Porosity, crack pattern and thermal conductivity can be included into MeCom for a best estimation of its thermal behavior. The result is the 3D deformed geometry of the pellets and the 3D maps of stresses and strains. At present, we are just including elasticity and thermal expansion into the FEM solver (Finite Element Method solver included in the MeCom package). The main mechanism for stress release is the presence of cracks in the geometry of the pellet. Without the crack pattern the stress-strain state results represent an extreme condition of behavior with the highest stress in the pellet greater than the most demanding condition. Nevertheless, the inclusion of cracks in the pellet geometry produces a most realistic result. The common pattern adopted for the cracks and the maximum number of them present in a fuel during irradiation is estimated from experiments. This pattern and amount of cracks were enough to release stresses via fracture of the UO2 pellet. The result is the increase of the pellet ridge height.
We find the stresses and the radial profiles of a fuel rod and the shape of the cracked pellet under irradiation showing the bamboo effect and others 3D effects as the presence of the secondary ridge. We mention: a) the reduction of the deformation of hollow pellets, b) the absence of ridging when conic shapes are used, c) the increment of ridging when a dishing is present, d) the reduction of ridging due to chamfers, and e) the trends of radial deformation by the variation of l/d (height / diameter of the pellet), as interesting examples by using BaCo + MECOM. Typically we calculated a pellet ridge height of ~10 µm in good agreement with the PWR fuel. Commercial, innovative or unusual pellet shapes can be analyzed by using this package. The results show a good agreement between experimental data and calculations particularly for the pellet radial profile after irradiation.
In this paper we present the BaCo + MeCom software package by using LWR fuel rod under demanding condition of irradiation. We are including as an experimental support of the calculation the cases of the CRP FUMEX II (Coordinated Research Project on Fuel Modeling at Extended Burnup II) of the IAEA (International Atomic Energy Agency) in order to demonstrate the good accuracy of our computer tools (BaCo + MeCom). The 3D calculations are experimentally supported by using experimental irradiations performed at the Halden Reactor Project. We find an excellent agreement with experimental data for the calculation of burnup, fission gas release and inner gas pressure. A good agreement is found for dimensional changes as the diameter and the length of the fuels. Then we can assume that the physical laws included in BaCo and the boundary conditions that we will use constitute a reasonable approach as a seed for the 3D calculations of fuel rods with the MeCom tools.