Fuel Pellet Cracking Research Articles

Multiphysics phase-field modeling of quasi-static cracking in urania ceramic nuclear fuel

This work presents phase-field modeling of quasi-static cracking in urania (UO2) ceramic nuclear fuel under neutron radiation at high temperatures. Considering the tightly coupled multi-physics processes within the fuel during reactor power operation, a diffusion model including Fickian and Soret effects is used to describe the oxygen hyper-stoichiometry (x in UO2+x), and the temperature field is given by a thermal model involving non-uniform fission-generated heat source and heat flow across fuel pellet, pellet-cladding gap and cladding to the outside heat sink. Both temperature and irradiation effects are taken into account for the basic thermo-mechanical properties and irradiation behaviors of the nuclear fuel. Especially, the acceleration of fuel thermal creep by oxygen hyper-stoichiometry is included. The fracture due to the above physical processes is approximated by a scalar phase-field variable based upon a cohesive phase-field fracture method. A granite fracture experiment is simulated to validate the thermo-fracture coupling approach. For the first time, the diffusion-thermo-mechanical-fracture coupling model is applied to UO2 fuel pellet cracking during reactor startup, power ramp and reactor shutdown. UO2 creep is found to play an important role on the fuel pellet fragmentation. The developed capability supports interpretation of experimental data and can guide material design of ceramic nuclear fuels.

Peridynamic modeling of fuel pellet cracking

This study presents the peridynamic simulation of thermal cracking behavior in uranium dioxide, UO2 fuel pellets that are used in light water reactors (LWR). The performance of the reactor is influenced by the thermo-mechanical behavior of the pellets. During the fission process, the pellets are subjected to high temperature gradients, and the oxygen diffusion significantly affects the temperature distribution. Therefore, a coupled analysis of temperature and oxygen diffusions and deformation is unavoidable in order to predict accurate cracking behavior in a fuel pellet. The accuracy of the predictions is verified qualitatively by comparing with the previous studies.

Finite element simulation of Pellet-Cladding Interaction (PCI) in nuclear fuel rods

This paper deals with the use of the finite element method to simulate locally the phenomenon of Pellet-Cladding Interaction (PCI) in nuclear fuel rods. The main physical processes involved in PCI are first presented. Then, the problem of PCI modeling is explained, putting the emphasis on finite element code TOUTATIS used for present analyses. Several results of two-dimensional FE analyses carried out by AREVA NP are shown. It appears that fuel pellet cracking during power ramp test is due to the friction between fuel pellet and cladding. The influence of fuel fracture properties is also investigated, focusing on the consequences on clad loading. These results are used to improve the modeling of PCI in AREVA NP’s future global fuel rod performance code COPERNIC3.

Power-to-melts of uranium–plutonium oxide fuel pins at a beginning-of-life condition in the experimental fast reactor JOYO

Power-to-melts of uranium–plutonium oxide fuel pins at an initial startup condition were experimentally obtained from the B5D-2 test in the experimental fast reactor JOYO in Oarai Engineering Center. MCNP code calculations were combined with burnup measurements to determine linear heat rating of the test fuel pins. To identify the axial incipient melting positions corresponding to the power-to-melts, solidified grain morphology and molten fuel axial movements were characterized. Extensive observations on longitudinal ceramographs allowed classifying molten fuel settlements near bottom and top extents of axial fuel melting into three types. The power-to-melts depended slightly on fuel-to-cladding gap sizes and clearly on both oxygen-to-metal ratios and densities of fuel pellets. These dependencies resulted from the fuel pellet cracking and relocation behavior, which fairly improves heat transfers across the gaps. Also, the power-to-melt at the bottom position was higher than that at the top position due to an axial gradient of cladding temperatures in each fuel pin.

Theoretical evaluation of rifled cladding for LWR fuel

Abstract Rifled Cladding deviates from regular fuel designs mainly by the inner surface of the cladding tube being prismatic, with a large number of faces. The design has been tested in the R2 test reactor at Studsvik, and is scheduled for testing in a Swedish BWR. Its observed technical merit is to reduce the fission gas release and increase the failure threshold during overpowers. The present study aims to find an explanation of the observed increase in failure threshold, i.e., the heat rating at which cladding failure occurs. Failure is strongly related to the maximum mechanical stress. The INTERPIN code was used to calculate the starting conditions for an overpower event, and the FREY-01 code was used for finite element calculations of cladding stress distributions during the overpower. FREY-01 accounts for the kinetics of fuel pellet cracking and its impact on cladding stresses. The study focuses on the conditions that produce the highest cladding stress. It was found that the worst case is associated with cracks opening close to the fuel-cladding contact loci, i.e., the midface positions of the prismatic surface, with no additional cracks at non-contact positions. This situation is similar to what holds for regular fuel. If, however, cracks also open at corner positions, then the calculated maximum stresses are considerably less. This latter situation is, in fact, observed in hot cell examinations of ramped rifled cladding rods. Therefore, the improved failure resistance of rifled cladding rods can be explained in terms of a reduction of the strong azimuthal mechanical interaction between fuel and cladding that may occur in fuel rods of regular design.

The Possible Impact of Fuel Pellet Cracking on Inferred Gap Conductance and Fuel Stored Energy

Thermally induced cracking of the UO2 fuel pellets undoubtedly results in some reduction of the effective fuel thermal conductivity, relative to that for solid UO2. This effect may be approximated ...

Pellet-Cladding Interaction of LMFBR Fuel Elements at Unsteady State: An Introduction to Computer Code ISUNE-5

The fuel pellet-cladding interaction (PCI) of liquid-metal fast breeder reactor (LMFBR) fuel elements or fuel rods at unsteady state is analyzed and discussed based on experimental results. In the analyses, the heat generation, fuel restructuring, temperature distribution, gap conductance, irradiation swelling, irradiation creep, fuel burnup, fission gas release, fuel pellet cracking, crack healing, cladding cracking, yield failure and fracture failure of the fuel elements are taken into consideration. To improve the sintered (U,Pu)O2 fuel performance and reactor core safety at high temperature and fuel burnup, it is desirable to (a) increase and maintain the ductility of cladding material, (b) provide sufficient gap thickness and plenum space for accommodating fission gas release, (c) keep ramps-power increase rate slow and gentle, and (d) reduce the intensity and frequency of transient PCI in order to avoid intense stress fatigue cracking (SFC) and stress corrosion cracking (SCC) due to fission product compounds CsI, CdI2, Cs2Te, etc. at the inner cladding surface of the fuel elements during PCI.

An empirical model of the effects of pellet cracking on the thermal conductivity of UO2 light water reactor fuel

Light water reactor fuel pellet cracking and pellet fragment relocation into the pellet-to-cladding gap during normal operation alters both the fuel thermal conductivity and the thermal resistance of the gap. Uranium dioxide fuel pellet thermal conductivity data from a series of tests being conducted in the Power Burst Facility to evaluate the thermal performance of LWR design fuel are presented. These data indicate that the effective thermal conductivity of a cracked and relocated light water reactor fuel pellet is strongly influenced by the closing or opening of the cracks as the rod power is increased or decreased and is dependent on the initial pellet and cladding dimensions. An empirical correlation is introduced which provides a means for calculating the effective thermal conductivity of cracked and relocated fuel within helium bonded fuel rods. The method also provides a means for estimating the relocated hot pellet-to-cladding gap width as a function of rod power.

MIPAC, a computer code for fuel performance analysis by the finite element method

An axisymmetric finite element computer code named MIPAC has been developed for analysis of the mechanical interaction behaviour between a fuel pellet and cladding. This computer code can deal with elastoplasticity of the pellet and cladding materials, creep effects for the both materials, pellet-cladding and pellet-pellet contact problems, hot pressing effect of the fuel pellet, fuel pellet cracking, and the cracked pellet's stiffness. A cyclical boundary condition is introduced to deal with one pellet length instead of the full-size fuel rod. The contact problems are solved without a fictitious contact element. In the fuel pellet cracking model the crack opening and closing behaviour under arbitrary power changes can be treated by introducing five kinds of crack modes. Mismatch of irregular crack surfaces is taken into account in the evaluation of the cracked pellet's stiffness. Finally, calculated results are compared with experimental data to show validity of the computer code.

Effect of Pellet Cracking on Light Water Reactor Fuel Temperatures

AbstractIt is postulated that typical light water reactor (LWR) fuel pellets will crack after a few power cycles and that the majority of the pellet segments will lie against the cladding. When there is a nominal cladding-to-pellet gap at operating conditions, pellet cracking will improve the fuel-to-cladding gap conductance but will reduce the fuel thermal conductivity. A model that accounts for the effects of fuel pellet cracking on both fuel conductivity and gap conductance has been formulated. Fuel centerline temperature measurements were made during the steady-state irradiation in the Halden Heavy Boiling Water Reactor of four typical LWR-type test rods with varying fuel density and pellet-to-cladding gap sizes. Calculations using the cracked pellet model were compared to the in-pile temperature measurements, and good agreement was obtained.