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It has the potential to pulverize the material and create fragments down to the size of individual grains, i.e. This fragmentation mechanism, which is the topic of this paper, is fundamentally different from the aforementioned processes that are related to thermally induced stresses. The fragments formed by thermally induced stresses during these accident conditions are typically 0.1−0.5 mm in size, and they are found predominantly at the pellet surface.Įven finer fuel fragments may form under certain accident conditions by overpressurization of gas-filled bubbles and pores in the solid.
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Similar crack patterns may also form at the end of a loss-of-coolant accident (LOCA), when the fuel is re-wetted (quenched) from high temperature. The steep temperature gradient that arises at the pellet periphery during the thermal shock induces a dense pattern of fine radial cracks close to the pellet surface. This is the case, for example, in a reactivity initiated accident (RIA), where the fuel pellets are heated to high temperature within tens of milliseconds. However, finer fragments may form in accident conditions that involve very steep temperature gradients in the fuel material. The observed number of cracks rarely exceeds 15 in solid UO 2 fuel pellets that have experienced normal operating conditions during their lifetime, which means that fuel pellet fragments formed by thermal stresses under normal operating conditions are generally larger than 2 mm. The number of cracks is also observed to increase slightly with fuel operating time or burnup ( 3). The number of radial cracks (or fragments) increases initially almost linearly with increasing LHGR, but the tendency for further cracking declines at an LHGR above 40−45 kWm −1, as a result of increased material plasticity at high temperature. The cracking proceeds as the power is increased, and the strength of the temperature gradient caused by the applied power dictates how many fragments need be created to keep the tensile stresses below the fracture threshold for the material. These stresses cause radial cracks to form at a linear heat generation rate (LHGR) of 5−6 kWm −1. Tensile thermo-elastic stresses are induced at the pellet periphery by the steep radial temperature gradient that arises in the low thermal conductivity material under normal reactor operation.
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There are several mechanisms for cracking and fragmentation of oxide (UO 2 or (U,Pu)O 2) nuclear fuel pellets.
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This is an Open Access article distributed under the terms of the Creative Commons Attribution License ( ), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Jernkvist, published by EDP Sciences, 2019 Calculated results suggest that the extent of fuel fragmentation and transient fission gas release depends strongly on the pre-accident fuel microstructure and fission gas distribution, but also on rapid changes in the external pressure exerted on the fuel pellets during the accident. The capabilities and shortcomings of the proposed models are discussed in light of selected results from this validation. Here, they have been implemented in the FRAPCON and FRAPTRAN programs and validated against experiments that simulate LOCA and RIA conditions. The models are intended for implementation in engineering type computer programs for thermal-mechanical analyses of LWR fuel rods. In this paper, we integrate rupture criteria for two kinds of cavities with models that calculate the aforementioned parameters in UO 2 LWR fuel for a given operating history. Analytical rupture criteria for various types of cavities exist, but application of these criteria requires that microstructural characteristics of the fuel, such as cavity size, shape and number density, are known together with the gas content of the cavities. In reactor accidents that involve rapid overheating of oxide fuel, overpressurization of gas-filled bubbles and pores may lead to rupture of these cavities, fine fragmentation of the fuel material, and burst-type release of the cavity gas.