Fission Gas Release Behavior of High Burnup UO2 Fuel under Reactivity Initiated Accident Conditions

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. 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 this paper, we integrate rupture criteria for two kinds of cavities with models that calculate the aforementioned parameters in UO2 LWR fuel for a given operating history. The models are intended for implementation in engineering type computer programs for thermal-mechanical analyses of LWR fuel rods. Here, they have been implemented in the FRAPCON and FRAPTRAN programs and validated against experiments that simulate LOCA and RIA conditions. The capabilities and shortcomings of the proposed models are discussed in light of selected results from this validation. 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.

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Instant Release Fractions from Corrosion Studies with High Burnup LWR Fuel.

MRS Proceedings ◽

10.1557/opl.2012.565 ◽

2012 ◽

Vol 1475 ◽

Cited By ~ 3

Author(s):

Ella Ekeroth ◽

Daqing Cui ◽

Jeanett Low ◽

Michael Granfors ◽

Hans-Urs Zwicky ◽

...

Keyword(s):

Preparation Method ◽

Gas Release ◽

Time Period ◽

Corrosion Studies ◽

Geological Repository ◽

Fission Gas ◽

High Burnup ◽

Product Inventory ◽

Fission Gas Release ◽

Fractional Release

ABSTRACTDuring irradiation in the reactor, a fraction of the fission product inventory will have segregated either to the gap between the fuel and the cladding or to the grain boundaries in the fuel. Of these nuclides, the behavior of the fission gases is best known. The part of the inventory that is rapidly released upon contact with water is designated the instant release fraction (IRF). Previous studies have shown that IRF and fission gas release (FGR) seem to be correlated. Studies of the instant release fraction from high burnup fuel is of interest for the assessment of the safety of a geological repository.The instant release fractions of 129I and 137Cs from five different light water reactor (LWR) fuel rods with a burnup range of 43 to 75 MWd/kgU and a fission gas release range from 0.9 to 5.0 % were studied. Four types of fuel samples (pellet, fragment, powder and fuel rodlet) have been used in the experiments. The results show that the fuel sample preparation method has a significant impact on the release from high burnup fuel samples over the time period covered by this study. Leaching of high burnup fuel samples with fuel detached from the cladding shows the highest release. The fractional 129I release from such fragment samples is similar to the FGR in the corresponding rod. On the other hand, corresponding fractional release of 137Cs is lower.

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