Model of Gas Pressure under the Cladding of WWER-1000 Fuel Rods after Operation

3/18/2024 2024 - #01 Modelling processes at nuclear facilities

Bokov A.A. Pavlov S.V. Ilyin P.A. Teplov V.G.

https://doi.org/10.26583/npe.2024.1.12

The paper summarizes and analyses the results of post-irradiation examinations of under-cladding gas pressure and composition in more than 300 fuel rods of different designs from 26 fuel assemblies of the VVER-1000 reactors operated at different nuclear power plants. In their design (the diameter of the fuel pellet, its central hole, and the cladding wall thickness), all the fuel rods fall into three groups. The fuel burnup in the fuel rods under study varied from 16 to 72 MW·day/kgU. Each group showed a growth of gas pressure under cladding with an increase of the fuel burnup and was well approximated by linear dependencies within the burnup range considered. The pressure increase rate for the fuel rods with thinner cladding and a fuel pellet 7.8 mm in diameter without a central hole (group 1) was higher than for the fuel rods with standard cladding and pellets with a central hole (groups 2 and 3). A phenomenological model of under-cladding gas pressure in spent fuel rods was developed. The model’s fundamental principle is Dalton’s law for gas mixtures and empirical dependencies of changes in the free volume of the fuel rods and changes in the quantity of krypton and xenon under cladding on the fuel burnup. For each group of fuel rods, the quantity of gaseous fission products (xenon and krypton) released under the cladding was well described by the exponential burning-up function corresponding to the specific group of fuel rods. The gas pressure calculations by the phenomenological model showed that for each group of fuel rods the rate of pressure rise inside the fuel rods started to increase with a growth of burnup beginning from ~ 55 MW·day/kgU. An experimental verification of this phenomenon requires an additional study of the fuel rods with increased fuel burnup. The phenomenological model developed can be used for the verification of the computer codes describing the behavior of the VVER-1000 fuel rods during normal operation in the reactor and long-term «wet» and «dry» storage of spent fuel assemblies.

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fuel rod burnup nuclear fuel water-water power reactor gas pressure model

Link for citing the article: Bokov A.A., Pavlov S.V., Ilyin P.A., Teplov V.G. Model of Gas Pressure under the Cladding of WWER-1000 Fuel Rods after Operation. Izvestiya vuzov. Yadernaya Energetika. 2024, no. 1, pp. 147-158; DOI: https://doi.org/10.26583/npe.2024.1.12 (in Russian).