Modelling of Helium release from the highly burned fuel during annealing and impact on its migration in the Uranium Dioxide fuel during neutron irradiation

Large amount of noble gases are retained in the high burn-up structure (HBS) located in the fuel surface. Release of both the helium atoms and the fission gas of xenon atoms behave alike during annealing process. That's way the transport (migration) and release of helium from the fuel under irradiation is also alike the fission gas products - the same mechanisms control these. Two stages of helium and xenon release from the highly burned fuel during annealing are observed. The first stage starts at the temperature about 930 K (657 °C) and the second stage starts at about 1350 K (1077 °C). The amount of gas released in the first stage is smaller in comparison with the amount in the second stage. It is believed that the helium atoms in the crystal structure of UO₂ occupy the most energetically favorable octahedral sites. If the potential barriers separating the given atom from the neighboring octahedral sites are sufficiently large, the atom can be effectively immobilized on millions of years. Helium atom located in the octahedral interstitial position of perfect crystal lattice UO₂ is submitted to strong repulsive forces from the surrounding metal and oxygen atoms, which means that it is in a deep potential well of depth preventing it from any movement in the crystals even at very high temperatures. Thus the octahedral interstitial positions in uranium dioxide, are effective traps for helium atoms. Applying the "Ab initio" calculations using the Wien2k program package the static energy barrier between interstitial sites in perfect lattice UO₂+He on about 4.15 eV was estimated. Associating the fact that xenon atoms chemically bond with the fuel and the helium atoms immobilized in the uranium dioxide fuel with the process of grain re-crystallization during which the xenon and helium atoms behave identically in terms of release, can be inferred that the hypothetical modeling of helium migration and release during irradiation is described by the defect trap model of fission gas behavior published earlier. It is proved that the gas release in the second stage is controlled by the grains recrystallization which starts at the temperature about 1100 °C for the highly burned fuel. According to authors' opinion in analysis of immobilization of helium atoms produced in the fuel, one should involve both the solid nuclear fuel material and its nano meter thick layer of surface – authors mean the total surface area of the fuel. Because the total surface area of the highly burned fuel is huge, the release of noble gases in the first stage is remarkable during annealing. In the area of nano superficial layer only good places for the immobilization of helium atoms are closest to the metallic surface octahedral sites, for which the barrier height is V0=1.9 eV, what corresponds to the temperature equal to about 690 °C what is evaluated by "Ab initio" calculation and the WKB (Wentzel, Kramers, Brillouin) approximation for the saturated vapor pressure in equilibrium equal to 1 atm (1.013 bar)