On the condition of UO2 nuclear fuel irradiated in a PWR to a burn-up in excess of 110 MWd/kgHM

Modelling of fine fragmentation and fission gas release of UO2 fuel in accident conditions

“…The gas pressure in the pores at time of formation is assumed to be P g = P eq + ∆P o = P h + 2γ f s /r p + ∆P o , where ∆P o is a constant model parameter that defines the pore overpressure at formation. It is determined such that the calculated pore gas pressure agrees with estimates from experimental studies [93,98,106,107] and also that the calculated volume fraction of porosity, φ 3 = n p V p = n p 4πr 3 p /3, in the newly formed HBS agrees with observations [93,97,100,108]. Here, the number density, n p , of the newly formed pores is calculated from the aforementioned assumptions regarding the pore initial size, gas pressure and gas content, in combination with the local temperature at time of pore formation.…”

“…Porosities in the range of 20-30 vol. % have been observed in fully restructured UO 2 with extremely high burnup [97,98], but the porosity is considerably lower for fuel with designs and burnup levels typical for commercial LWR fuel. Mechanical restraint from PCMI is known to suppress the porosity, and rim zone porosities above 15 vol % are rarely observed in commercial UO 2 fuel [93,99,100].…”

“…Estimates of the average gas pressure in the HBS pores, based on measured porosity volume fractions and estimates of the amount of gas retained in the pores at certain local burnups, have been reported. These estimates suggest that the pore gas pressure, under normal steadystate operating conditions and at temperatures typical for the fuel pellet rim in LWRs at these conditions, is in the range from about 50 to 150 MPa [93,98,106,107,118]. This pressure span is quite large, since the estimates are based on different assumptions and input data regarding the HBS porosity, temperature and partitioning of gas between the pores and the surrounding matrix.…”

“…Here, the number density, n p , of the newly formed pores is calculated from the aforementioned assumptions regarding the pore initial size, gas pressure and gas content, in combination with the local temperature at time of pore formation. Based on results from ceramographic studies of the HBS [97,98,109], the pore number density is then assumed to increase linearly with burnup, until it saturates at 10 17 pores/m 3 at a local fuel burnup of 100 MWd(kgU) −1 . At even higher burnup, experimental data [97] show that the pore number density starts to decrease, a phenomenon attributed to Ostwald ripening [110].…”

“…A fairly large number of experimental studies on pore growth and porosity evolution in the HBS are available in the open literature [93,97,98,100,108,109], but the phenomena are not yet fully understood. The experimental data are difficult to interpret, since characteristic properties of the HBS, such as porosity, pore size or pore number density, are presented as trends observed either with respect to local burnup or radial position in the fuel pellet.…”