Low burnup samples of U7wt%Mo with 10%, 15%, 20% and 25% 235U burnup were analysed by transmission electron microscopy in order to study the formation mechanism of the fission gas nanobubble lattice. The analysis showed that the bubble lattice formation starts at a burnup lower than 10%, preferentially at grain boundaries before extending towards the grain interior with increasing irradiation dose. The bubble lattice geometrical characteristics and degree of ordering seem to be independent of the fuel fission density. Pressure calculations indicate that bubbles are extremely underpressurized in the four samples analysed, thus suggesting a void-like nature of the cavities in their first stages of alignment. Finally, the first signs of bubble lattice disruption are observed at the grain boundaries when approaching 20% burnup due to the fuel recrystallization. The presented results provide further insights into the development of the fission gas nanobubble lattice in irradiated U(Mo).