Abstract Flash atomization can occur in rocket thrusters operated at near vaccum conditions just prior to ignition when cryogenic propellants are injected into the reaction chamber. Although the morphology of flashing sprays has been widely studied, empirical observations of the primary breakup mechanisms at the micro-scale are currently not feasible. Still, further analysis of the micro-scales is necessary for development of closure models in both empirical and numerical models. In this work direct numerical simulations are performed to provide insight into the microscopic processes that dominate the dynamics of the primary atomization. We use a multiphase solver (FS3D) based on the volume of fluid method and PLIC reconstruction to track the liquid-vapour interface with high fidelity, fully resolving viscous and capillary effects. Thermodynamic effects are introduced by calibration of the fluid properties and evaporation rate to a target range of thermodynamic conditions and nuclei number density. The use of an affordable incompressible scheme allows for highly resolved simulations comprising up to thousand bubbles across a range of Weber and Ohnesorge numbers. Using a randomized bubble arrangement it is generally observed that the qualitative characterization of the breakup dynamics corroborates previous results using regular bubble arrays. However, these differ substantially from the simplified mechanics hypothesised in the literature. A temporal analysis of the breakup provides estimates for the rate of change in the spray surface area and volume fraction. The droplet size distributions are compared in terms of droplet number, surface area and volume fraction. A model based on the thermodynamic conditions is proposed to estimate the SMD for different Weber numbers.