Analysis of energies of stationary bubbles at liquid pool surfaces and the subsequent droplet release due to bubble burst based on RESUS code calculations

In case of a hypothetical nuclear reactor accident involving a failure of the primary system, a liquid coolant pool may frequently be formed, which may be contaminated by suspended or solved fuel particles and fission products. For many accident sequences, in which gas is injected into the liquid domain or bubbles are generated by means of heating, pressure transients or chemical reactions, the release of low volatile species from liquid surfaces into a gas atmosphere due to bubble burst is identified as a decisive release mechanism. Generally, resuspension of radioactive species is caused by droplets ejected from the pool surface into the atmosphere, either dominated by direct momentum exchange mechanisms between gas flow and liquid for high gas fluxes or by bursting of single bubbles in case of low gas fluxes. In the latter case, the release of droplets containing radionuclides is governed by two mechanisms, namely micro-droplet generation due to bubble film cap collapse and formation and subsequent disintegration of liquid jets producing so-called jet droplets. Jet and jet droplet formation is modeled in the code system RESUS.MOD2 which contains models for the growth of the bubble in the pool, its shape while resting at the liquid surface, and jet and jet-droplet generation after bubble cap rupture. Using the module BUBSHAPE (BUBbleSHAPE) of the RESUS code, the characteristic profile of a bubble resting at the liquid surface as well as the mechanical energies available for droplet formation and consequently droplet- and particle release can be determined. The governing resuspension energies are identified to be the surface energy, the energy stored in the gas within the bubble due to its excess pressure, and the potential energy due to the displacement of liquid