Influence of steam generated by thermal radiation on the behavior of large two-phase UO2 bubbles in the water

Assessment of the behavior of a large two-phase UO2 bubble resulting from a hypothetical core disruptive accident in liquid metal-cooled fast breeder reactor is important from the standpoint of radiological risk. The Fuel Aerosol Simulant Tests (FAST) at the Oak Ridge National Laboratory were conducted to explore mechanisms important to the potential transport of radionuclides from such a bubble to the cover gas in a pool of either sodium or water. The bubble behavior was analyzed at the University of Virginia with the UVABUBL computer model which consists of a spherical heat transfer model with a two-dimensional hydrodynamic constraint. In the analyses previously reported on the underwater FAST experiments by using the UVABUBL computer model, penetration of the thermal radiation from the bubble into the surrounding water was not considered. A new model was developed to account for the absorption of radiation beyond the bubble-water interface. The current analysis also accounted for several other phenomena to obtain better agreement with the experiments. With the new model, a consistent set of input parameters was found that yielded agreement in bubble period, maximum radius, and pressure response between calculation and experiment for the underwater FAST test. The analysis indicated that heat transfer from the bubble led rapidly to a steam-driven bubble. Entrainment was believed, though not confirmed, to provide the source of steam for most of the tests. For those tests in which the water was initially close to saturation at the cover-gas pressure, significant vaporization occurred at the bubble surface