The UVABUBL program and its application to the fast underwater tests

The Fuel Aerosol Simulant Tests (FAST) at ORNL were conducted to explore mechanisms important to the potential transport of radioactive materials to the cover gas in an LMFBR hypothetical core disruptive accident. In these tests two-phase bubbles of UO2 were generated under water or sodium by the capacitor discharge vaporization technique. A computer code, UVABUBL, was developed to analyze the bubble behavior. The code consists of a two-dimensional constraint and a one-dimensional (spherical) heat transfer model. A consistent set of input parameters was found that provided agreement between calculation and experiment for the bubble period, radius, and pressure response for the FAST underwater tests. The analysis indicated that heat transfer from the bubble led rapidly to a steam driven bubble. It is believed, though not absolutely confirmed, that droplet entrainment into the bubble provided the source of steam for most of the tests. For those tests, however, for which the water was initially close to the saturation temperature at the cover-gas pressure, considerable vaporization occurred at the bubble surface. Significant dissociation of steam occurred at the bubble temperature and the resulting chemical reaction products had a small, though not insignificant, effect on the calculated bubble period. A one-dimensional-slab constraint provided close agreement for the period and radius with the two-dimensional results except for the smallest bubble. (orig./HP)