TMI-2 core debris bed coolability

This report studies the thermal behavior of debris beds impermeable to cooling water. It considers these debris beds as idealizations of the slumped TMI-2 core as conjectured between 2 h, 54 min and 3 h, 46 min into the accident. Section 2 calculates the decay heat levels as functions of time after shutdown, assuming various fractions of the more volatile fission products were released during the earlier high temperature transient up to 2 h, 54 min. Section 3 determines the thermal properties of the debris-bed material as function of fuel content in a mixture of uranium dioxide and zircaloy. With these parameters, Section 4 calculates the equilibrium temperature profiles in slab geometry for various sizes and compositions of the debris bed. Calculations were carried out to determine the minimum thickness of debris beds that have a molten interior under equilibrium conditions and the equilibrium thicknesses of the solid crusts of debris beds having molten interiors as functions of their overall dimension. Hopefully, these calculated parameters will help determine a lower size limit of the TMI-2 debris bed below which molten core material could not have flowed down to the lower plenum. In Section 5, a one-dimensional, slab geometry, transient heatup calculation was performed for a best-estimate configuration of the TMI-2 debris bed after its formation (about 2 h, 54 min into the accident)