Researchers at the Idaho National Engineering and Environmental Laboratory (INEEL), under the sponsorship of the U.S. Nuclear Regulatory Commission (USNRC), have used FIDAP to explore heat transfer in the lower head of a reactor vessel during the late phase of a severe accident. By this time during the accident, many of the vessel internals, including the fuel rods, have melted, resulting in a pool of heat-generating molten material at the base of the vessel. Cooling water is applied to the outside of the vessel, in hopes of preventing catastrophic failure of the container itself.

The goal of the ongoing project is to assess the ability of SCDAP/RELAP5/ MOD3.2 to solve a similar problem. This software has been widely used for transient analyses of accidents in nuclear power plants, especially those in which the cooling system contains multiple phases (water and vapor) in a number of different regimes. The RELAP5 models for simulation of molten metal are less sophisticated than those in FIDAP, so the simulation was performed as a validation for RELAP5.

The temperature distribution in the lower head of the reactor vessel as it approaches steady state, where a steady heat source in the molten metal is balanced by continuous cooling by room temperature water.

To date, the FIDAP results have provided helpful insight into the nature of natural convection currents in the molten metal, as well as the temperature distribution as the system approaches steady state. One of the most important results, however, is the computed heat flux through the surface of the container. If this exceeds a threshold value, known as the critical heat flux, or CHF, structural failure of the vessel will occur. The transient FIDAP results show a rapid reduction in the wall heat flux as cooling water is applied.

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