Percolation cooling of the Three Mile Island Unit 2 lower head by way of thermal cracking and gap formation

Two partial models have been developed to elucidate the Three Mile Island Unit 2 lower head coolability by water percolation from above into the thermally cracking debris bed and into a gap between the debris and the wall The bulk permeability of the cracked top crust is estimated based on simple fracture mechanics and application of Poiseuille's law to the fractures.

The gap is considered as an abstraction representing an initially rugged interface, which probably expanded by thermal deformation and cracking in connection with the water ingress. The coupled flow and heat conduction problem for the top crust is solved in slab geometry based on the I two-phase Darcy equations together with quasi-steady mass and energy conservation equations.

The resulting water penetration depth is in good agreement with the depth of the so-called loose debris bed The lower-head and bottom-crust problem is treated analogously by a two-dimensional axisymmetric model. The notion of a gap is maintained as a useful concept in the flow analysis. Simulations show that a central hot spot with a peak wall temperature of 1075 to 1100 degreesC can be obtained, but the quenching rates are not satisfactory.

It is concluded that a three-dimensional model with an additional mechanism to explain the sudden water ingress to the hot spot center would be more appropriate.