ИСТИНА

Coolability of an internally heated porous debris bed in a water pool is studied numerically, with emphasys on the effect of particle agglomeration due to incomplete solidification of melt fragments at the bed formation stage. Agglomerated debris can introduce excessive resistance for steam escaping from the bed and water entering into the bed, increasing the chances of large dry zone formation where the particle temperature can escalate. In this work, the coolability problem for a mound-shaped debris bed is considered with emphasis on the effect of distributed agglomerates reducing permeability in the top part of the debris. Vertical distribution of mass fraction of agglomerates is obtained as a generalization of physics-based surrogate model validated against DEFOR-A experiments. Numerical simulations are performed by DECOSIM code developed for coolability studies. A number of runs with different decay heat power are performed, and the size of dry zone and maximum particle temperature are obtained. These are compared with an analytical model for post-dryout maximum particle temperature developed for a homogeneous debris bed. By processing simulation results obtained in a wide range of parameters, a surrogate model for post-dryout behavior of a debris bed with agglomerates is developed. It is shown that dryout and post-dryout properties of the debris bed depend upon the maximum mass fraction of agglomerates on the top boundary of debris bed. The model provides an important link between the melt ejection from failed reactor pressure vessel and coolability of the debris bed formed upon fuel-coolant interaction.