An analysis of the aftereffects of the accident at the Three Mile Island (USA) nuclear power plant revealed the importance of heat and mass transfer in a heat-releasing porous medium for reactor safety [1]. The interaction of a porous heat-releasing bed with the body of the reactor is now considered to be one of the possible stages of serious accidents in lightwater reactors with a loss of cooling ability in the active zone [2]. The main danger for the body of the reactor is a thermal attack by molden material, when separate parts of the porous layer dry out and become overheated. To estimate this risk it is important to know the maximum thermal loads that do not lead to the formation of dry zones in the porous medium.The physical processes which govern the pattern of two-phase convection when boiling occurs in a porous medium under nonuniform heat-release conditions were investigated in [3]. It was shown that the critical heat flux depends very much on the possibility of a transverse redistribution of the liquid and vapor, due to the nonuniformity of the saturation of the medium by the liquid. First, the difference in the fraction of the vapor in neighboring regions leads to a variation in the pressure and to the development of natural convection of the liquid. In regions with the greatest fractions of vapor, a rising flow occurs and vice versa. A closing of the streamlines implies the setting up of return flow of the liquid from the region with a high fraction of vapor into a region with a low fraction of vapor, which provides an additional supply of liquid into the boiling region, i.e., it increases the limit permissible heat transfer. Second, variations in the fraction of the liquid leads to the occurrence of a nonuniformity in the capillary pressure. It is easy to understand the nature of this effect if we take into account the fact that in a porous medium the distribution of the vapor and liquid at the microlevel is regulated by capillary forces --water, which wets the solid matrix, tends to fill the finest capillaries, leaving the coarser capillaries to the vapor. The characteristic size of the capillaries filled with liquid, as a result, will depend on its fraction: the greater the fraction of the liquid the greater the mean size. Hence, when the fraction of the liquid changes, the size of the meniscus changes, and, correspondingly, the pressure difference in the vapor and liquid. It follows from this pattern that, in regions with a high fraction of vapor, the pressure is higher than in regions with a lower fraction of vapor. As a result, there is an additional removal of vapor from the active-boiling zone and an additional supply of water to it, produced by capillary effects. Hence, in a layer with nonuniform heat release, capillary effects, like convection, increase the maximum permissible thermal loads.One other factor which affects critical heat flows and the role of which has been insufficiently investigated is the transverse nonuniformity of a porous medium and the characteristic size of its channel...
