Thermal instability in freshwater lakes under ice: Effect of salt gradients or solar radiation?

The phenomenon of “temperature dichotomy”, or anomalous heating of surface water under the ice up to temperatures exceeding 4 °C is known to take place occasionally in solar-heated ice-covered freshwater lakes and has usually been explained by the stabilizing effect of the weak vertical salinity gradient created by the melt water flux from the ice and supporting the unstable temperature distribution. Here, we report an observation of the local temperature maximum in the upper part of the water column of ice-covered Lake Vendyurskoe (northwestern Russia). The observation was accompanied by vertically resolved measurements of the conductivity allowing estimation of the dissolved salts effect on the vertical density distribution. The results demonstrate insufficiency of the salt gradient to support the vertical stability of water column. Therefore, we suggest the vertically inhomogeneous radiation absorption to be the probable stabilizing mechanism here, similarly to radiatively heated boundary layers in the ocean, the atmosphere and star interiors. An analytical solution of the heat transfer equation is derived describing the temperature profile evolution in ice-covered lakes subject to solar radiation heating above the maximum density temperature. Observed daytime temperature profiles agree well with the analytical model that suggests the absence of convective mixing. According to the model, the temperature maximum is formed within a day that supports the hypothesis about the stabilizing effect of the solar radiation absorption. We conclude that in temperate lakes the warm layer should have diurnal character and should be destroyed during the nighttime by convection. In polar lakes, in turn, the warm layer can exist during essentially longer periods that is supported by application of the model to the previously published data from Lake Peters, Alaska. In this case, the ice melting rate can be significantly affected by the increased temperature gradient beneath the ice. Apart from potential effects on the ice melting rate and the spring plankton development in lakes, the regime represents a rare geophysical example of instability driven solely by radiative heating with many useful analogies in planetary and stellar physics.     

Optimization of the Materials Composition in External Core Catchers for Nuclear Reactors

Existing schemes of core melt retention apparatus for water-cooled water-moderated nuclear reactors are analyzed. In-shaft variants of melt catchers at nuclear power plants with VVÉR-1000 reactors are proposed. It is shown that TiO₂- and Nd₂O₃-based materials increase the operational reliability of the retention apparatus by modifying the processes occurring in the melt and by preserving the integrity of refractory coatings consisting of zirconium dioxide. TiO₂-based material not only decreases the effect of the melt on the refractory but also confines some fission products in geologically similar matrices which are synthesized as the melt cools.     

Rapid Solidification of ZrO2–8 mol % Y2O3 Melt under Conditions of Stepwise Reduction of Heating. Determination of the Liquidus and Solidus Temperatures

Investigations are conducted into the process of crystallization of a bath with a ZrO₂–8 mol % Y₂O₃ melt which is produced by local heating of ceramic by concentrated laser radiation and solidifies when the radiation power decreases by 20–30%. It is demonstrated that a gently sloping temperature plateau is observed in the process of solidification, which may be used to estimate the liquidus temperature.     

Titanate Composites for Core Catcher Donor Material in a Nuclear Reactor

Composites based on titanates compositions have been selected and tested. A technology for their production is proposed using high-alumina cement. Estimate of some of their properties is supplied making it possible to use these composites as donor materials in a reactor core catcher.     

Shock-wave compacting of a Fe-Ni-Al powder mixture and its study

The results are presented which concern measurements of X-ray diffraction spectra, microstructure, temperature dependences of the real part of the complex magnetic susceptibility, the elastic modulus, and internal friction of an Fe₆₅Ni₂₃Al₁₂ powder mixture obtained by shock-wave compacting. A series of anomalies of the magnetic and elastic properties have been found. It has been assumed that the observed features of magnetic and acoustic spectra of the composite can be caused by a number of factors, such as the formation of a metastable state, intermediate phases, and superstructures or by polymorphic transformations arising upon shockwave compacting at particle boundaries.