Core components and fuel rods may melt partially or completely during a major accident at a nuclear power reactor, so one gets a multicomponent mixture of liquids and solids, whose physical properties are very different from those of the pure components [1]. There is little information on the viscosities of liquids in binary and more complicated systems, and this makes it necessary to examine and test methods of measuring the kinematic viscosity for fuel-composition liquids.Out of the set of methods [2, 3], that of [4] is a good one, as it has a reliable theoretical basis and requires comparatively simple apparatus. That method has been applied to the viscosities of liquids formed by various metallic and ceramic fuel materials at temperatures up to 3050~ [5].The method involves exciting and recording low-frequency freely damped torsional oscillations in a cylindrical crucible filled with the liquid. The oscillation parameters are determined by an optoelectronic method from the displacement of a beam reflected from a mirror in the support, and from which the characteristics are calculated. The apparatus for examining the viscosity of liquids and fuel composites up to 2400~ was similar in design to the viscometer described in [6]. A hightemperature vertical vacuum resistance oven type SShVl~-l.25/2.5-IZ was used with a maximum working temperature of 2500~ The system included an automatic one for recording the torsional oscillations.The period and the damping decrement were recorded with a special interface board attached directly to the highway in an IBM-compatible personal computer. The software was based on an algorithm for the characteristics [8, 9] and enabled one to perform the experiment with one or two photocells. The light source was a helium-neon laser and the detectors were photodiodes.The maximum temperature could be increased to 2600"C by a minor modification of the screening in the heating zone and the use of tungsten as material for the rod and supporting wire, as well as the use of a crucible having special coatings preventing interaction with the molten material. The crucible had already been tested at that temperature.The apparatus was tested on model materials, namely tin and lead. Test studies were made on the melting points of the material up to 1000~ and gave values for the kinematic viscosity in accordance with published ones within the error of measurement, which did not exceed + 5 %, and which indicates that the design is good and the apparatus is viable.Preliminary experiments were performed on the application limits and use of the viscometer before extensive use and costly studies on the temperature dependence of the viscosity for reactor fuel materials, and also because there are no similar data.The model material was 99.9% pure tin with added f'mely-divided anhydrous corundum powder (A1203) on the assumption that there would be no reaction between those components at the moderate working temperature. The specimens were prepared by a method providing a uniform distribution of the corundu...