A comparative analysis is made of the computed and experimental data to confirm the adequacy of the heat-transfer model and the suitability of this model for determining the effective thermal condutivity of fibrous heat-insulation materials. Optimization calculations of the heat-insulation layer of quickly removable general-purpose heat insulation are performed for a wide range of the average diameter of fibrous materials as compared with the normative method.Thermal insulation is an integral part of the equipment which influences the reliability and cost-effectiveness of the operation of nuclear power plants with VVÉR reactors. Research and development work on upgrading such insulation has been conducted in the last few years. One promising structural solution is quickly removable general-purpose insulation, which makes it possible to quickly free of thermal insulation the equipment and pipeline parts which are to be monitored. This decreases the irradiation dose to workers during the period when the maintenance and preventative work is performed. The possibility of diverse applications also substantially increases the amount of radioactive wastes.Quickly removable thermal insulation consists of modules which are mounted by successive locking connections. A module consists of a protective casing with the heat-insulating material, based on basalt or glass fibers, inserted into it.As a rule, thermal insulation structures with a metal protective casing are used in the first loop; fabric jackets can be used in the second loop. Since thermal-insulation structures in operating nuclear power plants are part of the design, quickly removable general-purpose thermal insulation requires an individual design approach that takes into account the operational action, the site of the insulated object, and the dimensions and configuration of the object. The characteristics of the heat-insulating layer are optimized by performing a simulation during the development and design work in order to reduce the time and cost [1]. The thermotechnical characteristics were determined using a model of heat transfer in fibrous materials. In the process, their basic technical characteristics were taken into account (density, average diameter of the fibers, and operating temperature).During the development of the model heat transfer in a unit cell in the form of a parallelepiped, along one diagonal of which a fiber is placed, touching neighboring fibers at its vertices, were examined. It was assumed that the heat flux passing through the cell is perpendicular to one side of the parallelepiped, the content of the solid phase in the cell is equal to its average concentration, the temperature on the bottom and top surfaces of the unit cell is constant and heat is not transferred
The equipment and pipelines in operating nuclear power plants are, at the present time, insulated with heat-insulating materials (fiberglass, mineral wool, basaltic fiber). During operation, these heat-insulation materials absorb moisture from the surrounding air. In leaks, the coolant saturates the heat-insulating materials with moisture. The main technical characteristics of heat-insulating fiber materials and the quality of the fibers under these conditions have not been evaluated.This paper presents the results of investigations of prolonged one-time action of a medium (solution of boric acid) and temperature on a heat-insulating material made of basaltic fiber.The tests were conducted on fabrics made of basaltic fiber, which are widely used in nuclear power plants. To assess the effect of a boric acid solution and temperature acting together, the density, compressibility, elasticity, and fiber diameter were monitored. The chemical composition of a basaltic fiber is as follows (at. %): Na 3.5, Mg 1.8, Al 6.48, Si 16.3, P 0.26, K 0.51, Ca 3.7, Ti 0.36, Fe 4.3, O 61.1.The tests were conducted in two stages. The effect of a stationary medium and temperature acting together was assessed at the first stage and the effect of a moving medium together with temperature was assessed at the second stage.The samples in the shape of a parallelepiped, 100 ± 50 mm long and wide, were cut from heat-insulating fabric. The dimensions were determined by the procedures descibed in [1]. The process of holding heat-insulating materials in a stationary medium was conducted in two successive stages: 15 days in a boric acid solution at 40°C; some samples were eliminated and the remaining ones were held for an additional 15 days in a boric acid solution at 50°C. The samples were held in a moving medium for 15 days in a boric acid solution at 40°C and then 15 days in a boric acid solutions at 50°C. After the samples were removed from the medium, the solution was washed off them by successively replacing in a bath the solution with distilled water, and pH was determined. The washing process was terminated at pH = 7. Next, the samples were removed from the water and dried to a constant mass by the procedure described in [1].Methods of Investigation. The density, compressibility, elasticity, and diameter of the fibers before and after the heat-insulating material was exposed to the medium and heat were determined by the procedure of [1]. The micromorphology of the basaltic fibers was studied with high resoltion (to 10 nm) before and after exposure to the medium and heat by scanning electron microscopy in the secondary-electron regime with accelerating voltage 200 kV on the JEM-2000FXII electron microscope (Japan). The experimental samples of the fibers were glued with silver glue onto a metal plate in a holder and then metallized by depositing a 10-30 nm thick layer of amorphous carbon by thermal evaporation in vacuum (2·10 3 Pa) in the Univex-300 setup (Germany).
The quickly removable thermal insulation, which is now under development, improves labor conditions, decreases the time required for assembly and disassembly of thermal insulation, and decreases the irradiation dose to workers during maintenance and prophylactic work at nuclear power plants. It consists of removable modules, which are mounted by locking the modules together in succession. A module consists of a box (a protective metal case) containing thermal-insulation material whose shape is the same as that of the surface being insulated. Since the equipment at a nuclear power plant has, as a rule, a complicated configuration, the design of the thermal insulation requires an individual approach. Consequently, a decision was made to develop a universal computational model to estimate the main thermal insulation parameters (temperature on the surface, heat losses, and effective thermal conductivity). Fibrous materials (canvas made of basaltic or glass wool), which reproduce well the complicated shapes of metal casings, are to be used as the main thermal insulating layer.In the present paper, as a first step, a method for calculating the effective thermal conductivity of fibrous materials and the results of experimental investigations are presented.According to the theory of generalized conductivity [1], the effective thermal conductivity of fibrous thermal insulation can be determined by studying heat transfer in a unit cell in the form of a parallelepiped, with the fiber, touching neighboring fibers at the vertices of a parallelepiped, arranged along one of the diagonals. The heat flux which passes through the cell is perpendicular to one side of the parallelepiped. The content of the solid phase in the cell is equal to its average concentration.We shall examine three mechanisms of heat transfer through a unit cell of fibrous thermal insulation: heat transfer by radiation, heat transfer through the solid phase and filler gas by heat conduction, and convective heat transfer. The equation for radiation diffusion in which the nonuniformity of the incident and characteristic radiation is taken into account was used to determine the radiant component of the heat flux. The effective thermal conductivity of the solid phase was determined from the solution of the heat conduction equation for a fiber, arranged in a unit cell, and the heat exchange with the surrounding medium by radiation through the gas and neighboring fibers was taken into account. Scattering of the gas by fibers was taken into account in the calculation of heat transfer through the gas. As a result the following relations (for an optically thick layer) were obtained for the effect of thermal conductivity of fibrous thermal insulation (neglecting convection) [2, 3]: λ * eff = λ r + λ g + λ con ,
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