The Process And Mechanism Of Iodide Method For Zirconium Crystal Preparation

Zirconium is one of the most ideal cladding materials in nuclear reactors due to its low neutron absorption cross section and excellent high-temperature corrosion resistance. Meanwhile it has broad applications in aviation, aerospace and nuclear industry because of its excellent oxidation resistance, weldability and machining performance. Iodide method is an effective method to prepare zirconium with high purity and plasticity and low gas impurity content.For a long time, there have been only a few reports on zirconium preparation and purification by iodide method in terms of theory and technology because of different security measures for nuclear materials worldwide, which is detrimental to investigate zirconium preparation by iodide method deeply.In the aspects of theory, and thermodynamics require more accurate analysis. To resolve problems mentioned above and provide guidance for the application of this method, technical and theoretical research was systematically conducted, and results are as follows. On the other hand, in the aspects of technology, the effect of raw material temperature, the size of filament, charging methods, temperature controlling and iodine addition amount on the behavior of iodide method has not been thoroughly investigated, although all those influencing factors were adjusted to improve zirconium products.1. Theoretical aspects(l)In the standard condition, When low temperature zone temperature below 431 ℃, the reaction generates ZrI2. And it can make ZrI4 gas in the adequate iodine vapor. When low temperature zone temperature between 431 ℃ and 540 ℃, the reaction generates ZrI3, ZrI4 gas will exist in the reactor. As the low temperature zone temperature between 540 ℃ to 900 ℃, it is easy to generate ZrI2. Since ZrI4 is easy to volatilize, the atmosphere is almost ZrI4. When the low temperature zone temperature is higher than 900 ℃, it make almost all iodine vapor into ZrI4 gas.(2)Then the reactor of vapor pressure is calculated in the process of reaction, it is concluded that the higher the wire temperature the higher the content of iodine vapor. Through thermodynamics can determine the suitable conditions of reaction at low temperature reaction. Combining with the analysis of the actual production, when the temperature of the low temperature zone is lower, the difference of the concentration of ZrI4 can be maintained larger at high and low temperature zone. It is favorable to the diffusion.(3)In order to get the degree of difficulty of synthesis and decomposition reaction of each metal and iodine, it is calculated the thermodynamic data of different metals react with iodine. Zirconium and hafnium with iodine standard Gibbs free energy is very similar, therefore cannot be distinguished by iodide method. And nickel and chromium hardly react with iodine in standard condition, which can better remove.2. Technical aspects.(1)Through the experiment, in the high temperature zone, the deposition rate increases with the increase of K value. And the deposition rate will reach a constant value, K value is 90 when there will be the optimal solution. Controlling the molten salt temperature in low temperature area at about 250 ℃ is more appropriate. To add iodine properly in the experiment can improve the quality of each production, thus improving the operation efficiency. When the times of iodine, it can effectively improve the reaction time. The reaction rate is almost the same, so the final reaction efficiency is improved. Through experiments it is concluded that two kinds of temperature control mode deposition rate, comparing several temperature control methods, the molten salt deposition rate is more stability and faster than air cooling temperature control deposition rate. In terms of electrode, the experiment determines thicker electrodes can make the resulting product quality higher.(2)The optimal condition for zirconium preparation by iodide method was as follows, high K value(90), salt bath temperature control method, the low temperature zone is controlled at 250℃, molybdenum loading, add the iodine several times, under which high deposition rate(33.3g·M-1·-h-1), large grain size(>lmm) and low impurity content.