Research On Mo-based Lanthanide Oxides ?Tm₂O₃,Dy₂O₃ And Tb₂O₃? Neutron Absorber Materials

The core control rods are very important in the normal operation and accident conditions of the nuclear reactor for the control of nuclear reactivity,the key lies in the internal neutron absorber materials.The pellets of molybdenum-based thulium oxide,molybdenum-based dysprosium oxide and molybdenum-based terbium oxide used as neutron absorber materials of gray control rods in the nuclear reactor,were prepared by the process of high energy ball milling,cold isostatic pressing and argon protection sintering in this paper.The microstructural evolution of mixed powders during ball milling was analyzed,and the thermodynamic mechanism and kinetic driving mechanism were discussed.The effects of milling time and sintering temperature and sintering time on the phase and microhardness of sintered bulks were studied.The phase and microstructure of sintered bulks were analyzed,the microhardness thermophysical properties and density of sintered bulks were tested.After high energy ball milling,Mo-Tm₂O₃,Mo-Dy₂O₃ and Mo-Tb₄O₇ mixed homogeneous powders were obtained.The powder mixtures were fined,homogenized,nano-crystallized,and then converted into nanocrystalline supersaturated Mo-based solid solution by high-energy ball milling.In the Mo-Tm₂O₃,Mo-Tb₄O₇ and Mo-Dy₂O₃ systems,the diffraction peaks of oxides disappears after about 24h of ball milling,the Mo-phase diffraction peaks gradually broadened and reduced in intensity gradually with increasing milling,and shifted to a low diffraction angle.High energy ball milling induced oxides to be decomposed and then dissolved the rare earth atoms into the molybdenum crystal structure.The grain size of molybdenum phase decreased gradually with the increase of the ball milling time,and then became to be stable.The lattice constant remains basically unchanged before 24h of ball milling,and increases gradually after continued ball milling.The thermodynamic calculations show that the Gibbs free energy of the formation of solid solution and amorphization are both higher than zero for this chemical composition,and it is difficult to convert the alloy into solid solution and amorphization under the thermodynamic equilibrium condition.At the same time,the Gibbs free energy required to form the amorphous phase is larger than that to form the solid solution,so it is easier to flid solution.The mechanically driven mechanism of high energy ball milling promote the decomposition of the rare earth oxides and dissolved into the Mo crystal structure to form Mo-based supersaturated solid solution.High temperature sintering promoted the oxygen atoms and rare earth atoms precipitating from Mo crystal structure and forming rare earth oxides that uniformly distributed in the Mo matrix.After the milled powder pre-compacted billets were sintered,only the diffraction peaks of rare earth oxides?Tm₂O₃,Dy₂O₃ and Tb₂O₃?and Mo phases can be detected in the bulk materials,and no more other new phase peaks can be found.After sintering at 1400 to 1600?,there is no chemical reaction between rare earth oxides and molybdenum.The ball milling time and the sintering temperature have a certain influence on the phase of the sintered bulks,and have a significant effect on the microhardness value of the sintered bulks.The microhardness of the sintered bulks increased with increasing ball milling time.When the sintering temperature below the 0.7Tm?Tm is the melting point of the component?,the average microhardness of the sintered bulks increased with the increasing the sintering temperature.The average microhardness value of the 48h-milled mixed powder pre-compacted billets sintered at 1600 ? for 24h is the highest,the average microhardness value of molybdenum-based thulium oxide,molybdenum-based dysprosium oxide and molybdenum-based terbium oxide is 434HV_0.1,376HV_0.1 and 483HV_0.1,respectively.The experimental results showed that the density of the three series of sintered bulks is higher than 93%,the elongation of the molybdenum-based rare earth oxide bulks increases linearly with increasing temperature,and the elongation of the three series of sintered bulks is about 0.50%at 800?.The thermal conductivity of sintered bulks tested at room temperature,500? and 800 ? are much larger than the Dy₂TiO₅ bulk used as neutron absorber material,but the coefficient of thermal expansion are less than of the Dy₂TiO₅ bulk.The test results at 800? showed that the thermal expansion coefficients of molybdenum-based thulium oxide,molybdenum-based dysprosium oxide and molybdenum-based terbium oxide bulks were 6.55 × 10^-6/?,6.48 × 10^-6/? and 6.58 × 10^-6/?,respectively.