| The basic principle of magnetic confined fusion-fission hybrid reactor energy system (namely hybrid reactor) is that the sub-critical fission reactor with the fuel of238U and232Th driven by high energy neutron from D-T fusion in Tokamak system provides energy and produces tritium for D-T fusion. Its advantages are:fusion neutron has good breed property. They can react with238U and232Th to produce239Pu and233U. Water can make high energy slow down to thermo-neutron that can make239Pu and233U fission for energy emitting. The fuel, no matter the U, Th or spend fuel need not separation. The hybrid reactor can break energy source limitation and lift up the efficiencies of energy usability. It is a state-of-art technology.As one of key technologies and pre-conditions of hybrid reactor application, the design and preparation of nuclear fuel materials in sub-critical fission reactor is on the schedule. Fission fuel element is the core part of energy breeding in hybrid reactor, which irradiated by fusion neutrons and fission to give out energy. Hybrid reactor is a new energy system. With the difference from traditional fission reactor, the fission fuel element materials in hybrid reactor have a different working environment and demand different properties. According to the fusion-fission hybrid reactor energy system application, the properties demanding for fission nuclear fuel material have been analyzed. Material designing has been done from constitutions and structures. UMo alloy and UMoZr and UMoNb alloy have been selected as research direction.For U-10wt.%Mo alloy, designing and confirming a technology process of vacuum melting and casting-preparing powder by hydriding and dehydriding-vacuum solid phase sintering, nuclear fuel materials with certain void fraction have been prepared successfully. The y phase stability of U-Mo alloy, mechanism of U-10wt.%Mo alloy hydriding reaction, the regularity of powder metallurgy process affecting the void fraction in U-10wt.%Mo alloy, and the regularity of void fraction affecting the mechanical and thermal properties have been discussed. The main content lists as following:(1) The regularity of Mo concentration affecting the U-Mo alloy y phase stability. Thermodynamic phase diagram of U-Mo-Zr and U-Mo-Nb alloy have been studied by matured code. Several constitutions have been selected according calculation result and designing, and samples have been prepared for analysis of phase and micro structure. The result suggested that the y phase stability is increased with Mo concentration in U-Mo alloy. Nb is more useful for U-Mo y phase stability than Zr.(2) In-situ observation of hydriding reaction of U-10wt.%Mo alloy has been done, detailed surface morphology has been studied as well for mechanism of hydriding reaction. It is suggested that hydrogen in environment should diffuse through oxide layer and arrive at metal to hydride U-Mo alloy. So the thickness affected the hydriding reaction. The experiment results showed that the hydriding reaction process of U-10wt.%Mo alloy can be divided to three duration, namely, incubation nuclearating and growth. It is suggested that oxide layer has broken in phase decomposition area, which is the main cause for faster hydriding reaction than y phase U-10wt.%Mo alloy. The oxide layer on y phase U-10wt.%Mo alloy is dense and not broken. There is a longer incubation duration in hydriding reaction.(3) The room temperature mechanic properties and hardness of U-Mo alloy with different Mo concentration have been measured. The result showed that the mechanic properties are not related to Mo concentration directly, but affected by microstructure. According to hybrid reactor application environment, tensile tests of U-10wt.%Mo alloy have been done at400℃,500℃, and600℃. It is showed that the ultimate strength of the sample was decreased to about half of the strength at room temperature.It is found that there is plenty of carbide or oxide in grain boundary, which is the main cause for brittle fracture of the alloy.(4) The process of preparing low density porous U-Mo alloy by powder metallurgy was described. The results showed that densitify effects were not unremarkably at800℃and900℃sintering. It is proved that the density of sample increased with sinister time under1100℃. However, the sintering temperature cannot high than that to avoid sample melting.1100℃is the best sinter temperature. The void ratio can be controlled by adjusting sinister process conveniently. The goal of preparation of U-10wt.%Mo alloy with certain void fraction has been achieved.(5) The specific heat capacity, coefficient of linear thermal expansion, and thermal diffusivity of porous U-10wt.%Mo alloys with different density were measured and thermal conductivity from373K to873K were calculated according the experiment results. It is suggested that the thermal conductivity will increase with the temperature and decrease linearly with decreasing of sample density as Loeb equation described. As a candidate material for metallic fuel material application, the thermo-properties of U-10wt.%Mo alloys is extremely important and must be well determined for effectively model and predict fuel behavior under long term application duration. |
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