| Zirconium hydride in solid state with high thermal stability, high hydrogen content, low thermal neutron absorption cross section, good mechanical properties and negative reaction factor of temperature, is considered to be one of the most ideal moderators. The nuclear reactor using zirconium hydride as moderator can run in elevated temperature without high pressure vessels, therefore the zirconium hydride moderator is particularly suitable for the space nuclear power system. The United States and Russia leading the top air space technologies have all used zirconium hydride as moderator in their space nuclear power systems.However, the volume expansion and lattice distortion would accur during the preparation of the zirconium hydride moderator. Addirionally, the zirconium hydride itself is a brittle hydride phase. Those make the preparation of the high hydrogen crackfree zirconium hydride is very difficult. It is meaningful to resolve the cracking problem in the hydriding process of the zirconium hydride for improving the life and safefy of the space nuclear power system, which applies zirconium hydride as moderator. Due to technical blockade, currently the domestic research on the preparation of zirconium hydride and relative crack control mechanism is rather limited.The effect of Nb on the preparation of high hydrogen crackfree zirconium hydride and the existing status of Nb in the zirconium hydride were investigated at present. The thermodynamic properties and equilibrium phase information of zirconium hydride system can be used to guide the preparation of zirconium hydride moderator. Combining CALPHAD and experiment methods, the key problems such as the cracking and hydrogen content control, were tried to resolve in this paper. By means of advanced computer technologies, the efficiency on the investigations of preparation method and process parameters can be greatly improved.The Zr-H-M multielement thermodynamic database was established for the research needs of hydrogen absorption properties of zirconium alloys. The adding elements M in the database including11elements which were Al, Cu, Fe, La, Mg, Na, Nb, Ni, Pd, Sm, Ti. To aim at the preparation of Nb-containing zirconium hydride, the thermodynamic models of H-Nb binary system and Zr-H-Nb ternary system were optimized based on the literature information and the experimental data in this work, and the calculated results agreed well with the experimental data. The equilibrium phase information and thermodynamic properties of Zr-H-Nb system on the Zr-rich side (wt.%Nb≤2.5) could be calculated and predicted availably, to guide the preparation of Nb-containing crackfree zirconium hydride.To obtain the hydrogen absorption properties of Zr-Nb alloys, the PCT (Pressure-Composition-Temperature) measurements for Zr-Nb alloys with different Nb content (1wt.%～30wt.%) were carried out at low pressure in the temperature range of700℃～900℃. The experimental results indicated that the equilibrium hydrogen content of zirconium alloy decreased apparently with the Nb content increasing. As the Nb content of the alloys increased, the equilibrium hydrogen pressure also increased for any fixed concentration of hydrogen and the δ zirconium hydride precipitation from saturated βZr is unfavorable in presence of Nb. And Nb could delay the precipitation of s zirconium hydride.Nb affects the hydrogen content and cracking formation of the zirconium hydride, which is determined by the existing status of Nb in the zirconium hydride. The composition, structure, morphology and metallographic phase of hydriding products of Zr-Nb alloys with different Nb content (1wt.%～50wt.%) were investigated by the methods of XRD, SEM, EDS and metallographic test. In the case of fully absorption, the hydriding products of Zr-Nb alloys with different Nb content were all composed of mixed ε zirconium hydrides of ZrH2, ZrH1.95and ZrH1.801.When the Nb content was high, the products of NbHx solid solution with low hydrogen content downgraded the whole hydrogen content of the alloy, which was because of the δ-NbH2with high hydrogen level only existed steadily in the condition of high pressure above room temperature based on the thermodynamic calculated results. The solubility of Nb in zirconium hydride is limited and the majority of Nb disperes on the surface of zirconium hydride in the form of small white particles of H-contaning Zr-Nb solid solution under the condition of low hydrogen level. The existence of Nb doesn't significantly change the lattice parameter of zirconium hydride, but improves the organizational structure of zirconium hydride, which reduces the H-concentration defect sites and the possibility of cracking. Although Nb can help control the formation of crack, but Nb also affects the hydrogen content of the zirconium hydride, so the dosage of Nb should not be too much. In addition, more hydrogen content contained in the zirconium hydride with1wt.%Nb, the more obvious features of twin stucture can be observed. The crack forms mainly along grain boundaries.Through the above analysis, combined with thermodynamic calculations and experiments for the preparation of zirconium hydride, the key issues in the preparation of crackfree zirconium hydride with high hydrogen content were investigated systematacially. The reasonable parameters, related mechanisms and phenomenons in the preparation process were tried to obtain and analysis at present. The results showed that the upper limit of hydrogen pressure in the hydriding process should be restricted in a low level. The hydrogen pressure in this work was programmed not to exceed105kPa. The formation of solid solutions during the preparation of zirconium hydride had no effects on cracking. In order to control the cracking, slow cooling rate and slow hydrogen flow during the βZr→δ and8→εe two phase transition processes were the key factors. The precipitation of hydride phases, which were the source of cracking, destroyed the plastic nature of the material. According to thermodynamic calculations,βZr→δ transition temperature was872℃and8δ→ε transition temperature was794℃. A reasonable multi-step hydriding temperature curve was made based on the calculations and experiments. The hydrogen absorption of the alloys was in a slow rate and the cracking was inhibited efficiently.Temperature plays a decisive role on the hydrogen content of zirconium hydride. Keep pressure in a constant level, a rule of correspondence between equilibrium hydrogen concentration in the alloy and temperature was concluded. As the temperature decreased, the hydrogen composition increased and the equilibrium phases of the system also changed. For Zr-1Nb-H system, the zirconium hydride with hydrogen concentration of1.8H/Zr in atom ratio was fct structure in equilibrium at788℃, and the zirconium hydride with1.6H/Zr was fcc structure at820℃. During the hydriding process, enough hydriding time should be given to make hydrogen diffuse completely and ensure the uniform and saturation of hydrogen content in the alloys, especially for the large size samples. This process also played the annealing role to remove the internal stress and inhibit the cracking.
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