| Nuclear energy plays an important role in the energy structure of the world, and has a huge space for development in our country. Nuclear reactor is the core of the system. In a nuclear reactor, a variety of radiation rays are produced by nuclear fission or fusion (including different levels of neutrons, y-rays, secondary y-rays, other charged particles, high-energy rays). On the one hand, these rays can damage the internal organs and skin of human; on the other hand, they pollute the air, soil, water and food. In addition, they radiate the structural materials and equipment and reduce their life. Thus, the reactor protection is the most important issue of nuclear safety.Currently, the lead-based, cement-based materials are common nuclear shielding material used mostly. However, the lead-based material is toxic and ineffective for neutron shielding; the weight and volume of the cement-based material are huge and not easy to move. It has low mechanical strength. Metal matrix composites (MMCs) are considered as one of the most promising materials with high properties in 21st century. The boron is widely used in nuclear shielding, aerospace, automotive and other fields because of its high neutron absorption ability. However, the reports of the preparation for the boronated metal neutron shielding materials about foreign aspects are very limited for the technological security. The technology of neutron shielding materials in China is not mature enough.Based on the comprehensive consideration of nuclear shielding and mechanical properties, the design route of metal matrix composite was adopted in this research for neutron absorbing material design ideas. In this study,10B was selected as neutron absorber nuclide as well as 304 stainless steel and aluminum were selected as the matrix for the storage of radionuclide. Three types of shielded metal composites with high boron were prepared by referring to the ASTM standards and handbook on neutron absorber materials for spent nuclear fuel application edited by Electric Power Research Institute and the microstructure, mechanical properties and shielding property (10B areal density) of them were analyzed. In this work, the high boron 304 stainless steel laminated composite plates prepared by coated casting, hot deformation is designed for spent fuel storage racks and transport containers; the aluminum matrix composites prepared by stirring casting followed by hot rolling and powder metallurgy followed by hot rolling are designed for spacer grid in spent fuel storage chamber and civil radiation protection walls. The main research contents and results were as follows:(1) The boron stainless steel core with 2.25wt% boron and different titanium content (0,2.2,5.7,7.9wt%) were melt in vacuum furnace and casted as core materials, whereas the molten 304 stainless steel was poured as cladding. The layer-metal composite plates with different microstructure and properties were fabricated by hot forging, hot rolling at 1100-950℃ followed by solution treatment at for 1100℃ 2-5 hours. It is indicated that the boride in Ti-free sample is (Fe, Cr)2B, the borides decreases and become coarse. The (Fe, Cr)2B eventually disappears with Ti increasing and the small dark TiB2 phase appears and grows into petals, located in the vicinity of large pieces of borides and petals borides. After hot rolling molding, core layer and the cladding layer to form a good metallurgical bonding, casting defects (such as osteoporosis, shrinkage, etc.) are eliminated. After the pressing between both sides of the metal at high temperatures, the chemical bonding formed between atoms, the transition region formed by diffusion concentration gradient. Compared with the hot-rolled state, the strength of high boron steel clad plate decreased after solution treatment, but the elongation is greatly improved. The tensile and yield strengths firstly increased and then decreased as the Ti content increases after the solution treatment at 1100℃ for 4h, the changing tendency of elongation is contrary. The tensile strength, yield strength and elongation are 526.88MPa,219.36 MPa and 29%, respectively for the sample containing 5.7wt% Ti after the solution treatment at 1100℃ for 4h. The performance of high boron 304 stainless steel composite plate reached the level of 304B7 in the US standards.(2) The boron aluminum alloy contained 2.0wt% B was produced by stir casting. The billet was hot rolled at 450-500℃ followed by solution treatment at 450℃ for 2 and 4 hours respectively. It is found that the AIB2 plies with large aspect ratio were in-situ synthesised by the reaction of Al and B in the casting process, and the AIB2 particles distributed uniformly in the matrix. The results showed that, compared with the matrix alloy, the hardness and tensile strength of the in-situ AIB2 reinforced composite material are significantly improved. The tensile property of sample treated at 450℃ for 2h is lower than those of the 6351 alloy (T5 heat treatment) and 6351+Boron produced by the Eagle Picher, but the elongation of the sample is higher. The calculations show that, the B-10 areal density of the aluminum boron alloy is equivalent to one-fifth of 6351 aluminum alloy prepared with enriched boron. However, the price of the aluminum boron alloy in this research has certain advantages.(3) The AlB2 particle reinforced aluminum matrix composites were fabricated by using of powder metallurgy (mechanical alloying + spark plasma sintering), hot rolling method and solution treatment. The raw materials are aerosolized aluminum powder and crystalline boron powder (the content of boron powder is 7,12wt% respectively). It can be found that the number and distribution of reinforcement in Al-7wt% B composites are superior to Al-12wt% B composites under the same mechanical milling time (five minutes). The sintered density of the samples is improved and the boron powder reunion is reduced by hot-rolling. It is found that the tensile and yield strength of the samples increase generally by hot rolling and solution treatment. The 10B areal density of aluminum matrix composites were calculated theoretically comparing with BORALTM. The 10B areal density of the hot-rolled Al-7wt%B and Al-12wt%B composites can reach to the level of BORALTM with apparent thickness. |
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