傾斜機能材料の概念を用いたラジオアイソトープ熱源の構造最適化に関する研究

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An energy conversion system with radioisotope (RI) heat source is promising for a maintenance-free generator for long-term uses including deep sea missions. Strontium-90, a β-decay radioisotope, is one of the most prospective heat source materials. The β-ray itself can be readily captured within the vicinity of the decaying strontium, while bremsstrahlung, generated by an interaction between the β-ray and the surrounding atomic nuclei, needs to be shielded for the protection of human bodies and electronic devices. Since the converted energy from β-ray to bremsstrahlung depends on the atomic number of the nucleus of interaction, an addition of light-element materials is expected to reduce the energy. In this study, conceptional design is made for minimizing the dose rate of strontium titanate (SrTiO_3) heat source using the concept of Functionally Graded Materials (FGM). Boron nitride (BN) is selected as the addition because it is a light ceramic and has high heat conductivity which may enhance the surface temperature of the heat source. A cylindrical heat source of SrTiO_3 with BN addition is adopted as a simulation model. A two-layer structure of SrTiO_3 and BN for inner and outer layers, respectively, is employed as the base structure. In order to design the FGM of the minimum radiation dose with a multilayer structure, the behavior of β-ray and bremsstrahlung is estimated by a continuous-energy Monte Carlo code MCNP 4A. The optimized five-layer structure exhibits the dose rate of 78% to the base (two-layer) structure and approximately one-third of the uniformly mixed one. Surface temperature is estimated by a one-dimensional heat conduction equation. The results of the estimation reveal that the surface temperature of the five-layer structure is approximately 350℃ higher than that of the base structure. Sintering studies of SrTiO_3-BN composite ceramics are also made for investigating the possibility of manufacturing the materials.
1997-07-09

傾斜機能材料の概念を用いたラジオアイソトープ熱源の構造最適化に関する研究

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