Study on thermal electric conversion system for sodium cooled FBR. Investigation for development of thermoelectric materials and systematic technology

Recently, it has been important to reuse discharged heat energy from present nuclear plants in the view of reduction of environmental burden and improvement of heat efficiency for plant. For practical use in future of sodium cooled FBRs, which are typical high temperature system, this issue must be given priority. The thermal electric conversion system has been applied to the limited uses such as space or military, however, that results show good merits for reliability, maintenance free, and so on. Recently, this technology has been reconsidered in the view of saving energy in general industry. In this study, we made an investigation for applicability of the thermal electric conversion system to sodium cooled FBR as a heat recovery technology. Exactly, we have carried out the fundamental research and development for thermoelectric materials and elements, development of modules, and sodium tests with those modules, and then, we acquired the fundamental knowledge to estimate the efficiencies of thermal electric conversion system or modules for a sodium cooled FBR. The results were, (1) Study on using of discharged heat in the MONJU plant as an example: Condenser, decay heat from core or spent fuels, and decay heat removal in an accident were extracted as candidates to apply, nad decay heat system from spent fuels were selected as a subject because these have few restrictions relatively. (2) Study on maximum electric output power of the fundamental thermal electric conversion systems in case of using of discharged heat: For a condenser in a 700 MW class plant, mega watts of electric power can be generated by BiTe elements, however, the volume of the condenser will be 10 to 20 times of the present one. And, in case of an iron based elements setting on a 1-inch pipe flow in 500degC sodium, it is possible to generate the electric power of 31.6 kW maximum by the 16.9 m panel length. (3) Research and development of thermal electric conversion elements and modules composed by iron based materials: Focus on the iron based materials, which are plentiful and economical, we investigated systematically thermal electrical characteristics in those materials, and find out the FeVAl alloy which has -165 μV/K of zeebeck coefficient. (4) Characteristics conformation tests using sodium loop: Prototype modules of FeAl and FeVAl had been fabricated, and success to light a small lamp. The electric power generation was estimated to maximum 3.1W for the FeAl elements, maximum 7.5 W for the FeVAl elements by the modules of 1.8 m length, respectively. (author)