Design study on core and fuel properties of sodium cooled fast reactor (Mixed oxide and metal fuel core). Results in JFY2004

Sodium cooled fast reactor (mixed oxide and metal fuel core) is one of promising candidates in Feasibility Study on Commercialized Fast Reactor Cycle System in Japan (FS). Its design study has been conducted through FS. In JFY2004's mixed oxide fuel core design study, a core with improved FAIDUS (Fuel Assembly with Inner Duct Structure) type fuel assemblies was examined. The improved FAIDUS type fuel assembly is newly designed, and the inner duct is installed at corner of wrapper tube. That is expected to have superior performance for molten fuel release at CDA to usual ABLE type and have less issue for fabrication. The structural design including thermal hydraulic characteristics evaluation for the improved FAIDUS type fuel assembly and design studies for the core with improved FAIDUS type fuel assembly were carried out. In the core design study, two types of core concepts were examined as well as JFY2003's study. One is 'Compact type' that follows a conventional type core and the other is 'High internal conversion type (HIC type)' that aims to reduce fuel cycle cost by increasing total discharge average burn-up (including blankets). As a result, the HIC type core is revealed to attain higher total discharge average burn-up and longer operation cycle length compared with the 'Compact type' core, which decreases the fuel cycle cost and increases the availability factor. The HIC type core was selected from this as representatives of large scale (1500 MWe) and medium scale (750 MWe) reactors of FS phase II. The HIC type large scale core is able to attain the total discharge average burn-up of 90-115 GWd/t and the operation cycle length of about 26 months. The HIC type medium scale core is able to attain the total discharge average burn-up of 86-104 GWd/t and the operation cycle length of 26-27 months. These results indicate that HIC type cores (large and medium scale cores) have capability to achieve sufficiently main design requirements of FS such as the total discharge average burn-up (60 GWd/t) and the operation cycle length (18 months). In the metal fuel core design study, a core with 'High core outlet temperature type' was investigated for improving economics. 'High core outlet temperature' core is aimed at achieving the identical core outlet and inlet temperatures to those of sodium cooled mixed oxide fuel cores (550deg C/395deg C). In the new measure for flatting the radial power distribution, Pu-enrichment and fuel pin-diameter are unified and the levels of Zr-contents and fuel smeared densities of metal fuel are changed in the radial direction. For avoiding the difficulty in the fuel fabrication compared with conventional core, the Zr-content was set less than 10 wt%. This new measure produced cores with smaller core size compared with the mixed oxide fuel 'Compact type' cores. The core flow distribution design was performed by thermal hydraulic analysis under the condition of outlet/inlet temperatures of 550deg C/395deg C with adoption of the wire-type spacer because of its less development issues. The results of maximum cladding temperature were 650deg C for large-scale core and 652deg C for middle-scale core, which nearly satisfied the design limit of 650deg C. Rationalization of hot spot factor has a potential for further increase of the core outlet temperature. (author)