The development of the fast reactor (FR) cycle is being preceded in Japan to utilize the plutonium and trans-uranium (TRU) efficiently. As the fuel generates heat due to decay of the TRU, it is required to develop an efficient system for cooling of the wire wrapped fuel pin bundle during the fabrication process where the bundle is being laid in horizontal state. To meet the requirement the four year study has started in 2006 sponsored by the Japanese government. In the first year the fabrications of the full mock-up and the enlarged- partial model were completed and the steady state heat removal tests have been performed. The temperature distribution data in the fuel bundle have been obtained and analyses have been also performed to compare with those measured data. The steady state experiments have revealed that the air cooling has enough function to maintain the fuel soundness keeping cladding temperature below the design criterion of 200 °C. The experiments will be continued to clarify transient thermal behaviors along with the development of an analysis code, and the designing work to get an efficient system for cooling fuel bundles will be also continued.