[en] A test series on FP/structural material interaction has been performed in WIND project at Japan Atomic Energy Research Institute to investigate the interaction between CsI and type 316 stainless steel, and the CsI instability. Three tests on the interaction of cesium iodide with type 316 stainless steel were performed under steam/noble gas, hydrogen/steam/noble gas and air/steam/noble gas atmospheric conditions. In the test with air ingress condition, EDX (Energy Dispersive X-ray spectroscopy) measurements showed that the retained mass of cesium was larger than that of iodine in the structural surface at a high temperature. Retained cesium might have taken chemical forms that were different from cesium iodide. However, the cesium compounds were not confirmed by XPS (X-ray photoelectron spectroscopy) analysis. In all the tests, it was confirmed by XPS analysis that the molecular iodine migrated to a depth of 100 nm maximum of the surface layer of stainless steel. It is considered that the amount of cesium and iodine retained in the type 316 stainless steel surface was too small to influence the source term evaluation. It was also found that a decomposition rate of cesium iodide in the air ingress condition was higher than that in the hydrogen injection condition. (author)