[en] In order to understand the transient boiling phenomena during PCM (power-cooling-mismatch) in light water reactors, transient burnout experiments were performed using a vertical annular test section under atmospheric pressure. The gap between the electrically heated stainless steel tube and the outer glass shroud was 2.0 mm and 1.4 mm, and the heated tube outer diameter and length were 10 mm and 800 mm respectively. Experiment ranges were as follows. Heat flux: 0.65 -- 1.36 x 10⁶ kcal/hm², inlet water temperature: 30 -- 76⁰C, initial mass velocity: 2.4 -- 6.0 x 10⁶ kg/hm², flow reduction time: 0.1 -- 68 sec, and flow reduction rate: 0.29 -- 752 cm/sec/sec. The experimental results showed that beyond a flow reduction rate of about 5 cm/sec/sec (1.4 mm gap) and about 1 cm/sec/sec (2.0 mm gap), burnout mass velocity became lower than the steady state one. When the flow reduction rates were further increased to 20 -- 40 cm/sec/sec or beyond, the burnout delay time became constant at about 0.4 sec. (author)