[en] This paper describes experimental results of 200 hours running and high heat fluxes over 300 W/cm² durability tests for a 96% purity BeO ceramic cold type insulator. By the operation with an ETL Mark III and VI, the electrical and thermophysical characteristics of the wall were obtained, and failure of the ceramic elements was analyzed. The maximum heat fluxes were 180 W/cm² in Mark III and 120 W/cm² in Mark VI and heat fluxes distribution along the channel axis agreed approximately with a theoretical values, and a time elaps characteristics of the open circuit voltage was stabilized after about 4 hours from the starting of the power generation. This transient phenomena coresponded with the decay time of Si-Al₂O₃ system clearance filler. A surface deterioration of the ceramic elements near the cathodes was examined with a SEI figures and XMA analysis, and evidence of chemical reaction with potassium and some impurities diffused in it was obtained. By the use of high heat fluxes testing apparatus, the limitation of heat fluxes for the wall was determined. The experiments showed that good durability against the heat flux around 400 W/cm² was promised with the use of high heat fluxes structure wall and element thickness under 5 mm. (auth.)