A reference vessel (decay heat 504 W, water level 24 cm, hydrogen production 20.5 ℓ/d, absorbed dose rate at the bottom 755 Gy/h) was evaluated. Air is introduced by the water inlet and released through the vent plug. The maximum temperature and hydrogen concentration are 211 ℃ and 1.6%, respectively. The temperature of the residual water is below 47 ℃.