Abstract: Based on the innovative water-cooled pebble-bed nuclear reactor,an experimental investigation was performed to study the characteristics of subcooled flow boiling incipience in packed bed with internal heat generation.Oxidized carbon steel balls with 8 mm diameter were used to form a random packed bed,which was volumetrically heated by induction.Effects of interstice mass flux G,inlet temperature T_inand vertical position on the heat flux of ONB was investigated.Results indicate that there is a turning point on the relation curve of wall temperature T_w and heat flux q,which is the subcooled flow boiling incipience in volumetrically heated packed bed.The heat flux of ONB increases with the increasing of mass flux and decreases with the increasing of inlet temperature,respectively.The heat flux of ONB is smaller as the measurement position is closer to the exit of packed bed.A dimensionless relationship was developed to predict the q_ONB in packed bed with internal heat generation,and the deviation between predicted and experimental data is in the range of ±20%.