Present status of research on boiling critical heat flux at low flow velocity

The flowing boiling critical heat flux at low flow velocity, for example in natural circulation boiling, is important in relation to the safety of the nuclear reactors of liquid cooling type under various conditions. The circumstance of removing the decay heat generated in a core by natural circulation has the larger possibility to occur than the condition of high flow velocity and high thermal load which has been studied in wide range so far, in addition, though the phenomena are gentle, sometimes range effect is caused. For example, the core cooling after a loss of coolant accident in a LWR is the low flow velocity condition being considered here. The published literatures on the critical heat flux under high pressure, low flow velocity condition are few. In this report, a number of the results of the research carried out at the Kyoto University Reactor Research Institute are summarized. The mechanism of critical heat flux and the basic equations, the critical heat flux in circular pipes, double walled pipes and rectangular pipes, the critical heat flux in the channel with a group of tubes, and the minimum value of critical heat flux are reported. (K.I.)