Transient behavior of natural circulation for boiling two-phase flow - experimental results

The safety of current light water reactors (LWRs) is too dependent on active engineered safety features to enhance their reliability greatly. Many concepts have been proposed for the next generation of LWRs in which passive safety functions are pursued. A natural-circulation boiling water reactor (BWR), such as the simplified BWR SBWR, is one such proposal. From the viewpoint of core stability, the power density of natural-circulation BWRs is lower than that of current BWRs and their fuel pin length is shorter, so that reactor diameter is larger. Moreover, the size of a reactor vessel is limited by the ability of the present machine, and its output power may be lower than 600 MW(electric). As Japan is a relatively small country, it is difficult to find reactor sites, and Japanese electric power companies are not inclined to introduce small or medium-sized reactors. If the merits of eliminating circulation pumps are truly understood, however, it seems that a series of power generators will be supported by industry and natural-circulation BWRs may thus be introduced in Japan. The purpose of this paper is to introduce a discussion on the merits and drawbacks of natural-circulation BWRs. The thermohydraulic behavior of developing processes of naturalmore » circulation in boiling two-phase flow has been investigated experimentally by simulating normal and abnormal start-up conditions.« less