Fundamental two-phase flow behavior under natural circulation conditio
ns is studied both experimentally and analytically. Natural circulatio
n characteristics for both drum-type and chimney-type reactors are exp
erimentally investigated focusing on the flow patterns, flow rate and
two-phase behavior inside the vessel. Analytical studies are performed
to further understand the natural circulation characteristics of an a
ctual plant and the mechanisms of geysering-like instabilities. Experi
mental and analytical results confirm that sufficient and stable flow
is achievable in a natural circulation BWR.