The development of fast neutron spectrum boiling water reactors requires understanding of flow behavior in a tight lattice bundle. We conducted experiments to visualize single-channel flow under atmospheric conditions and to measure the critical heat flux (CHF) location and liquid film behavior. The observed flow pattern in the tight lattice was in agreement with the Hewitt-Roberts flow pattern map. CHF occurred at the wider rod gap location. Furthermore, the liquid film flow rate at this location was less than that at the narrow rod gap location. The CHF mechanism in a tight lattice would be the film dryout occurring by decreasing the liquid film flow rate.
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