In-vessel retention is a key severe accident management strategy now widely adopted by some nuclear power plants. The saturated pool boiling heat transfer coefficients and critical heat flux (CHF) were measured from downward facing structured surface with grooves in deionized water to enhance the CHF. The orientations were 5°

30°

45°

60° and 90° (vertical). The results showed that the nucleate boiling heat transfer coefficients and the local CHF for structured surface with grooves were consistently higher than those for plain surface. Compared with plain surface

the CHF increase could reach 65%-90% for structured surface. The vapor blanket and wavy vapor layer bubble structures with different CHF trigger mechanisms on the downward facing surface at high heat fluxes were observed on structured surface. The enhancement of the local CHF and the nucleate boiling heat transfer coefficients was mainly due to the significantly increase of heat transfer area and the grooves that effectively improve surface wettability.