In order to conform to the new regulatory standard in Japan, seawater is regarded as the alternative water source both for BWRs (Boiling water reactors) and PWRs (Pressurized water reactors). For preventing further accident evolutions occurred in Fukushima Daiichi nuclear power plants, seawater was injected into the reactors for more than one week. With long-term seawater injection, sea salt compositions are condensed and many of them will precipitate when saturated concentrations are exceeded. Unlike corrosion issues, impacts of sea salt precipitation on the heat removal has not been studied widely in the past because it has not been regarded as the alternative water source before Fukushima Daiichi nuclear power plants accident. The existent knowledge base of boric acid precipitation under LOCA conditions was studied. Based on the existent study on impacts of boric acid precipitation under LOCA conditions, the experimental project of seawater consisting of four experiments was proposed. The existent database of physical properties, such as viscosity, of seawater is rather poor under severe accident conditions. In addition, it is likely that boric acid will be injected with water or seawater to prevent re-criticality. It is known that physical properties of boric acid vary widely under high temperature conditions. Measurement of viscosity of the seawater-boric acid mixture was conducted in high temperature using a rotational viscometer. Under conditions equivalent to the estimated bulk coolant conditions under a long-term cooling phase of severe accidents. In this range, no obvious change of viscosity is expected. Then a detailed structure of seawater precipitates was observed using a mockup fuel bundle with 5x5 in the square lattice and 500 mm in the length. Images of precipitates were taken using the X-ray CT. The water level, the concentration of sea salt and the heat flux are employed as experimental parameters. The heat flux, bubble stirring in downstream of spacers and heat loss by a non-heated channel box were identified as influential factors to local and overall precipitates in fuel bundles.