Why did Japan inject water into the nuclear power plant after the Fukushima nuclear power plant leak

Due to tsunamis and earthquakes, the cooling system at the Fukushima nuclear power plant failed and the nuclear reactor could not be cooled. The explosion of the Fukushima nuclear power plant was also due to excessive temperature and the explosion of some flammable gases. like hydrogen. After the cooling system fails, because the core of the nuclear reaction column is not removed, the nuclear reaction continues, resulting in the nuclear reactor temperature being too high. The nuclear material will melt due to the high temperature and form gas. smoke that will float in the atmosphere and cause nuclear pollution. The purpose of water injection is to cool the reactor to prevent nuclear materials from melting.