In-Vessel Retention (IVR) strategy with water injection on the molten pool can ensure the nuclear safety and reduce the possibility of vessel failure. The wATer injectiOn on molten Zirconium-Stainless steel Metallic pool experiment (ATOM) has been performed to investigate the effect of water injection on the heat transfer of the molten pool. The simulant of the molten pool contains the low mass fraction of Zirconium (13%). Two water injections are carried out with the same mass flow. When water is injected on the melt surface, the heat flux transferred to water of the first and the second water injection are about 1.3 MW/m2 and 1.7 MW/m2, respectively. The heat flux in ATOM test is larger than that of the calculated result of Berenson film boiling correlation by 5.3 times for the first water injection and 8.0 times for the second water injection. When water is injected, the melt temperature could be cooled effectively and the heat flux to the sidewall is reduced too. The results indicated that water injection on the surface of the light metallic layer could effectively cool the molten pool and reduce the thermal focusing on the RPV. Neither vapor explosion nor melt ejection is found for the low mass fraction of the Zirconium test.