In order to investigate the pressure transient at the later phase of a large leak sodium-water reaction event in KALIMER(Korea Advanced LIquid MEtal Reactor), an experimental study has been carried out. The experiment was performed to simulate the long-term system transient response of a large leak sodium-water reaction event due to double ended guillotine failure of three tubes in a 1/10 scale-down mock-up test facility with sufficient considerations of the quasi-steady state features of the SWR event.