In order to investigate the process of nuclear power plant accidents, an accident scenario of Fukushima Daiichi Nuclear Power Plant, Unit 1 is analyzed using data available to the public. A phase equilibrium process model and adiabatic expansion model were introduced. Original data reported in the first stage of the accident were examined to clarify the behavior of the isolation condensers (ICs), which are generally believed to have become nonfunctional after the tsunami and station blackout. The original data and observation reports verified that the so called "fail safe" system to close the valves in the IC did not work properly owing to the shutdown of AC power. The reports also showed evidence that the operators injected water into the IC reservoir tank. In the author's previous report, the initial leak from the reactor pressure vessel (RPV) was assumed just after the earthquake. This report assumes that the leakage of the RPV occurred at 3/11 20:26 owing to small dry out of nuclear fuel clusters. In the present accident scenario, the leakage of the primary containment vessel (PCV) occurred at 3/12 3:30. A large break in the RPV occurred at 6:20 and again at 16:00. It is estimated that most of the fuel remains in the RPV, whereas, Tokyo Electric Power Company, Inc. (TEPCO) estimates that most of the fuel melted out through the RPV. The present analysis model and the accident scenario describe the data measured at the accident, many evidences and witnesses reported at the early stage of the accident.
