The criticality accident that occurred on September 30, 1999 at a uranium processing plant in Tokai-mura was an unprecedented nuclear accident in Japan, not only because it caused deaths of two workers due to radiation casualty but also because it called for evacuation and sheltering indoors to nearby residents. The accident was not directly caused by failures or malfunctions of hardware but by workers' unsafe action deviated from the approved procedure. It was a typical organizational accident in that several organizational factors worked behind. This article is to analyze various causal factors that lead to the accident, including situational factors of workers' unsafe action that triggered the accident, operational and business management of the company, and nuclear safety regulation by the government. It also discusses problems of emergency response after the accident.

The debris coolability analysis module in the severe accident analysis code 'SAMPSON' has been enhanced to predict more mechanistically the safety margin of present reactor pressure vessels in a severe accident. The module calculates debris three-dimensional natural convection with simultaneous spreading, melting and solidification using the 'debris spreading-cooling model' in combination with the temperature distribution of the vessel wall and it evaluates the wall failure. Debris spreading is solved by the free surface calculation method in which the height function is applied. The model makes possible a multiplex heat and mass transfer analysis with flow spearhead and melt front transportation for a single-phase flow analysis code through the resetting of two types of mesh attributions and re-arrangement of the pressure matrix at each time step. The results calculated with the present model are compared with the results from a water spreading experiment. The comparisons verify the model capability for predictions of debris flow in the spreading process. The module provides a good tool for prediction of the reactor safety margin in a severe accident through the three-dimensional natural convection analysis of debris with simultaneous spreading, melting and solidification.

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