Accounting for steel stock in Japan

During the last two decades, the total material input in Japan has been about 2 billion tons, and approximately 50%, i.e., about 1 billion tons has accumulated as the net addition to stock in the form of buildings, social infrastructure, and various kinds of products. The amount of the net addition to stock is calculated annually from the differential between the input and the output. Therefore, the contents of the accumulated stock are unknown. It is said that these unknown contents include material that has already been discarded as invisible waste. In this study, dynamic material flow analysis was conducted to quantify the amount of stock that is not associated with social activities and cannot possibly be collected as scrap in the future. First, we defined some terms: in-use stock, hibernating stock, and overall stock. Hibernating stock comprises the steel used for the constructional material of landfill site, the steel dissipated by corrosion and erosion, etc. Furthermore, we defined the system boundary to account,for the steel stock and constructed equations to calculate the three types of stock. To calculate the amount of in-use stock, a dynamic model-the population balance model-was adopted. The amounts of in-use stock and hibernating stock in Japan from 1980 to 2000 were calculated. The result shows that 0.96 billion tons of the in-use stock is included in the 1.22 billion tons of overall stock in the year 2000.