Accounting for steel stock in Japan

During the last two decades, the total material input in Japan has been about 2 billion tons per year, 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 output. However, the detailed contents of the accumulated stock are unknown. It is said that these unknown contents of stocks include material that has already been discarded as invisible waste. In this study, the following terms are defined: in-use stocks, obsolete stocks, and overall stocks. The materials that are currently used in society are known as in-use stocks. Obsolete stocks comprise the steels used for the constructional material in a landfill site, the steels dissipated by corrosion and erosion, etc., which are not associated with social activities and cannot possibly be collected as scrap in the future. Overall stocks are the total of these two types of stock. In this study, a dynamic material flow analysis was conducted to quantify the amount of in-use stocks and obsolete stocks. Furthermore, we defined a system boundary to account for the steel stocks and constructed equations to calculate the three types of stock. The amounts of in-use stock and obsolete stock in Japan from 1980 to 2000 were calculated. The result shows that 0.94 billion tons of the in-use stock is included in the 1.22 billion tons of overall stock in the year 2000.