Abstract
In Japan, after the Fukushima Daiichi Nuclear Power Plant accident, many nuclear power plants should be considered operation period and life spans, including decommissioning. To realize safe and appropriate decommissioning of many nuclear plants, suitable management of radioactive materials produced in the reactors is one of the most important issues.
Classification of radioactive materials into clearance level and higher levels is also quite important as for the total cost of decommissioning.
Also, the accurate and cost-effective measurement for radioactive materials classification is highly desired. The standard source method has been applied to radioactivity measurements for the efficiency calibration of the measurement object. However, the objects to be measured in decommissioning are generally large and heavy. Therefore, this method requires shredding and molding. But these preparations are limited or cost prohibitive. Another method uses numerical calculations. For example, MCNPTM is a general purpose Monte Carlo code utilized worldwide for these efficiency calculations. However, this code requires experience and knowledge of a trained user. Also, quite long calculation time should be expected. The In Situ Object Counting System (ISOCSTM) technology was developed to solve this problem. This technology is already widely used around the world.
Prior to the Fukushima Daiichi NPP accident, ISOCS had only been applied to a few cases in Japan, such as the measurement of primary loop recirculation (PLR) piping in nuclear power plants. However, since the Fukushima NPP accident, the demand for on-site and off-site radioactivity measurement has rapidly increased, and the number of situations that make it difficult to create the efficiency calibration using a standard source has increased due to the high throughput and the large size of measurement targets. This is especially true for the measurement of removed soil and incinerated ash, off-site of Fukushima Diichi NPP. In addition, MCNP calculations are typically not adequately fast for real-time modeling. On the other hand, the time required for efficiency calibration using ISOCS is only a few seconds. At the Fukushima NPP off-site, ISOCS was also applied in the TRUCKSCAN and BULKSCAN systems. At the sites of other decommissioning projects, ISOCS has been applied to measurements of turbines, contaminated pipes, condensers, concrete wall of turbine buildings and more. In this report, we show the results of these various projects and the ISOCS benefits for cost, accuracy, and uncertainty.
