Alpha Radioactivity Monitor Using Ionized Air Transportation for Large Size Uranium Waste: Part 1—Large Measurement Chamber and Evaluation of Detection Performance

We present an ionized air transportation type alpha radioactivity monitor to efficiently perform the clearance level inspection for large size uranium waste and its detection performance. In previous work, we developed a prototype monitor with an about 1000 mm cubic measurement chamber to measure the cut waste. However, in a survey of target waste, we found that it is desired to measure not only the cut waste but also the lengthy waste such as uncut cylinders. Therefore, we developed an alpha radioactivity monitor with a long and large measurement chamber (effective sizes: 500 mm x900 mm x3200 mm) for long and large cylindrically-shaped waste (maximum size: 300 mm in diameter and 3000 mm in length, weight: 10 to 200 kg). We aimed <1000 Bq as the target value of Alpha radioactivity Detection Limit (ADL), which is one-tenth of the clearance level (1 Bq/g) for 10 kg waste. The issue to size up the measurement chamber was to suppress the reduction in sensitivity of alpha radioactivity. To overcome this, we enhanced an air fan power and optimized an ion sensor design. Using this monitor, we measured and evaluated ADLs for several cases supposing the practical applications (long cylinders with a smooth surface, bump, or concavity and convexity, and pipes with several small diameters). The resulting ALD ranged from 60 Bq to 120 Bq and sufficiently satisfied the target ALD (<1000 Bq). In conclusion, this monitor has sufficient performance for the clearance level inspection for large uranium waste.