The development of a clearance level radioactive waste measurement system using an array of NaI(Tl) detectors

In order to meet the needs of rapid classification and measurement of the radioactive waste, a clearance level radioactive waste measurement system has been successfully developed and applied, achieving the goal of waste minimization. The measurement system consisted of 34 NaI(Tl) detectors taking into account the detector layout, detection efficiency and shielding chamber (to reduce the background of the environment). The NaI(Tl) detector array was built by a geometric condition of spatially symmetric to measure gamma rays. The shielding chamber composed of 2 mm copper and 10 mm lead is used as an passive shielding, and the radioactive waste is placed inside the shielding chamber, which suppress the gamma rays from the decay of 40K, 208Tl and other nuclides in the environment. A method to determine the counting of energy interval for the gamma rays is proposed to obtain the radioactivity of the radioactive waste package. The performance testing results of the measurement system show that the energy resolution and efficiency for 661.7 keV (137Cs) are 7.84% and 69.49% respectively, and the minimum detectable activity of 137Cs is 24.52 Bq. All these indicate that the indices of the clearance level radioactive waste measurement system have reached the international recommended level.