PRINCIPLES OF OPTIMAL SAMPLING FOR CHARACTERIZATION OF SOLID RADIOACTIVE WASTE OF THE IGNALINA NUCLEAR POWER PLANT

Operation of the Ignalina Nuclear Power Plant (INPP) resulted in about 7500 tons of solid radioactive waste kept in 45 sections of four storage facilities (No 155, 155/1, 157, 157/1) at the INPP site. For safe and efficient decommissioning of these storages it is important to know the waste inventory. The nuclide vector (NV) method was used for characterization of the radionuclide inventory in the storages of solid radioactive waste (SRW). A well planned sampling and radionuclide composition measurements were needed to evaluate scaling factors of the NV. The optimal number of the samples N, to be taken from every storage section of SRW, was evaluated by using a well-known relationship between error of the mean Delta and dispersion sigma: Delta = sigma/root N. The specific activity ratio (scaling factor) of (137)Cs to (60)Co radionuclides in the radioactive waste was selected as the most informative ratio, reflecting dispersion of the nuclide vector. Computer modelling of the radioactive waste streams and Monte Carlo simulations of the most representative (137)Cs/(60)Co activity ratio dispersion in each waste storage section were done based on the historical data about the SRW management at the INPP and spectrometric measurements of the radionuclide activity of SRW at the main waste collection points. The evaluated optimal average number of required samples from the SRW storage sections was 7, whereas the number of required samples varied from 4 (storage facility No 157/1, section 1) to 19 (storage facility No 157/1, sections No 9 and 19/2).