RADIOLOGICAL CHARACTERIZATION AND CLEARANCE OF RADIOACTIVE WASTE AT NCSR "DEMOKRITOS"

At the present paper, projects about the radiological characterization of waste and the Greek research reactor components that were carried out at the National Centre for Scientific Research “Demokritos” (NSCR “Demokritos”) are presented. Remarkable examples are the radiological characterization of the reactor ion exchange resin waste [1, 2] as well as the characterization of the reactor primary cooling system [3, 4]. Furthermore, the technique for verification of compliance with the clearance limits which was developed for pipes [5] and also the procedure for verification of clearance of an old reactor delay tank segments [6] are mentioned. The estimates of total radioactivity present in materials are derived from non-destructive radiological measurements and in some cases through computational models as well as by sampling and laboratory analyses. 1. Resin waste project The radiological characterization of the ion exchange resin waste of the Greek Research Reactor (GRR-1) aimed at selecting the most cost effective management program with the view of removal from the regulatory control at the proper time: (i) after verification of the general clearance [7, 8] or (ii) in order to be disposed at a landfill. Additionally, it should be stressed, that the full data of resin characterization makes easier the characterization of the research reactor, since these data give the information about the radionuclides of concern inside the primary circuit of the reactor. The ion exchange resin was used for purification of the reactor coolant water at the demineralization co-flow regenerated units. After the end of its useful life, the resin material of total mass 300 kg was stored for one year for radioactivity decay at the reactor premises and then was put in 3 steel drums coated with plastic. Each drum was labelled as regards the year of receipt by the radioactive waste management team. The waste is kept in primary form, in drums, at the radioactive waste interim storage facility of the NCSR “Demokritos. The storage period of resin, until 2011 (the year of decay correction), ranged between 9 to 48 years, thus the short lived radionuclides have been decayed. The dose rate on the surface of the drums ranged between the level of natural background and 4 μSv h -1 . During the life time of resin 15 regeneration procedures were performed. After each regeneration process, about 10 m 3 of slightly acid aqueous liquid waste was produced and collected in a retention tank for decay and monitoring. Then, one sample of 0.5 l was collected and put into a 30 l plastic vessel that was used for accumulation of the composite sample for the specific year. Firstly, the radiological characterization was carried out by collecting samples of primary waste from each of the resin drum and performing analysis by gamma spectrometry. The analysis results indicated that the waste belongs to the classes VLLW and LLW [9]. The activity concentrations are: Ag-108m < 4.70 Bq/g, Cs-137 < 160 Bq/g, Eu-152 < 2 Bq/g, Co-60 < 6 Bq/g. Furthermore, the data of gross beta and gamma spectrometry analysis of liquid waste that was being produced during the