Research On The On-site Monitoring Technique For Carborne Radioactive Waste Container

Identifying isotope species and activities of radioactive waste is very important for the designment of waste disposal project. In order to measure rapidly carborne waste container whose volume is very large and in which radioactive isotopes are non-uniformly distributed, an on-site monitoring technique was established.The main researches and results are listed below:(1) On-site monitoring model of carborne radioactive waste container was established. According to the general characteristics of large-scale radioactive decommissioned facility and measurement requirements, the tomographic gamma scanning (TGS) and waste box scanning technology were used for reference simutaneously, the extending distance and continuum scanning model for carborne waste container was established. Then the influence of non-uniform distribution was discussed, and the measurement parameters were selected.(2) Calibration method for on-site monitoring model was established. The sampling algorithm for the efficiency calibration which was combined with the Monte Carlo integral method and one numeric method based on the Beck formula was built, and which could be used to calculate the conversion coefficients of specific activtity in the case of complicated geometrical structure. Accuracy for the calculation method was tested by experiment with uniform volumetric source simulated by plane source and numerical calculation using Beck formula. The results indicated that the relative deviation between experiment values and theoretic values was in the range of±2%.(3) Experiments used to verify the measurement model were conducted. The model was verified respectively by linear, plane, volumetric sources, which were uniformly or non-uniformly distributed. The results indicated that:if isotopes were non-uniform in the scanning direction, the influence of non-unifrom distribution could be reduced effectively by increasing scanning horizontal distance, but if the isotopes were non-uniform in the axis direction of detector, increasing scanning horizontal distance couldn't reduce the influence. So scanning must be done in three direction.(4) Minimum detective specific activities (MDSA)for the interested isotopes were estimated, and the uncertainties of measurement results were assessed. Background count rates caused by natural nuclides were calculated with Monte Carlo method. Then MDSAs of 241Am, 137Cs,60Co and 152Eu were calculated when the measurement time was 1000s. The results were 170 Bq/kg,l Bq/kg,1 Bq/kg and 5Bq/kg respectively. According to uncertainties of the parameters, then the combined uncertainties were obtained with Monte Carlo synthesization method. The relative uncertainty was less than 40%(1σ) for 241Am,34%(1σ) for 137Cs,60Co and 152Eu.