Evaluation of the Radiological Consequences of C Due to Contaminated Ignalina Npp Graphite Incineration

After decommissioning of Unit 1 of the Ignalina Nuclear Power Plant, the problem of the radioactive waste management emerged. Among radioactive waste there is an inventory of about 1700 tons of the graphite containing C radioisotope as an activation product. The estimates show that the maximal total inventory of C in graphite from Unit 1 is around 7 · 10 Bq. One of the possible ways for utilization of the graphite is its incineration in the radioactive waste processing plant. Unfortunately, in this case a significant amount of the radionuclide would be released into the atmosphere in the form of CO2 and the released radiocarbon would cause additional exposure of the population. Possible radiological consequences for the Lithuanian inhabitants are evaluated using the model of radiocarbon dispersion in the environment and considering several scenarios of the graphite incineration. Dispersion of the incineration gas is modelled using the Gaussian dispersion model. Assimilation of CO2 by the vegetation due to photosynthesis as well as washout of CO2 from the atmosphere by rain, uptake of the deposited C by the plants from soil, and the eventual contamination of food products are considered. An estimated additional exposure effective dose to the critical group of the local population due to continuous releases of the total inventory of C from the incinerator is of the order of 2.7 mSv. The consumption of the contaminated locally produced food products is the main contribution to the dose. Such continuous incineration of graphite would be acceptable if it were extended for at least 14 years in order not to exceed the annual dose limit of 0.2 mSv·y. The incineration of graphite would cause the least radiological consequences if it functioned only in the dark time of the day or in winter when plants do not perform photosynthesis. In this case the effective dose for the population would be of the order of 5.2 μSv. World population would receive an average lifetime (∼50 years) dose of 0.43 μSv per person which is negligibly small.