Estimation of the inventory of 14C and other key radionuclides in irradiated RBMK-1500 graphite based on limited measurements and full 3D core modeling

Ignalina NPP contains two Units with RBMK-1500 reactors. After shutdown, several Unit 1 systems and equipment were radiologically characterized and dismantled. The highest volume of reactor structures is attributed to the graphite stack of the reactor core, radiological characterization of which has not yet been performed. The stack can be visualized as a vertical cylinder 8 m high and 14 m diameter, made up of 2488 columns where each column is made up from several graphite blocks. The total mass of the graphite stack blocks is about 1700 tonnes. Therefore, the main goal of work reported in this paper was to estimate the inventory of C-14 and other key radionuclides in the irradiated graphite by a combination of activity measurements and full 3D reactor graphite stack neutron activation modeling. Obtained results show that, based on the combination of modeling and measurement techniques, the total inventory of C-14 in graphite stack is estimated at 3.22x10(14) Bq at 9 years after Unit 1 reactor final shutdown. C-14 activity is the highest among the analyzed radionuclides; the second highest is Co-60 (similar to 6 times lower).