Actinides input to the dose in the irradiated graphite of RBMK-1500 reactor

The purpose of this work is to indicate the actinides input to the total radiation dose caused by the irradiated graphite of the RBMK-1500 reactor in comparison to the dose delivered by other nuclides. We used computer codes (SCALE 6.1 and MCNPX 2.7) to estimate the dose rate delivered by actinides giving special attention to the 244Cm isotope as a critical contributor to the total activity of actinides in the spent graphite for approximately up to 200 years. The concentrations of Pu, Am and Cm isotopes in the graphite sample from the Ignalina Nuclear Power Plant (NPP) Unit 1 have been measured with the inductively coupled plasma mass spectrometer and specific isotope ratios have been compared with alpha spectrometric results as well as with the values simulated by the computer codes. Good agreement of the experimental results and the simulated ratios serves as an additional confirmation of validity of the calculation models. The effective doses rates of inhalation and ingestion for personnel, gamma radiation power, and nuclides, which constitute the neutron source in the irradiated RBMK-1500 graphite constructions, have also been identified. The obtained results are important for decommissioning of the Ignalina NPP and other NPPs with graphite-moderated reactors.