ESTIMATION OF THE GENERATION OF ¹³C AND ¹⁴C IN THE REACTOR GRAPHITE USING MCNP6 MODELLING, ISOTOPE RATIO MASS SPECTROMETRY AND ¹⁴C MEASUREMENT TECHNIQUE

Center for Physical Sciences and Technology, Savanorių pr. 231, LT-02300 Vilnius, Lithuania

rita.plukiene@ftmc.lt

Published online: 22 February 2021

Abstract

Characterization of irradiated graphite in terms of ¹⁴C activity is crucial for the optimization of treatment technology: geological disposal, landfill storage, recycling, etc. The main contributor to ¹⁴C generation in the RBMK reactor graphite is ¹⁴N(n, p)¹⁴C reaction. The generation of carbon isotopes ¹³C and ¹⁴C in the virgin RBMK graphite samples irradiated at the LVR-15 research reactor (Research Centre Řež, Ltd.) were investigated in order to obtain the impurity concentration level of ¹⁴N. Afterwards the modeling of graphite activation in the RBMK-1500 reactor was performed by computer code MCNP6 using obtained ¹⁴N impurity concentrations and new nuclear data libraries. The irradiation parameters – neutron fluence have been checked by method based on coupling of stable isotope ratio mass spectrometry and computer modelling. The activity of ¹⁴C in the different constructions of irradiated graphite of the RBMK-1500 reactor has been measured by the β spectrometry technique (LSC) and has been compared with the simulated one. Obtained results have indicated the importance of ¹⁴C production from ¹⁴N in the RBMK-1500 reactor and in the LVR-15 neutron spectrum. Measured ¹⁴C specific activity values in the samples varied from 130-700 kBq/g in the RBMK-1500 irradiated samples and from 3-12.5 Bq/g in the LVR-15 irradiated graphite samples. This corresponds to 15±4 - 80±10 ppm impurity of ¹⁴N in various graphite samples of RBMK reactor.