Method based on isotope ratio mass spectrometry for evaluation of carbon activation in the reactor graphite

The general idea of this work is to introduce an evaluation method to restore the irradiation parameters of graphite or other carbonaceous materials using experimental and modelling results of 13C generation in the irradiated material. The method is based on coupling of stable isotope ratio mass spectrometry and computer modelling of the reactor core to evaluate the realistic characteristics of the reactor core such as the neutron fluence in any position of the reactor graphite stack or other graphite constructions. The generation of carbon isotopes 13C and 14C in the irradiated graphite of the RBMK-1500 reactor has been estimated by modelling of the reactor core with computer codes MCNPX and CINDER90. Good agreement of simulated and measured Δ13C/12C values in graphite of the central part of the reactor core indicates that the neutron flux (1.40 x 1014 n/cm2 s) is modelled accurately in the graphite sleeve of the fuel channel. The simulated activity of 14C is compared with the one measured by the β spectrometry technique. Results indicate that production of 14C from 14N in the RBMK-1500 reactor is considerable and has to be taken into account in order to make proper evaluation of 14C activity. Measured 14C specific activity values correspond to 15 ± 4 ppm impurity of 14N in graphite samples from the RBMK-1500 reactor core.