Assessment of different mechanisms of C-14 production in irradiated graphite of RBMK-1500 reactors

Abstract

Two RBMK-1500 water-cooled graphite-moderated channel-type power reactors at the Ignalina Nuclear Power Plant (INPP) are under decommissioning now. The total mass of irradiated graphite in the cores of both units is more than 3600 tons. The main source of uncertainty in the numerical assessment of graphite activity is the uncertainty of the initial impurities content in graphite. Nitrogen is one of the most important impurities, having a large neutron capture cross-section. This impurity may become the dominant source of C-14 production. RBMK reactors graphite stacks operate in the cooling mixture of helium-nitrogen gases and this may additionally increase the quantity of the nitrogen impurity. In this paper the results of the numerical modelling of graphite activation for the INPP Unit 1 reactor are presented. In order to evaluate the C-14 activity dependence on the nitrogen impurity content, several cases with different nitrogen content were modelled taking into account initial nitrogen impurity quantities in the graphite matrix and possible nitrogen quantities entrapped in the graphite pores from cooling gases.

Rapid analysis method for the determination of 14C specific activity in irradiated graphite

“…Many of the old reactors are shut down and their decommissioning is in progress. Irradiated graphite from the reactor core, moderators and reflectors represents the greatest volume of waste materials [ 2 , 3 ]. Graphite waste can be disposed of in the near surface or in a deep geological repository depending on its specific activity [ 4 ].…”

“…The main contributor to 14 C generation in the RBMK reactor is the 14 N(n, p) 14 C reaction. This occurs mostly on the graphite surface because of the sufficient isotopic abundance of 14 N due to presence of a helium-nitrogen mixture that is used to flush the graphite stack in the core, as well as the high neutron reaction cross section [ 2 , 3 , 5 ]. Assuming that a considerable part of 14 C is located in the outer graphite layer, removal of 14 C from the graphite surface would allow decreasing the total activity of graphite.…”

Estimation of the inventory of14C and other key radionuclides in irradiated RBMK-1500 graphite based on limited measurements and full 3D core modeling

“…The highest volume of reactor structures is attributed to the stack of the reactor core, which is constructed from graphite blocks; the radiological characterization of this is of great importance. Earlier studies (Ancius et al 2005; Smaizys et al 2005; Narkunas et al 2010) revealed that the radiological inventory in irradiated RBMK-1500 reactor graphite could be dominated by the radionuclide 14 C, however estimations were based on conservative assumptions and lacked experimental activity measurement data. The production of 14 C in nuclear reactors is typically dominated by neutron activation of 14 N and 17 O (Svetlik et al 2017), but for graphite moderated reactors, the influence of 13 C could also be significant (Narkunas et al 2010).…”

“…Earlier studies (Ancius et al 2005; Smaizys et al 2005; Narkunas et al 2010) revealed that the radiological inventory in irradiated RBMK-1500 reactor graphite could be dominated by the radionuclide 14 C, however estimations were based on conservative assumptions and lacked experimental activity measurement data. The production of 14 C in nuclear reactors is typically dominated by neutron activation of 14 N and 17 O (Svetlik et al 2017), but for graphite moderated reactors, the influence of 13 C could also be significant (Narkunas et al 2010). The importance of 14 C in the radioactive waste of Ignalina NPP was also presented in (Vaitkeviciene et al 2013).…”