Modelling of Impurity Activation in the RBMK Reactor Graphite Using MCNPX

Numerical modelling is an important tool for estimation of the radioactive waste generation at the nuclear energy sites. In this work we have applied a Monte Carlo code, MCNPX version 2.6 for calculation of activation of the graphite stack in the RBMK-1500 reactor. The simplified 3D model of the RBMK-1500 reactor core fragment with 14 fuel assemblies and 2 control rods distributed according to the real RBMK-1500 reactor core geometry has been created. Use of the Monte Carlo method for estimation of impurity activation in the reactor core faces a problem of large uncertainties of effective cross-sections if a calculation set is insufficient. Satisfactory results can be obtained by using parallel computing algorithms. The ICP-MS mass spectrometry for identification of the impurity concentration in virgin graphite from the RBMK-1500 reactor has been performed to support MCNPX modelling of the realistic neutron irradiation conditions of the graphite. Simulated radiological characteristics of the graphite have been compared with the previous calculations made with different impurity concentrations obtained by neutron activation analysis and GDMS. The analysis of radioactive impurity content shows that C, Co, Fe, Pu, Am and Cm make the major contribution to graphite activity and radiotoxicity during hundreds of years. The obtained results are important for decommissioning of the Ignalina NPP and other NPP with RBMK type reactors.