In this work concentrations of minor graphite impurities in virgin graphite of the Ignalina Nuclear Power Plant (Ignalina NPP) measured by high resolution inductively coupled plasma mass spectrometry (ICP-MS) are presented and applicability of the method for the evaluation of minor graphite impurities is discussed. It is shown that ICP-MS is a promising technique for determination of graphite impurities in 10 ppm -10 ppt concentration range as a rapid and relatively cheap method. The most important nuclides for evaluation of short-and long-lived activation products in the Ignalina NPP graphite were measured with the accuracy sufficient for the radioactive waste classification. These results could be used for planning of dismantling and disposal of the Ignalina NPP graphite.
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 14 C, 60 Co, 55 Fe, 238 Pu, 241 Am and 244 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.
Operation of the Ignalina Nuclear Power Plant (INPP) resulted in about 7500 tons of solid radioactive waste kept in 45 sections of four storage facilities (No 155, 155/1, 157, 157/1) at the INPP site. For safe and efficient decommissioning of these storages it is important to know the waste inventory. The nuclide vector (NV) method was used for characterization of the radionuclide inventory in the storages of solid radioactive waste (SRW). A well planned sampling and radionuclide composition measurements were needed to evaluate scaling factors of the NV.The optimal number of the samples N , to be taken from every storage section of SRW, was evaluated by using a wellknown relationship between error of the mean ∆ and dispersion σ: ∆ = σ/ √ N . The specific activity ratio (scaling factor) of 137 Cs to 60 Co radionuclides in the radioactive waste was selected as the most informative ratio, reflecting dispersion of the nuclide vector. Computer modelling of the radioactive waste streams and Monte Carlo simulations of the most representative 137 Cs/ 60 Co activity ratio dispersion in each waste storage section were done based on the historical data about the SRW management at the INPP and spectrometric measurements of the radionuclide activity of SRW at the main waste collection points. The evaluated optimal average number of required samples from the SRW storage sections was 7, whereas the number of required samples varied from 4 (storage facility No 157/1, section 1) to 19 (storage facility No 157/1, sections No 9 and 19/2).
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