(e) Nuclear graphite waste management strategy during decommissioning of Ignalina NPP is the pending decision in Lithuania. In the RBMK type reactor graphite is a neutron moderator and reflector. The total mass of radioactive graphite from the both Ignalina NPP units is up to 3,800 t. 14 C is the limiting radionuclide for long-term disposal of irradiated graphite due to half-life of 5730 years and relatively high activity as well as mobility in geological media. Characterization of irradiated graphite in terms of both 14 C activity and chemical bonds in the lattice is crucial for the optimization of treatment technology (e.g. geological disposal, landfill storage, recycling, etc.). For this purpose numerical simulations and experimental analysis are performed. Elena Lagzdina, Danielius Lingis, Jevgenij Garankin, Rita Plukienė, Andrius Garbaras, Arūnas Gudelis, Laurynas Juodis, Mindaugas Gaspariūnas, Vitalij Kovalevskij, Ieva Matulaitienė, Gediminas Niaura, Artūras Plukis, Vidmantas Remeikis Center for Physical Sciences and Technology, Savanorių pr. 231, LT-02300 Vilnius, Lithuania [email protected] 1. Plukiene et al., 2014, Nuc. Eng. Des. 277, 95-105 2. Remeikis et al., 2009, Nucl. Eng. Des. 239, 813-818 3. Remeikis et al., 2018, PLOS ONE, In Press Conclusions References Determination of 14 C specific activity in irradiated graphite Experimentally validated numerical 3D model of RBMK-1500 is used for 14 C profile determination in different graphite constructions (stack, sleeve, top, bottom, side reflectors). 14 C activity measurements in graphite samples is carried out by using express method or liquid scintillation counting (LSC) technique. The evolution of graphitic sp 2 -related content as well as formation of an amorphous structure serves for understanding of location and stability of 14 C in graphite matrix, while the thermal treatment carries information about recrystallization process. Further structural investigations are currently in progress. 12 C + ion implantation and thermal treatment 14 C measurements are usually carried out by using liquid scintillation counting (LSC) technique after time consuming sample preparation procedure. Recently we proposed an express analysis method for the specific 14 C activity determination in small graphite samples in the range of 1- 100 μg [3]. This method is based on the graphite sample combustion in the commercial elemental analyzer and determination of 14 C specific activity by using the semiconductor detectors. This method is planned to apply for determination of the graphite homogeneity profile in terms of 14 C activity. Characterization of 14 C in neutron irradiated RBMK-1500 graphite For further graphite treatment technology optimization the structural investigations of graphite should be performed. 14 C mobility and position in graphite matrix is determined by neutron irradiation in the reactor at certain operation conditions. In order to understand the processes in the irradiated graphite we observe the propagation of defects induced by 12 C + ion implantation at energy of 700 keV at varying fluences. The structural changes after implantation and thermal treatment later on are investigated by Raman spectroscopy. The SRIM-2013 code is also used to estimate the damage profile in the surface of the graphite samples. RBMK-1500 graphite Fig. 4. 700 keV 12 C + ion implantation parameters according to SRIM: (a) the projected range of implanted ions; (b) the damage profile. Fig. 1. (a) Neutron flux distribution in the RBMK reactor graphite horizontal and (b) vertical cross sections. (c) Horizontal cross section full scale reactor core; (d) vertical cross section of reactor core with bottom, top reflectors, metal plates and cooling tubes system on the top. (e) Magnified view of RBMK-1500 core fragment (3x3) with fuel assemblies, inserted and extracted control rods. Fuel channel Control rod channel Graphite reflector Top and bottom reflector Stack Sleeve Channel Fuel Coolant B 4 C absorber Al tube Control rod graphite Experimentally validated numerical 3D model of RBMK-1500 (MCNP6 and SCALE 6.1.) is effective for description of the change of radiological characteristics of different parts of nuclear reactor during operation and decommissioning periods [1]. Both experimental measurements and modeling data are used for scaling factor determination [2], which subsequently could be used for sorting of spent graphite radioactive waste. 0 50 100 150 200 250 300 350 0 50 100 150 200 250 300 350 1 2 3 4 5 6 7 8 9 1 sample number linear aproximation m TCD (g) m weighing (g) 0 50 100 150 200 0 20 40 60 80 100 120 140 160 sample No. linear fit Counts by detector 14 C activity (Bq) (c) (d) (b) (a) (c) (b) (a) Fig. 2. (a) Rapid system for 14 C specific activity determination in the sample. (b) The correlation of the graphite sample mass as determined by two independent methods: weighing and combustion in the elemental analyzer. (c) Correlation between LSC and semiconductor detector data. 1000 1200 1400 1600 1800 0.0 0.2 0.4 0.6 0.8 1.0 Intensity, a.u. Raman shift, cm -1 Virgin graphite 3.5x10 15 C + /cm 2 1.18x10 16 C + /cm 2 7.2x10 15 C + /cm 2 1.15x10 15 C + /cm 2 1000 1200 1400 1600 1800 0.0 0.2 0.4 0.6 0.8 1.0 Virgin graphite 1,2x10 16 C + /cm 2 1,2x10 16 C + /cm 2 , 400 o C, 5h 1,2x10 16 C + /cm 2 , 600 o C, 5h 1,2x10 16 C + /cm 2 , 800 o C, 5h Intensity, a.u. Raman shift, cm -1 Theoretical calculations (SRIM) Sample preparation Ion implantation (ion accelerator Tandetron 4110A) Raman characterization Thermal treatment Raman characterization Fig. 3. SEM images of raw RBMK-1500 stack graphite samples. (b) (a) Fig. 5. Raman spectra of raw RBMK-1500 stack graphite samples (a) implanted at a fluence of (b) 1.2×10 15 ions/cm 2 (c) 3.5×10 15 ions/cm 2 (d) 7.2×10 15 ions/cm 2 (e) 1.2×10 16 ions/cm 2 and subsequently annealed at various temperatures ranging from 400ºC to 800ºC for 5h. 1000 1200 1400 1600 1800 2000 0.0 0.2 0.4 0.6 0.8 1.0 Intensity, a.u. Raman shift, cm -1 7.2x10 15 C + /cm 2 7.2x10 15 C + /cm 2 , 400 o C, 5h 7.2x10 15 C + /cm 2 , 600 o C, 5h 7.2x10 15 C + /cm 2 , 800 o C, 5h Virgin graphite 800 1000 1200 1400 1600 1800 2000 0.0 0.2 0.4 0.6 0.8 1.0 Raman shift, cm -1 Intensity, a.u. Virgin graphite 3,5x10 15 C + /cm 2 3,5x10 15 C + /cm 2 , 400 o C, 5h 3,5x10 15 C + /cm 2 , 600 o C, 5h 3,5x10 15 C + /cm 2 , 800 o C, 5h 800 1000 1200 1400 1600 1800 2000 0.0 0.2 0.4 0.6 0.8 1.0 Intensity, a.u. Raman shift, cm -1 Virgin graphite 1,2x10 15 C + /cm 2 1,2x10 15 C + /cm 2 , 400 o C, 5h 1,2x10 15 C + /cm 2 , 600 o C, 5h 1,2x10 15 C + /cm 2 , 800 o C, 5h (a) (b) (c) (d) (e)