Investigation of graphite matrix activation in the fuel pebble of Reaktor Daya Eskperimental

Program of Reaktor Daya Eksperimental (RDE) construction in Indonesia is still on progress and it is continuously supported by research and development activities. RDE will be built based on pebble-bed type of High Temperature Gas-cooled Reactor utilizing UO2 fuel microkernel dispersed in a 6-diameter spherical solid graphite matrix. Graphite is the major composition of the fuel pebble. In addition to the fuel pebbles, there are also dummy pebbles in the RDE core which are fully composed of graphite without fuel kernel inside. During irradiation in the core, there happen activation of graphite due to neutron captured reaction. In this study, the activation of graphite was investigated through the simulation of ORIGEN2.1 code. The graphite matrix in the pebble fuel was irradiated for five cycles and the dummy pebble was irradiated only for one cycle. From the ORIGEN2.1 simulation, the activation of graphite matrix produces many isotopes of light nuclei but the isotopes that have significant half-life and activity are only H-3, Be-10, and C-14. The activities of H-3, Be-10, and C-14 inside the graphite matrix of one fuel pebble are 3.98E-08 Ci, 2.45E-08 Ci, and 8.99E-07 Ci, respectively. The results for the activation of graphite of one dummy pebble for the same isotopes are 3.48E-10 Ci, 5.13E-09 Ci, and 1.89E-07 Ci, respectively. These isotopes deposit in the graphite matrix and might be released into the primary coolant through some mechanisms such as pebble crack, graphite corrosion, and graphite abrasion due to the friction during the pebble shuffling in the core. However, since the activity of isotopes is small, it can be stated that the fuel pebble of RDE is safe.