D. Mulyana S. Chirayath, “Fuel Pebbles Theft Analysis For Physical Protection System Development”, INMM and ESARDA Joint Annual Meeting, Virtual Meeting, August 23 – September 1, 2021.
Pebble bed reactor (PBR) is not a new technological concept, but it is a relatively new marketable option. As with other reactor types, it needs unique safety, security, and safeguard approach. With a unique fuel design and all materials (fuel, moderator, and coating) contained within a 6 cm-diameter sphere, PBR fuel should be able to achieve a burnup of up to 90 GWd/MTU. A fuel pebble may consist 8,000 to 15,000 TRISOs as fuel kernels, which can contain fission products during and after irradiation. A set of Monte Carlo n-Particle (MCNP) neutronic simulations on PBR’s fresh and spent fuel pebbles are completed to investigate how much radiation would introduce a potential harming effect to personal. This type of understanding is needed for safety, security, and safeguard analysis. With a 5 g mass of uranium per pebble enriched to 17 wt% of 235U, the radiation dose rate from a fresh fuel pebble is only 0.38 μrem/h at 1 m. At the same distance, a bare 90 GWd/MTU spent fuel pebble cooled for one year ends up with a gamma dose rate of 980,000 μrem/h. A lead sphere shielding (density of 11.34 g/cm3) with a thickness of approx. 12.5 cm reduced the dose rate back to 0.328 μrem/h (similar to that of fresh fuel pebble). This may or may not be detected by a radiation detector. However, the 12.5 cm thick lead is about 175.6 kg, while the pebble itself is only 100.5 g. This gives a significant security barrier to the irradiated pebble theft. This study provides technical information of possible ways to minimize the probability of detection of the fresh and spent fuel. This study provides a technical output that is useful for a physical protection system development, especially to deal with an insider threat, for a PBR system. To simplify the analysis, the study is limited to gamma radiation dose rate analysis only.