Neutronic characteristics of the GAMA aqueous homogeneous reactor (GAMA-AHR)

The GAMA aqueous homogeneous reactor (GAMA-AHR) is an AHR design developed by the Nuclear Reactor Research Team at the Department of Nuclear Engineering and Engineering Physics, Universitas Gadjah Mada. This nonpower reactor produces valuable radioisotopes, mostly molybdenum-99, that are intended for medical purposes. The standard GAMA-AHR design utilizes 3.0 mole of uranyl nitrate as seed (fissile) fuel and 4 mole of thorium nitrate as blanket (fertile) fuel, and both are dissolved in light water as solutions. Compared with other AHR designs, the GAMA-AHR utilizes light water as the reflector instead of graphite. This design allows the use of a completely passive post-shutdown cooling system. Criticality calculation and neutronic parameter analysis were performed utilizing SCALE6.2 and OpenMC computer codes with the ENDF/B-VII.1 neutron cross section library, respectively. The analyzed parameters comprised power distribution, excess reactivity, shutdown margin, shutdown capability using a fuel drain tank, control-rod worth, temperature reactivity coefficient, and void reactivity coefficient (αθ). From the calculation results, the GAMA-AHR exhibited an excess reactivity of 0.07843 Δk/k at zero burn-up in 3.0 mole of uranyl nitrate solution. The excess reactivity at zero burn-up increased with the increase in the mole fraction of uranyl nitrate until the maximum value of 0.215 Δk/k at 15 mole of uranyl nitrate. The GAMA-AHR exhibited the control-rod shutdown margin of −0.03072 Δk/k at a normal operation temperature (80 °C) and −0.01958 Δk/k at 40 °C. Draining of the seed-fuel solution shut down the reactor with a shutdown reactivity of −1.40385 Δk/k. The GAMA-AHR showed a negative feedback for temperature and αθ, with values of −0.000155 Δk/(k.°C) and −0.00353 Δk/(k.), respectively. These results prove the inherent safety characteristic of the GAMA-AHR. The GAMA-AHR with the thermal power of 200 kWth capable to produce 2500 six day Ci per week of Mo-99. © 2024 Elsevier B.V.