•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.•Compared with other AHR designs, the GAMA-AHR utilizes light water as the reflector instead of graphite.•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.•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. 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.