Behavior of reactivity change in AHR reactor

One of the phenomena at Aqueous Homogeneous Reactor (AHR) of uranium solution fuel is radiolysis process that breaks the water molecules into oxygen and hydrogen gases by both neutron and gamma radiations. The reduction of water molecules in the solution causes the concentration of uranium solution increases and eventually leads to core reactivity change. As AHR is equipped with recombiner to recombine the oxygen and hydrogen gases into water, the fuel concentration will become lower. This will also induce core reactivity change. The periodic changes in reactivity due to fluctuation in uranium concentration will lead to power fluctuation. The paper aims at determining reactivity change due to the change in uranium concentration, and other reactivity change such as doppler effect. The analyses were performed using two Monte Carlo method codes, MCNP and MVP. For the reactor being analyzed, with power of 20 kW and 1 kW/l of power density, has the doppler reactivity coefficient -1,01 × 10-2 %dk/k. This system also has strong negative feedback reactivity from void generation. (author)