Monte Carlo simulations of the water draining experimentof Giacint critical assembly

•The MCNP6 code is extended for simulating reactor dynamic problems with moving parts of the geometries.•The geometrical motion is accounted in each particle random walk.•The water draining experiment in the Giacint critical assembly was simulated.•The MCNP simulation results were consistent with results from Serpent.•The MCNP simulation results were also compared with the measured data and predicted a faster transient than the experiment data. The MCNP6 computer program has been successfully extended to simulate reactor dynamics problems with moving parts of the geometries. Different from the dynamic method developed in other Monte Carlo codes, a movement scheme has been developed to account for the geometrical parts motion during the particle random walk. The MCNP6 computer program has been used to simulate two transient experiments of the Giacint critical assembly. The MCNP6 calculated total neutron flux was compared with that from the Serpent simulation. An excellent agreement was obtained between the results of the two Monte Caro computer programs. The MCNP6 calculated total neutron flux was also compared with the two measured transients. The MCNP6 results predicted a faster transient than the experimental data. The MCNP6 transient simulation was improved with an adjusted geometrical model which shifts the fuel rods slightly to match the measured reactivity worth due to the drained water.