Investigation on neutronics changes in tungsten granular blockage accident on VENUS-II light water reactor

Tungsten granular blockage accident is one of standard accidents in the CiADS system. In this paper, the benchmark experiments for the tungsten granular blockage accident have been performed on the VENUS-II light water zero-power facility of CIAE in Beijing. The effective multiplication factor K_eff values of the reactor were measured by the source jerk method for the tungsten granular targets with different heights, 5 cm, 10 cm, 20 cm, 30 cm and 40 cm. To achieve the proper sub-critical status, 904 fuel rods were loaded in the reactor core. The results were analyzed with the Monte Carlo code NECP-MCX using the ENDF/B-VII.0 library. In the NECP-MCX simulations, the efficient homogeneous modeling method was adopted to build the tungsten granular target model. The trend of the simulated K_eff values was found to be similar with those of the experiment during tungsten granular blockage accident, and the minimum and maximum relative deviations are respectively about 0.38 % and 0.51 %. The results show that the effective multiplication factor K_eff decrease with the increase of tungsten particle height and the K_eff curves are generally inverse S-shaped. The sensitivity analyses in NECP-MCX indicate that the negative reactivity worth of tungsten granular target is mainly caused by the radiation capture reaction channel in the tungsten alloy grains. Through further analyses, it was found that the tungsten (n, gamma) reaction channel in the thermal and epithermal neutron regions is most related to the K_eff variation of the VENUS-II light water zero-power facility. Meanwhile, the tungsten granular target has the great influence on the neutron flux and neutron spectra around the spallation target zone. To improve the security of the CiADS integration system, the efficient method to avoid the tungsten granular target blockage accident should be recommended.