IRIS is an advanced, smaller-power pressurized water reactor, with aggressive dose reduction objectives. Because of its integral configuration, IRIS has a thick downcomer region that significantly reduces the radiation field outside the reactor vessel, forming the technical basis for achieving the objectives. However, this feature also makes the shielding analysis very challenging. The goal of evaluating the dose rate distribution throughout the IRIS nuclear power plant and, in particular, in all accessible areas further amplifies the problem.

The MAVRIC sequence of the SCALE6 code system was selected for this analysis. MAVRIC employs a hybrid deterministic-stochastic approach, with CADIS and Forward-CADIS methods being used to develop variance-reduction parameters for Monte Carlo simulations. MAVRIC was successfully applied to determine the dose rate distribution throughout a large portion of the IRIS nuclear power plant including the control room. The obtained results confirmed that the dose rate is below the set target limit in the relevant plant areas and, in particular, in the control room.