Preliminary Evaluation of the Scale MAVRIC Code and FW-CADIS Method for Efficient Scoping of the Radiation Field Throughout the IRIS Containment

The integral configuration of the International Reactor Innovative and Secure (IRIS) with its relatively thick downcomer region within the reactor vessel and compact spherical steel containment offers potential for a significant dose reduction but also presents challenges for the related deep-penetration shielding analyses due to the large spatial domain. It is necessary to determine the radiation field throughout the 25-m-diam spherical containment and into the adjoining auxiliary building.

The shielding analysis is being performed using the "traditional" deterministic S_N and Monte Carlo approaches and codes (TORT and MCNP, respectively). In the preliminary, scoping phase, the radiation field is sought "everywhere" throughout the power plant to identify any possible shielding issues. This is very challenging for typical Monte Carlo variance-reduction methods, which are devised and may work very well to provide results in a limited region or for individual "detectors" rather than everywhere. However, the recently developed FW-CADIS method, implemented within the MAVRIC sequence of the SCALE code system, aims to address this problem. It uses forward and adjoint deterministic S_N calculations to generate effective biasing parameters for Monte Carlo simulations throughout the problem. Previous studies have confirmed its potential for obtaining Monte Carlo solutions with acceptable statistics over large spatial domains.

The objective of this work was to evaluate the applicability of FW-CADIS/MAVRIC to efficiently perform the required shielding analysis of IRIS. For that purpose, a representative model was prepared, retaining the main problem characteristics, i.e., a large spatial domain (>10 m in each dimension) and significant attenuation (more than 12 orders of magnitude), but geometrically rather simplified at this stage of evaluation. The obtained preliminary results indicate that the FW-CADIS method implemented through the MAVRIC sequence in SCALE will enable determination of the radiation field throughout the large spatial domain of the IRIS nuclear power plant.