Impact of the Detailed Fission Source Distribution on IRIS Shielding Analyses

International Reactor Innovative and Secure (IRIS) is an advanced pressurized water reactor with an integral primary system. It features an integral reactor vessel surrounded by a spherical steel containment 25 m in diameter. Both deterministic and Monte Carlo methods are used to characterize its radiation environment. This paper focuses on the generation of the neutron fission source that is employed as the fixed source in radiation transport calculations. To facilitate radiation shielding analysis, a technique is proposed to synthesize fission source data from the IRIS depletion history into an average and a limiting (maximum) source distribution. The average source preserves the time-integrated, spatially dependent fission neutrons and is suitable for evaluation of long-term irradiation effects, such as the radiation damage on the reactor vessel. The maximum source gives a bounding fission neutron distribution that is suitable for calculation of the maximum instantaneous dose to the personnel. Spatial and spectral effects are also taken into consideration in the source representation. Pinwise axial distributions of the neutron fission source and the associated contribution from primary fissionable isotopes have been generated to allow evaluation of neutron leakage in the critical regions, such as at the core periphery. Less detailed assemblywise axial distributions are also made available to simplify their implementation in the MCNP and TORT models. A comparison of the results obtained with the latter distributions against the reference results (employing the most detailed distribution) will show the impact of simplifications and help identify strategic features and locations where preserving the detailed information is beneficial for meeting specific shielding objectives.

The judicious postprocessing and interpretation of the fission source distribution proposed by this approach make the subsequent radiation analysis practical while retaining the critical details needed to achieve high accuracy.