NEA Organised Validation of Multi-Dimensional Transport Codes and Cross-Section Sets Used Presently in Reactor Shielding

Abstract

A Task Force ofthe OECDINEA Nuclear Science Committee performed benchmark calculations to validate two-and threedimensional transport codes and cross-section sets used for radiation shielding. Based on careful measurements from the VENUS critical facility of SCKlCEN-Mol/Belgium, the Task Force started with two-dimensional benchmark calculations representing the arrangement of VENUS-I , which is a mock-up of the Westinghouse three loop reactor. About 20 different calculations were presented. SeveraI2D-S N -codes were used including some in-house codes and the Monte Carlo program MCNP-4A. The transport cross-sections were taken from ENDFIB-VI or JEF 2.2 and the dosimetry data mainly from IRDF-90v2. The most challenging task was the validation of the latest versions of three-dimensional transport codes with the complex arrangement ofVENUS-3. Approximately 14 independent benchmark calculations were contributed world-wide. Several versions of3D-S N -codes and the Monte Carlo code MCNP were applied. In conclusion the results of the three-dimensional benchmark (VENUS-3) are in general much closer to the experimental values than for the two-dimensional benchmark (VENUS-I). This shows the inherent deficiencies oftwo-dimensional transport calculations, due to approximations such as diffusion based axial buckling corrections and improper averaging of source and shielding materials, since real configurations are always three-dimensional.

Radiation Shielding Activities at the OECD/Nuclear Energy Agency

“…The results of this study involving both 2-D and 3-D models are the subject of a separate paper at this ICRS9 conference (11). The results indicate that 2-D models suffer from underestimation of the neutron flux due to approximations such as diffusion based axial buckling corrections in the material layers outside the reactor core, and improper averaging of source and shielding materials.…”