Modeling a TRIGA Mark II Biological Shield using MCNP5

The 250kW TRIGA Mark II research reactor, Vienna, is equipped with experimental facilities inside and outside the reactor core. Outside the core, there are four beam tubes (three radial and one tangential beam tube), a thermal column and a neutron radiography collimator, supplying neutrons for irradiation experiments. These experiments include neutron radiography, neutron tomography, neutron activation and academic training purposes. This paper describes calculations of the neutron flux density in the thermal column and in one of the selected beam tubes (i.e. BT-A), using the MCNP5 neutron transport code. To validate these calculations with experiments, neutron flux measurements using the gold foil activation method were performed on 13 selected positions in the thermal column. After comparison between calculations and measurements on thermal column, this paper also presents the thermal flux distribution in BT-A calculated by MCNP5. For this task, the already developed MCNP model for the TRIGA core was extended to the biological concrete shield including the four irradiation beam tubes and the above mentioned irradiation facilities. To save computational costs and incorporate the accurate and complete information for the individual MC particle tracks, the surface source writing capability of MCNP was applied to the TRIGA shielding model. The variance reduction techniques were also applied to improve the statistics of the problem and to save computational efforts.