Mcnp Calculations Supporting The Start-Up Of The New Core At The Triga Vienna

During 2012 the TRIGA Mk II reactor in Vienna was converted from a highly heterogeneous core which included HEU fuel elements to an all LEU core. Because after 45 years of operation a simple exchange of the HEU with new LEU elements would have been insufficient to reach the necessary excess reactivity, most old pins were sent back to the US. As replacement 77 slightly used (burn up < 1.3%) 20% enriched stainless steel fuel pins were leased from the US Department of Energy (DOE), which together with the 13 retained elements should ensure another 20 years of operation. The previously developed MCNP model was adapted for the new core and used to calculate the basic core load. As it turned out the reactivity was considerably overestimated. This paper presents a discussion of these results together with the measured values and changes in the neutron flux at several experimental positions compared to the 2011 values.