Simplified MC Simulation of Neutron Spectra at C-Pb, C-Unat, C-Fe Interfaces Induced by Fast Neutron Source.

One of the peculiarities of HTGR systems is the variability of the neutron spectrum in the core and reflector regions. Neutronic codes for reactor physics design and burnup analysis may then experience difficulties in reproducing correctly the flux behaviour in proximity of interfaces with the reflector, or absorbing control materials. In order to reduce these problems, benchmark experiments are proposed on the fast source reactor TAPIRO at the ENEA Casaccia Center. The experiments would consist in detection traverses in a graphite column, starting from near external reflector boundary, where a sector of the outer copper reflector has been removed and then characterized by a very hard neutron spectrum. Along the column the spectrum gradually softens up to thermal values. Different materials would be interposed, such as Unat, Pb, Fe, etc. to reproduce spectrum transition conditions at interface points between regions with different compositions. Activation foils would be used for activation analysis with threshold energies in the fast, intermediate and epithermal regions. A preliminary, simplified simulation exercise has been done aiming at showing the spectral changings occurring at material interface points for a number of configurations of the graphite column. The results have been calculated using a continuous energy Monte Carlo code (MCNP).