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Description
This study investigates the qualitative and quantitative properties of the neutron flux near the geometry boundary of the neutron subcritical piles of the Nuclear Engineering Laboratory of NTUA (NEL-NTUA) using the freely available OpenMC Code. The Laboratory operates two subcritical piles. The first one has a cubic geometry tank of 1 m3 volume. A fast neutron Am-Be source with nominal radioactivity of 10 Ci is placed at the mass center of the tank. The tank is filled with water. The second pile has a cubic geometry box of 0.2 m3 volume. A fast neutron Am-Be source with nominal radioactivity of 5 Ci is placed at its mass center. The box is filled with solid paraffin. The properties of these neutron sources inside the subcritical piles (dimensions, composition, radioactivity etc.) were prepared in detail in order to be used as input to the OpenMC code. The neutronic analysis for the water pile and the paraffin pile followed. The results showed that the neutron leakage spectrum includes thermal neutrons (assumed to have energy ranging from 0 eV to 0.5 eV), but also epithermal (assumed to have energy ranging from 0.5 eV to 0.1 MeV) and fast neutrons (assumed to have energy ranging from 0.1 MeV to infinity). In the case of the water pile, the percentages were estimated to be ~37, 11 and 52% respectively. In the case of paraffin, the percentages were estimated to be ~45, 11 and 44% respectively. The overall neutron flux in the water pile was estimated to be 3-4 neutrons cm2s-1 in the least favorable part of the tank external surface. The overall neutron flux in the paraffin pile was estimated to be 25-30 neutrons cm2s-1 in the least favorable part of the of the box external surface. Results were brutally verified with some portable instruments available in the Lab and with neutron flux results coming from the Greek Regulator's dosimetry services. After that, a series of several neutron moderators/absorbers for the water subcritical pile were computationally examined in order to test the possibility of replacing the water of the first pile with a different moderating material, not prone to leakage in case of small tank breaches. It was found that the most suitable materials for the substitution of the water are organic polymers (i.e. plastics) in the form of small diameter granules, with density slightly greater than that of water and also borax decahydrate as well as mixtures of sodium polyacrylate with water.