Characterization of a fast to thermal neutron spectrum converter on PROSPERO reactor

The PROSPERO reactor is located at CEA Valduc Center in France. The reactor is composed of an internal core made of High Enriched Uranium metal alloy surrounded by a reflector of depleted uranium. The size of the assembled core is 352 mm length and 314 mm in diameter. The reactor is used as a fast neutron spectrum source and is operated in delayed critical state with a continuous and steady power for several hours, which can vary from 3 mW to 3 kW, which is the nominal power. The core of the PROSPERO reactor is made critical by driving depleted uranium control rods through the length of the reflector. The PROSPERO reactor is operated in a cell 10 m × 8 m × 6 m high, with 1.4-meter-thick concrete walls. This large cell offers a wide range of irradiation sites from the reflector surface to 6 meter away from the reactor central axis. The flux at nominal power varies from 5.10⁺¹⁰ n.cm-2/s at the reflector surface to 10⁺⁷ n.cm-2/s at 5 meters from reactor axis. Due to scattering events in the assembly, the mean energy of the leakage neutron spectrum is 0.8 MeV. This experimental tool is devoted to mixed neutron and gamma fluxes production. The neutron over gamma fluences ratio near the reflector surface is approximately equal to 10. Within the framework of a diversified use of the irradiation tools and in accordance with a request of the scientific community to offers new opportunities, our laboratory, the "Criticality and Neutronics Research Laboratory", decided to build a neutron energy converter allowing the production of a neutron thermal spectrum using PROSPERO. As the core produces fast neutrons spectrum, we built a hollow cubic box of 50 cm × 50 cm × 50 cm with 10-cm-thick polyethylene bricks and placed one meter away from central reactor axis to moderate as much as possible neutrons to lower energies (E<;0.6 eV). Analysis of the moderated flux inside the converter was performed using different activation f- - oils such as indium or gold, encapsulated or not with cadmium and an additional specific French activation detector named SNAC2. We have developed a model of the experiment in the Monte Carlo neutron transport code TRIPOLI-4. A non-analogous transport calculation scheme was necessary to reproduce properly the experimental activities . The results of the calculated activations are within 4% of the experimental measurements given with 10% uncertainty (2 sigma). We show results of the neutron spectrum converter we built that realizes thermalisation of 80% of the PROSPERO reactor fast neutrons below the cadmium threshold of 0.6 eV. Epithermal neutrons represent 15% of the spectrum and only 5% are in the fast neutron range above 1 MeV. The total flux at the center of the converter is 1.4 10⁺⁹ n.cm^-2/s at 3000 W. The neutron converter was successfully used during several electronic hardness programs in the two last years. To improve the thermalisation of the spectrum of the PROSPERO reactor, complementary calculations have been carried out using optimized material compositions and thickness values of the neutron energy converter.