Neutrons propagation in Lead: a feasibility study for experiments in the RSV TAPIRO fast research reactor

The increasing worldwide interest in Lead-cooled Fast Reactors (LFRs) substantiates the need to validate the analytical codes and methods used to support their design. For neutronic analyses, this is chiefly reflected in assessing the impact of nuclear data uncertainties on the integral and local parameters resulting from such analyses. Besides this driving interest, the aim of refining nuclear data moves continuous efforts for more accurate measurements, be them differential or integral, for which adequate facilities are required. A preliminary attempt at adjusting the ENDF/B-VIII.0 neutron data libraries for application to ALFRED – the demonstration reactor of future European LFRs – provided clear indications on the need to refine the cross-sections of Lead isotopes, and particularly of the elastic and inelastic ones at high energies. However, experiments dealing specifically with Lead's neutronics are not frequent in literature and even scarce in international databases establishing benchmark cases for the validation of neutronic codes or the adjustment of nuclear data libraries. Therefore, the availability at the ENEA's Casaccia research center of a fast source reactor – RSV TAPIRO – provides a unique opportunity to perform new integral experiments in support of fast reactors, including LFRs, owing to the well-characterized neutron spectrum of the thermal column, a spacious experimental slot within the biological shield of the reactor in the proximity of its external Copper reflector. Accordingly, a series of experiments – which could be recognized as benchmarks of international value – has been envisaged, dealing with the use of Lead in a reactor. The experiments concern the propagation of neutrons through blocks of materials representing relevant elements of a reactor core, and ranging from pure Lead to mixtures reproducing portions of the reflector and shield in LFRs. The paper is focused on the feasibility study of some of these experiments in which Lead and mixture blocks are inserted in the so-called thermal column of the RSV TAPIRO reactor and irradiated by the neutron flux emerging from the Copper reflector surrounding the reactor core. Despite the name of the experimental opening, the spectrum of the incident flux is relative hard, owing to the almost-pure fission spectrum in the core. The scope of the feasibility study includes the conceptual design of the blocks, moving from representative studies with target LFRs. An in-depth investigation was also performed concerning the required instrumentation: appropriate neutron detectors and threshold activation foils were selected aiming at characterizing the assembly's neutron flux intensity and energy spectrum, so that the measured reaction rates and spectral indexes could be used to assess the impact of nuclear data on these parameters, retrieving information for ad-hoc adjustments applied to LFRs.