Design of an Instrumented Fuel Rod for Intra-pin Neutron Flux Measurements in the CROCUS Experimental Reactor

In response to the need for validating high-fidelity deterministic neutronics solvers capable of pin-resolved neutron flux distributions, an instrumented fuel rod was designed for the experimental reactor CROCUS, operated by EPFL’s Laboratory for Reactor Physics and Systems Behaviour. This rod aims at obtaining intra-pin data using activation dosimetry techniques. Placed in the outer region of the reactor core, the rod utilizes spaces between four metal uranium cigars (25 cm each) to house various disk dosimeters, targeting different neutron energy ranges and allowing retrieval of radial and azimuthal intra-pin neutron reaction rates. A design of experiment (DOE) study, aided by Serpent2 Monte Carlo calculations, facilitated the selection of dosimeter material and rod design, ensuring detection of within-pin neutron flux v ariations w hile adhering to mechanical and regulatory constraints. Azimuthal divisions of activation dosimeters were irradiated at the core center and subsequently subjected to activity determination using an HPGe gamma spectrometer. The objectives were met, detecting expected azimuthal variations within dosimeters and confirming uncertainties on reconstructed activity to be smaller than the amplitude of the observed variations. A permit from the Swiss nuclear regulatory authority ENSI-IFSN was obtained, allowing installation and usage of the instrumented fuel rod in CROCUS, known as the NECTAR experiments.