The NUCLEUS experiment aims to detect reactor anti-neutrinos through coherent elastic neutrino-nucleus scattering (CEvNS) at the Chooz nuclear power plant in France using a 10-g cryogenic detection system made of CaWO4 and Al2O3 target crystals featuring unprecedented low energy thresholds of 20 eV. Although being exposed to a high neutrino flux of 1.7x10^12 /(s cm²), the NUCLEUS experimental site offers challenging background conditions for CEvNS detection.
With a 3 meters water equivalent overburden, secondary cosmic-rays and gammas from natural radioactivity are expected to be the main contributors to the backgrounds in the region of interest, below 100 eV, where the NUCLEUS experiment is most sensitive to CEvNS. To reach a sufficient signal over background ratio in order to observe the neutrino signal, passive and active shielding have then to be added around the cryogenic detectors. This poster presents preliminary results of extensive Monte Carlo simulations combined to its dedicated on-site background measurements for both optimizing the design of the NUCLEUS experimental setup and for achieving a first background prediction at sub-keV energies.