We propose to perform the first measurement of the neutron-neutron scattering cross section in the through-channel of the pulsed aperiodic reactor YAGUAR (Snezhinsk, Russia). Such a measurement directly determines the neutron-neutron scattering length, and by comparison with the proton-proton scattering length bears upon the issue of charge symmetry of the nuclear force. The proposed experimental set-up, as well as modelling of the neutron density and of the frequency of neutron-neutron collisions is described. Experimental results are reported on the formation and optimization of the thermal neutron field inside the through-channel of the reactor YAGUAR. The instantaneous value of 1.1 × 1018 cm-2 s-1 obtained for the thermal neutron flux density is large enough to perform the first direct neutron-neutron scattering length measurement.