The design and development of Accelerator-Driven Systems (ADS) is a new branch of nuclear technology. It is known that the reactor-physical modelling of subcritical ADS requires a different approach from the modelling of critical cores due the presence of the so-called higher modes which limits the applicability of the commonly used reactivity measurement techniques that rely on the point kinetics approximation. In order to investigate possible methods for the effective computation of these modes MUTANTS (MUltimodal Three-Dimensional Advanced Neutron Transport Simulator), a few-group diffusion code is being developed. The code uses a finite difference mesh for spatial discretization and incorporates various routines from the SLEPc and PETSc libraries. Its features include general reactor physics calculations (such as criticality iteration, transient calculation and adjoint problems) as well as the computation of kinetic and static eigenmodes either by the standard multitudinous approach, or a filtered search based on data-driven solutions in case of the former. Although the application of diffusion theory has some benefits due to its simplicity and the existence of analytical solutions for various cases, the aim is to implement adequate eigenvalue algorithms alongside higher-order transport approximations in the future. This would allow either an optimal placement of detectors or the calculation of correction factors for reactivity measurements.