Clusterization of flare radio spectra is performed using two clusterization methods, dividing the flares into three groups with different durations (t < 3, 3 < t < 10, t > 10 min). Independent of whether the flares are divided according to duration, the most powerful flares have very different amplitudes but similar spectra. Less powerful flares have more diverse spectra. Special classes of flare spectra are revealed - delta-like and sine-like - which are distinguished by a number of special properties. The envelope for delta-like spectra is similar to the shape of the spectra of powerful flares. The amplitudes of delta-like spectra do not depend on the flare duration. Sine-like spectra vary n a regular way in the transition to flares with longer durations. It is suggested that the stability of the shape of the spectra for powerful flares is associated with microstructure of the radio flare generation region. The characteristics of the microstructures are derived in the framework of gyrosynchrotron radiation as the mechanism for the radio emission. It is shown that powerful impulsive flares (t < 3 min) arise in open magnetic structures, while weaker flares arise in closed, low-lying magnetic loops.