Why spontaneous nuclear fission not classified as a kind of radioactive decay? In radioactive decays, one considers the alpha, beta, gamma and electronic capture scenarios.
Why is the spontaneous nuclear fission not considered as a kind of radioactive decay ?
 A: At least in the field of radiation protection, spontaneous fission actually is considered as a decay mode. For example in ICRP, 2008. Nuclear Decay Data for Dosimetric Calculations. ICRP Publication 107. Ann. ICRP 38 (3).:

Spontaneous fission
Spontaneous fission is a decay mechanism resulting in splitting of the nucleus into lighter nuclei, referred to as ‘fission fragments’, with the emission of neutrons.

In the ICRP-07 collection, 28 radionuclies decay by spontaneous fission.

The decay modes are abbreviated as: A – alpha, B− – beta minus, B+ – beta plus, EC – electron capture, IT – isomeric transition, SF-spontaneous fission. The contribution of fission fragments and neutrons associated with spontaneous fission are included in the total energy.

Note that spontaneous fission has a half-life and a branching fraction like any other radioactive decay. For example, Cf-252 has a half life of 2.645 years and branching fractions of 96.908 % for alpha decay and 3.092 % for spontaneous fission decay. Excluding the contribution of spontaneous fission decay and only looking at the alpha decay would result in a wrong total half-life for Cf-252.
