I understand your confusion, since particles are ejected from the nucleus in both processes. Alpha decay may be a bit of a grey zone, since it expels a helium nucleus, so the atom is split into two very asymmetric parts, but it is generally not considered a fission process. Alpha decay and spontaneous fission are actually competing processes.
Beta decay is something else entirely where no protons or neutrons leave the core, so there is no fission to speak of.
And spontaneous fission is possible, but it’s extremely rare. For your example of Uranium, which is overwhelmingly (99.3%) U-238:
Spontaneous fission is extraordinarily rare on a per-atom basis. Uranium-238, the most common uranium isotope on Earth, has a spontaneous fission half-life of about 8.4 × 10¹⁵ years.
There are no atoms splitting inside the body. Nuclear decay happens, but not nuclear fission.
Alpha and beta decay are types of fission.
I understand your confusion, since particles are ejected from the nucleus in both processes. Alpha decay may be a bit of a grey zone, since it expels a helium nucleus, so the atom is split into two very asymmetric parts, but it is generally not considered a fission process. Alpha decay and spontaneous fission are actually competing processes.
Beta decay is something else entirely where no protons or neutrons leave the core, so there is no fission to speak of.
And spontaneous fission is possible, but it’s extremely rare. For your example of Uranium, which is overwhelmingly (99.3%) U-238:
Source
So with the uranium in your example there is a non-zero chance of spontaneous fission occurring, but it’s very unlikely.