Nuclear fission is a process by which a large nucleus is split into two smaller nuclei, or fission fragments. It takes place after the nucleus absorbs a neutron that usually is a product of another atom's radioactive decay.
A common fission reaction bombards uranium-235 with a single neutron. This creates a uranium-236 atom which is unstable. Its nucleus tends to split into more stable products like Krypton-90 & Barium-143.
If you compare the masses before and after reaction, notice that Kr-90 + Ba-143 only = 233. But they were produced from U-236. This is because 3 neutrons were also released.
