Fission of U-235 ..... There are a number of possible fission fragments from the fissioning of U-235. But the important "left-overs" from the splitting apart of an atom of U-235 are neutrons (sometimes up to three per event). The neutrons go on to interact with other U-235 atoms causing them to undergo fission. The fission fragments are immaterial, it's the neutrons which make the chain reaction possible. -- See the diagram below. Control rods in a reactor absorb neutrons and regulate or even stop the fission process. Chain reaction - fission of U-235
1, 235_U = 231 - Th + alpha 231-Th = 231-Pa + beta(-) 231-Pa = 227-Ac + alpha 227-Ac = 223-Fr + alpha & 227-Th + beta 223-Fr = 223-Ra + beta(-) & 227-Th = 223-Ra + alpha 223-Ra = 219-Rn + alpha 219-Rn = 215-Po + alpha 215-Po = 211-Pb + alpha & 215-Po = 215-At + beta(-) 211-Pb = 211-Bi + beta(-) & 215-At = 211-Bi + alpha 211-Bi = 207-Tl + alpha & 211-Bi = 211-Po + beta(-) 207-Tl = 207-Pb + beta(-) & 211-Po = 207-Pb + alpha. That's the full disintegration series down to lead(207-Pb) ,which is not radioactive. Your correspondent ' pisgahchemist' has given a very good description of the use of control rods.