What are examples of fission?

Example 1If a 235U atom splits up into two nuclides with mass number 117 and 118, estimate the energy released in the process.

SolutionA search of stable nuclides with mass numbers 117 and 118 are 117Sn50, and 118Sn50, their masses being 116.902956 and 117.901609 amu respectively. The mass of 235U is 235.043924 amu. The difference in mass 235.043924 - (116.902956 + 117.901609)

= 0.2394 amu (931.5 MeV) / (1 amu)

= 223 MeV.

Discussion

Actually, the fission is induced by neutrons, and usually the split is uneven. In reality, two neutrons are also released, but they were ignored in this example to make the estimate simple. Furthermore, the fission products are beta emitters as illustrated by example 2.

Example 2Assume the neutron induced fission reaction to be, 235U + n ® 142Cs55 + 90Rb35 + 4 n.

explain the results and estimate the energy released.

Solution

The neutron-rich fission products are beta emitters:142Cs55 ( , b) 142Ba56 ( , b) 142La57 ( , b) 142Ce58 ( , b) 142Pr59 ( , b) 142Nd60 (stable)

90Rb37 ( , b) 90Sr38 ( , b) 90Y39 ( , b) 90Zr40 (stable)

The masses of n, 142Nd60 and 90Zr40 are 1.008665, 141.907719 and 89.904703 amu respectively. The energy per fission and the decay energy are estimated as follows. Energy = 235.04924 - (89.904703 + 141.907719 + 3 x 1.008665)

= 0.210823 amu (931.5 MeV / amu)

= 196 MeV (1.6022e-13 J / MeV)

= 3.15e-11 J