Fusion

Fusion Energy

Nuclides with mass number less than 20 contain more energy than nuclides with twice their masses. Thus, fusion releases energy. This is evident from their mass excesses shown below.

Mass excess (MeV)
of light nuclides
n 8.071
H 7.289
D 13.136
3T 14.950
3He 14.931
4He 2.425
5He 11.390
5Li 11.680
6Li 14.086
7Li 14.907
8Li 20.946
9He 40.810
9Li 24.954
9Be 11.348
9B 12.415
9C 28.914
9N 12.415
The energy of fusion Q for the D-T fusion can be evaluated from the mass excesses of D, T, He, and n (neutron) as shown in the Table,

D + T ® 4He + n + Q

Q = (13.136 + 14.950) - (2.425 + 8.071)
    = 17.6 MeV


The T-D Fusion
By the same method, the energy of fusion for other reactions can be evaluated. We have D + T ® 4He + n + 17.6 MeV

D + 3He ® 4He + p + 18.4 MeV

D + D ® 3He + n + 3.3 MeV

D + D ® 3T + p + 4.0 MeV

From engineering point of view, the most favourable fusion reaction to consider is the D + T fusion, because its cross section is much higher than that of D + 3He fusion, although the latter releases a little more energy.

Hydrogen Fusion

Fusion of hydrogen takes place at very high temperatures. The reaction, H + H ® D + b+ + n (+ e-) The positron annihilates with the electron and release 1.02 MeV in gamma rays. As we shall see, hydrogen fusion in stars follows several steps, but the overall reaction may be represented by the reaction: 4 H ® 4He + 2 (b+ + n + (+ e-) + 25.7 MeV The following reaction is rare if it happens at all, D + D ® 4He + 23.85 MeV. Instead, fusion reactions of deuterium are D + D ® 3He + n + 3.3 MeV

D + D ® 3T + p + 4.0 MeV

In these two reactions, a proton or neutron is released.

© cchieh@uwaterloo.ca