Question 25.11: The Nuclear Difference Compare the approximate energy releas...
The Nuclear Difference
Compare the approximate energy released in this fusion reaction with (a) a typical fission reaction and (b) a chemical reaction like the burning of coal \left( C + O _{2} \rightarrow CO _{2}\right) which releases 4.1 eV. Also compare these reactions on the basis of energy released per mass of reactants.
You’ve seen that fission reactions release around 200 MeV. Our fusion reaction’s energy is about 20 MeV, so the fission reaction releases 10 times the energy. But the fission reactants \left({ }^{235} U \text { and a neutron }\right) have a mass around 236 u, while the fusion reactants involve five nucleons, or about 5 u. Then the energy per unit mass is just under 1 MeV/u (200 MeV/236 u) for fission and about 4 MeV/u (20 MeV/5 u) for fusion. So fusion releases about four times as much energy on a per-mass basis. Both nuclear reactions energy releases vastly exceed that from the chemical reaction: 200 MeV/4 eV. or a factor of 50 million for fission and 5 million for fusion, per reaction. \text { One }{ }^{12} C \text { and }{ }^{16} O _{2} have a total mass of 44 u, so burning carbon gives about 0.09 eV per u. Then fission’s per-mass yield is about 10^{7} times greater than that of carbon burning, while fusion’s is about 4×10^{7} times greater.
REFLECT The curve of binding energy (Figure 25.5) confirms our fission-fusion comparison; going up the steep left-hand side of the curve (fusion) clearly results in a greater energy release per nucleon. The comparison with the reaction C + O _{2} \rightarrow CO _{2} shows why nuclear fuels are so much more potent than chemical fuels. That’s why a nuclear power plant is refueled once a year with a few truckloads of uranium, while a comparable coal plant receives many 110-car trainloads of coal each week. It s also the reason that a single nuclear bomb can destroy an entire city.
