E.4.1—Fission energy release

Model fission

A heavy nucleus can split into two lighter nuclei after absorbing a neutron, or spontaneously in an unstable state. The products have a greater binding energy per nucleon than the original heavy nucleus.

Track the release

The increase in total binding energy appears as kinetic energy of the fission products, neutron energy and radiation. The mass of the products is slightly smaller, with the mass difference converted to energy.

Worked example — use binding energy per nucleon

For $^{235}\mathrm{U}$ splitting into $^{89}\mathrm{Kr}$ and $^{144}\mathrm{Ba}$, use $B=A(B/A)$. With values $7.59$, $8.72$ and $8.27\,\mathrm{MeV}$ per nucleon, $E=[89(8.72)+144(8.27)]-235(7.59)=1.83\times10^2\,\mathrm{MeV}$. The products are more tightly bound, so this positive difference is released.

Understand fissile material

Enrichment increases the fraction of uranium-235 relative to uranium-238, making a sustained fission process more feasible.

Common trap

Do not say energy is created from nothing. It comes from the mass defect and the change in nuclear binding energy.