Nuclear Fusion (OCR A Level Physics): Revision Note

Nuclear Fusion

• Fusion is defined as:

Small nuclides that combine together to make larger nuclei, releasing energy

• Low mass nuclei, such as hydrogen and helium, can undergo fusion and release energy

• On Earth, research is focused on achieving the deuterium-tritium (D-T) reaction

• This involves fusing a deuterium nucleus and a tritium nucleus together to produce a helium nucleus and a neutron

${}_1{}^2\mathrm{H}+\hspace{0.17em}{}_1{}^3\mathrm{H}\to {}_2{}^4\mathrm{He}+\hspace{0.17em}{}_0{}^1\mathrm{n}$

Deuterium-tritium fusion

• For two nuclei to fuse, both nuclei must have high kinetic energy

  • This is because protons are positively charged, which means that they repel one another

• It takes a great deal of energy to overcome the electrostatic force, hence, fusion can only be achieved in an extremely hot, dense environment, such as the core of a star

• When two hydrogen nuclei (protons) fuse, a deuterium nucleus is produced

  • A positron and an electron neutrino are also produced as one of the protons converts into a neutron through beta-plus decay

${}_1{}^1\mathrm{H}+\hspace{0.17em}{}_1{}^1\mathrm{H}\to {}_1{}^2\mathrm{H}+\hspace{0.17em}{\mathrm{e}}^{+}+\hspace{0.17em}{\mathrm{\nu }}_{\mathrm{e}}$

• In the centres of stars, four hydrogen nuclei $\left({}_1{}^1\mathrm{H}\right)$ fuse to produce a helium nucleus $\left({}_2{}^4\mathrm{He}\right)$, plus the release of energy

  • This provides fuel for the star to continue burning