Metal-foil fusion and quantum tunnelling A-level challenge

A deuterium nucleus contains one proton and one neutron. Two deuterium nuclei approach one another inside a metal foil. Explain why the nuclei repel as they approach and why increasing their separation reduces this repulsive force.

The strong nuclear force can bind the nuclei only when they are separated by a few femtometres. In a classical model, a low-energy nucleus cannot pass over the electrostatic potential-energy barrier. Quantum mechanics gives the nucleus a small probability of being found on the other side of the barrier. Use this information to explain why fusion can still occur at energies below the classical barrier and why the fusion probability remains small.

A fusion reaction produces a helium nucleus whose binding energy per nucleon is greater than that of the two original deuterium nuclei. Explain why energy is released in the reaction and state why the metal-foil result is not evidence for a useful fusion power source.