It is called the Coulomb barrier.

A proton requires more energy to be incorporated (captured) into an atomic nucleus when the Coulomb barrier is higher.

The higher the Coulomb barrier the more kinetic energy a proton requires to get close to a nucleus and be captured by it.

The amount of energy that is needed to overcome this repulsion is known as the Coulomb barrier.

The amount of energy released by the fusion reaction when it occurs may be greater or less than the Coulomb barrier.

Even using these isotopes, the Coulomb barrier is large enough that the nuclei must be given great amounts of energy before they will fuse.

Due to the increased mass, the Coulomb barrier decreases.

At the time, the temperature of the Sun was considered too low to overcome the Coulomb barrier.

The amount of energy that needs to be applied to force the nuclei together is termed the Coulomb barrier or fusion barrier energy.

The nuclei do not actually have to have enough energy to overcome the Coulomb barrier completely.