What kind of force exists between two neutrons?

Two protons are kept at a separation of $10 \, nm$. Let $F_{n}$ and $F_{e}$ be the nuclear force and the electromagnetic force between them, respectively.

What is the approximate percentage of the total mass of an atom that is concentrated in its nucleus (in $\%$)?

$\pi$ mesons can be

$STATEMENT-1$: The plot of atomic number ($y$-axis) versus number of neutrons ($x$-axis) for stable nuclei shows a curvature towards the $x$-axis from the line of $45^{\circ}$ slope as the atomic number is increased.
$STATEMENT-2$: Proton-proton electrostatic repulsions begin to overcome attractive nuclear forces in heavier nuclides,requiring more neutrons to maintain stability.

If the radius of a nucleus with mass number $125$ is $1.5 \text{ fermi}$, then the radius of a nucleus with mass number $64$ is: (in $\text{ fermi}$)