The strong force is sufficiently strong to stabilize the uranium
nucleus against decay. However, if a couple of protons team up with
a couple of neutrons to form a helium nucleus inside the uranium
nucleus (in a picture popularized by George Gamow, author of the
Mr. Tompkins books) they would have enough energy to escape to
infinity. The strong force still poses a barrier to their escape,
but after many tries and billions of years, on average, the helium
nucleus escapes by quantum mechanical tunneling, the same mechanism
that allows a car to leave a garage without opening the door in the
Mr. Tompkins books.