Yes, the thermal expansion is very important in physics and technology.
the earth was a big hot ball made up of many rocks and gases. After millions of years the outer portion of the earth coooled down. We live on this outer layer of the earth called the Crust.
Because the neutron economics are against it, a critical reactor and a chain reaction would not be possible. Ordinary water absorbs too many neutrons. For natural uranium you must use graphite or heavy water for the moderator.
There is new rocks that build up they first start as lava and they build up and make new crust