What is a nuclear and concentric crossword puzzle?

A nuclear crossword puzzle is a type of crossword puzzle that consists of essentially two different crossword puzzles that share the same grid. It works like a classic or an Italian crossword puzzle (by definition) and can be a combination of both. The first is located in the center of the grid and is solved independently. The second is solved after the first is solved and develops around the first crossword puzzle. Parts or even whole words are common among crossword puzzles. It is usually one of the difficult crossword puzzles. There are also variants of the nucleus such as one called concentric which, in addition to the two different crossword puzzles, also includes words in concentric circles around the perimeter, defined by separate definitions.