The Chrysalids is set in Waknuk in a time deep into the distant future.  There has been some kind of apocalyptic event, known as the Tribulation, and it has left the people of Waknuk in a 19th century level of technology.

Waknuk is in what today is Canda, in Labrador.

The civilized part of it -- of which Waknuk was only a small district -- was called Labrador. (ch 4)

Labrador is in the extreme eastern tip of Canada, near Newfoundland. It is bordered by the Atlantic Ocean.  The areas around Waknuk are known as the Fringes, where an uncivilized people who have been kicked out of Waknuk are living.

The people of Sealand (New Zealand) live far away from Waknuk.  They have more advanced technology, and can communicate telepathically.

Waknuk is a deeply religious society where no one who is different from their narrowly-defined view of human nature is allowed.  They even destroy crops and livestock that show differences.  As a result, there is no growth.  Waknuk’s isolation allows it to control its people and prevent any evolution of its society.

Your father and his kind are a part of those fragments. They have become history without being aware of it. (ch 16)

By setting the story in the future, an allegory of our time is achieved.  Waknuk is not so different from some of our countries and localities.  An apocalypse is not so out of sight, and neither is a tyrannical theocracy.