Christopher Coates 
The Anvil [EPUB ebook] 

Deadly radiation from a passing comet is about to destroy life on Earth.
In desperation, the U.S. Government constructs a series of underground shelters to protect as many people as possible. They are expected to be underground for 20 years before returning back to the surface.
But can they survive their time within the confines of the underground shelter… and what will they find when they finally emerge?
A parallel story to ’The Ark’, ’The Anvil’ is a riveting story of survival and courage in a post-apocalyptic sci-fi setting.