Domain by James Herbert

The long-dreaded nuclear conflict. The city torn apart, shattered, its people destroyed or mutilated beyond hope. For just a few, survival is possible only beneath the wrecked streets - if there is time to avoid the slow-descending poisonous ashes. But below, the rats, demonic offspring of irradiated forebears, are waiting. They know that Man is weakened, become frail. Has become their prey...