Can the population be protected in a nuclear attack?
It is virtually impossible to protect any population from a nuclear attack with even a single nuclear weapon. Once a weapon is detonated, those inside the fire zone will not survive the ensuing firestorm, even if they are in a deep basement or underground shelter. Since the firestorm requires only tens of minutes to develop and create air temperatures well above the boiling point of water, along with hurricane force winds which drive the flames horizontally at ground level, there is virtually no time to escape. Strategic nuclear weapons create firestorms over total areas of hundreds or thousands square kilometers or miles.
Those outside the firestorm and downwind from radioactive fallout would need to be in underground shelters stocked with enough food and water to last for about one to three weeks, since it would take that much time before the radioactivity levels come down to levels low enough to preclude immediate sickness or death.
However, in a large nuclear war, deadly climate change would cause average surface temperatures on Earth to become so cold that it would become impossible to grow food crops for many years. Even those who escaped the initial fire, blast and radioactivity would eventually starve to death.