Its not just surviving the nuclear blast and then the nuclear fallout after, which would be tough as it is...
Its also the sh!t that comes with it, long term ill health, economy falls on its ass, crops fail, food and water shortages, pollution in the air and water etc, hospitals overrun, lack of healthcare in general, mental health shot to pieces from seeing friends and family microwaved, public running riot, powercuts, no jobs or income, poor world/global infrastructure in general, the list goes on... The world barely copes with COVID, forget one of our countries being nuked... Weve seen how selfish the people (and politicians) act within a week of the pandemic, forget an everyman for himself nuclear war...
Russia tested the Tsar Bomba way back in the 60's, which was 4,000 times more powerful than the bomb dropped on Hiroshima, the fallout would had been so dangerous they actually had to modify the test from 100 megatons to 50 megatons, so cut the strength in half... That was in the 60's, think what Russia, USA and the UK have now, they prolly have a 'if we cant survive, neither can you' bomb that can end the world...