One nuke would not be enough. The largest nuclear warhead ever built was the Tsar Bomba, built and tested by the Soviet Union in 1961, with a yield of around 54 megatons (that’s about three THOUSAND times larger than the bomb that destroyed Hiroshima, and about 200 – 300 times larger than most ICBM and cruise missile warheads.
And the thing the Soviets found about this was that, yes, it was a massively powerful blast – light from the blast was seen as much as 1,000 km from the test site, the magnitude 5.2 seismic shockwave and the atmospheric pressure wave from the blast circled the earth three times over the next two to three days, and windows were broken in wooden buildings almost 500 miles from the blast site
– but they noted that a great deal of the energy from the detonation was simply reflected straight up from the ground and sent into space, reducing the power of the blast on the ground.
What Russia does have, though, are missiles capable of carrying multiple large warheads which combined have a yield approximately the same as the Tsar Bomba, but would be able to cause massive damage over a much wider area.
The effect of detonating the Tsar Bomba directly over Big Ben (via Nukemap). As you can see, all of London would be flattened (the darker grey circle), with extensive damage as far out as Chelmsford and Reading.
But a lot of protected targets within the lighter grey and the outermost yellow circles could remain operational, andof course there are many targets beyond the radius of destruction that would remain virtually untouched.
Ten 5-megaton warheads, with a combined yield as great as the Tsar Bomba, would cause much more widespread damage, taking out multiple targets. London would be equally flattened, along with Heathrow, Stansted, Luton, Southend and Farnborough airports, the port of Dover (and Folkestone), hte major transport and industrial facilities around Dartford and Gravesend, and the Royal Navy centres of Portsmouth and Southampton.
Far more damaging to the country as a whole, with the same power of weapons.
Reduce the size of the warhead again to around 150 kilotonnes, and you have something that can still very effectively destroy the important targets above (airports, ports, industrial centres, major transport networks, military targets etc) but that can be built in much larger numbers for the same cost, and carried in much larger numbers by missiles and aircraft.
One Russian ballistic missile-carrying submarine can potentially launch up to 128 smaller warheads which would saturate the UK, destroying almost 100% of the heavy industry, airports, ports, power stations, major transport hubs and military targets.
The country would not be “destroyed” in the sense that a decent percentage, probably even a majority, of the population would have survived.
But there would be no trains, no electricity, no running water, no food going from distribution centres to supermarkets, nothing that enables the UK to operate as a vibrant country with a large population, so in that respect, one SSBN could “destroy” the country.
