How Close Can You Be to Survive a Nuclear Bomb Blast?

This is the first part of a two-part article about surviving nuclear blasts.  In this first part, we look at the immediate effects of nuclear blasts, in the second part, we will look at longer term effects.

Few things are more horrific in many people’s minds than the thought of being close to a nuclear explosion.  Some people have gone to great lengths, constructing massive bunkers/shelters in their basements, to do what they believe may be necessary to optimize their chances of survival in such cases.  But – two questions :  Are such things really necessary?  And, if they are necessary, will they truly protect you?

Sure, we agree that ground zero would not be a nice place to be at, but the horror and the power of nuclear weapons are often overstated and misunderstood – especially by the ‘anti-nuke’ campaigners; oh yes, and by bunker salesmen, too!  So, let’s first investigate the question – how survivable is a nuclear explosion, and then in a subsequent article series we’ll evaluate the best type of bunker or other shelter structure that would be appropriate for most of us.

The survivability of a nuclear blast depends on several variables (of course).  In particular, it depends on how powerful the nuclear bomb is – and that’s the first variable most civilians fail to account for.  A second variable is how far you are likely to be from the blast (and we consider some of the surprising unexpected considerations related to determining that in the second part of this two-part article).

Other variables include the weather (obviously wind has a massive impact on fallout patterns, so too does rain), the time of day (the nuclear flash will blind more people at night), topography (you might be sheltered by a hill) and ‘urban clutter’ (buildings and other things that occlude and slow down a blast wave more quickly than most theoretical models allow for).

One more huge variable is whether the blast is an air blast (most likely), a surface blast (less blast effect but massively more fallout) or a sub-surface blast (effects depend on how deep the blast is).

How Powerful Are Nuclear Weapons?

Nuclear bombs are measured in terms of the equivalent amount of TNT required to create a similar blast.  Actually, due to various imprecisions, these days they are measured in terms of total energy released which is converted to a theoretical equivalent amount of TNT to make it sound more scary and also more meaningful – if you were told that a bomb had a power of 4.184 petajoules you’d have no idea what that meant, but most people can vaguely comprehend that a one megaton bomb is awesomely powerful.

The 1 MT rating is equivalent to the 4.184 petajoule rating.  You might not be familiar with the ‘peta’ prefix – a petajoule is  1000 terajoules, or 1,000,000 gigajoules or 1,000,000,000 megajoules, or, in the ultimate, 1,000,000,000,000,000 joules – a very big number indeed!

But, back to the usual common measurement of nuclear weapons.  The power of such weapons is usually measured either in kilotons (kT) or megatons (MT), being respectively 1000 tons or 1,000,000 tons of TNT equivalent.

Nuclear bombs range in size from a few kilotons of TNT equivalent power to possibly over 100 megatons of TNT equivalent power.  The smallest that we are more or less aware of were the (withdrawn from inventory more than 30 years ago) W54 series of warheads, with explosive blasts measured in the mere tons or tens of tons of TNT equivalent.

The biggest ever exploded was a Russian bomb, called  the Tsar Bomba, which created an estimated 57 megaton blast, in 1961.

To put these sizes into context, conventional ‘high explosive’ type bombs range from some tens of pounds of TNT equivalent up to the largest GBU-43/B bombs with an 11 ton yield.  Russia might have an even larger bomb with a 44 ton yield.  Most conventional bombs have an under half ton yield.

So that’s the first take-away point.  A ‘nuclear bomb’ can range from something less powerful than a conventional technology bomb, to something of hard to comprehend power and magnitude.

There’s as much as a million times difference in power between a small nuclear bomb and a huge one – that’s like comparing the tiniest firework cracker with a huge 6000 lb conventional ‘bunker buster’ bomb.  Except that, of course, even the smallest nuclear weapon is sort of like a huge 6,000 lb conventional bunker buster bomb, and they just go up from there in scale!

Nuclear Bombs Are Getting Smaller

A related piece of good news.  Although the first decade or two of nuclear bomb development saw a steady increase in size/power, that trend has now reversed.  The two bombs used against Japan were approximately 13 – 18 kT for the Hiroshima bomb and 20 – 22 kT for the Nagasaki bomb; and then for the next fifteen years or so after that, bomb sizes got bigger and bigger.

The largest bombs ever tested were the US Castle Bravo test in 1954 (15 MT – this was actually a mistake, it was planned to be only half that size) and the Russian Tsar Bomba test in 1961 (57 MT).

Since that time, the typical warhead size has gone down again rather than up.  Happily, bigger is not necessarily ‘better’ when it comes to nuclear weapons.  There are several reasons for this.

Due to the increased accuracy of the delivery systems, there has become less need for a massively powerful bomb – a smaller bomb delivered with precision would generally have the same or better effect than a bigger bomb that arrives some distance off target.  Earlier missiles were only accurate to within a mile or so of their target, the latest generation are thought to be accurate to 200 ft or so, so there is no longer a need to have a weapon so powerful that it will be capable of destroying its target, even if it is a mile further away than expected.

Secondly, the evolution of multi-warheaded missiles means that instead of a missile delivering one big bomb to one target, they can now deliver two, three, or many bombs to many different targets, but this requires each warhead to be smaller and lighter (ie less powerful) than otherwise would be the case.

With a single missile having a limited amount of space available and weight carrying capability to transport warheads, and with a fairly direct relationship between a bomb’s power and its weight (and lesserly space), there has been a general favoring to the smaller warheads, although Russia still has a few enormous 20 MT warheads in its inventory.

There is also the surprising and counter-intuitive fact that the effects of a nuclear explosion do not increase directly with the increase in its power – that is to say, a bomb with twice the rated TNT equivalent explosive power does not also have twice as much destructive power; it has more like perhaps 1.6 times the destructive power (the actual relationship is x^0.67).

This means it is better to have two bombs, each of half the power of a single bomb (and better still to have four bombs, each of one-quarter the power).  In terms of maximizing the total destroyed area, if you have a single missile that could have, say one 8 MT warhead, two 4 MT warheads, or four 2 MT warheads, generally this last option would be the most desirable one.  It also means the attacker can choose between sending multiple warheads to one target, or being able to attack more targets.

Furthermore, having four warheads all splitting off from the one missile gives the enemy four times as many objects to intercept.  It is much harder to safely defend against four incoming warheads than one.

So, for all these reasons, multiple small bombs are now usually the preferred approach.

Bigger Bombs Don’t Have Proportionally Greater Destructive Ranges

This statement needs explaining.  There are two factors at play here – the first is that if a bomb is eight times bigger than another bomb, it doesn’t destroy eight times as many square miles (due to the power of the bomb not increasing linearly with its TNT equivalent, as explained in the preceding section).  At the bottom of this page it says that eight small bombs might cover 160 sq miles of area (ie 20 sq miles each), whereas one single bomb, eight times the size, would only cover 80 sq miles.

The second factor is to do with the difference between a bomb’s destructive area and its destructive range.  A bomb’s destructive area spreads out more or less in a circular pattern, but the area of a circle is proportional to the square of its radius.  In other words, for a bomb to have a radius of destruction twice as far as another bomb, it would need to be four times more powerful, not two times as powerful.

So, continuing this example, 80 square miles require a circle with a radius of 5.0 miles, and a 20 sq mile circle has a radius of 2.5 miles.  In other words, to double the distance within which a bomb will destroy everything, and after allowing for both the square relationship between distance and area, and the less than doubling of explosive effect when you double the power of a bomb, you have to increase its explosive power not twice, not four times, but eight times.

This is presented visually in the following diagram, which shows the radius of the fireball created by bombs of different sizes, ranging from small to the largest ever detonated (sourced from this page).

Don’t go getting too complacent, though.  This is only the close-in fireball – the blast and temperature effects would extend much further than this (although subject to the same proportionality).

Actual Effects and Safe Distances

Now that we start to talk about actual damage and death, it is important to realize that these things are not clear-cut.  Apart from extremely close to a bomb’s detonation, where everyone will be killed, and everything destroyed, and extremely far from its detonation, where no-one will be killed and nothing destroyed, in the range between ‘very close’ and ‘safely far away’ there is a sliding scale of death and destruction.  There are zones where 90% of ‘average’ buildings will be destroyed, and other zones where only 10% of average buildings will be destroyed, and the same for where varying percentages of people may be killed or injured.

As can be seen from pictures taken after the explosions in Hiroshima and Nagasaki, even very close to the blast centers, some buildings remained standing, while other buildings, relatively far away, were destroyed.  There’s a lot more to whether buildings and people survive than just distance from the blast, and one of the factors is best described as ‘luck’.

So the numbers we give below are very approximate.

To be specific, a 20 MT warhead (the largest in Russia’s arsenal) would send lethal radiation about 3 miles, almost all buildings and many people would be killed by blast effects up to 4 miles away, and third degree burns (the most serious) would be inflicted on people in direct line of the blast up to 24 miles away (see the table below, taken from the Wikipedia article on this page).

 

Effects	Explosive yield / Height of Burst
	1 kt / 200 m	20 kt / 540 m	1 Mt / 2.0 km	20 Mt / 5.4 km
Blast—effective ground range GR / measured in km
Urban areas completely levelled (20 psi or 140 kPa)	0.2	0.6	2.4	6.4
Destruction of most civilian buildings (5 psi or 34 kPa)	0.6	1.7	6.2	17
Moderate damage to civilian buildings (1 psi or 6.9 kPa)	1.7	4.7	17	47
Railway cars thrown from tracks and crushed (62 kPa; values for other than 20 kt are extrapolated using the cube-root scaling)	≈0.4	1.0	≈4	≈10
Thermal radiation—effective ground range GR / measured in km
Conflagration	0.5	2.0	10	30
Third degree burns	0.6	2.5	12	38
Second degree burns	0.8	3.2	15	44
First degree burns	1.1	4.2	19	53
Effects of instant nuclear radiation—effective slant range SR / in km
Lethal total dose (neutrons and gamma rays)	0.8	1.4	2.3	4.7
Total dose for acute radiation syndrome	1.2	1.8	2.9	5.4

 

With most bombs likely to be 1 MT or less, the column in the table for 1 MT devices is perhaps most relevant.  If you have a well-built retreat, then as long as you are, say, 5 miles or more away from the detonation, your retreat will remain standing.

As for yourself, it would be nice to be a similar distance away to keep your own overpressure experience to a minimum (ie under 20 psi, although the body may survive up to 30 psi according to page 4-5 of this FEMA document).

There is also a need to avoid the lethal radiation, which will reach out about 2 miles, with diminishing degrees of lethality as you get further away from the blast – for example, you’ll have a 50% chance of dying from radiation (but not so quickly) if you are within 5 miles.

But your biggest worry (ie the threat reaching out the furthest) will be the flash and temperature effects.  If you are outside, you don’t want to have the bad luck to be looking at the bomb (especially at night), and ideally you’d be more than 13 miles from it to avoid even first degree burns.  At 10 miles, you’ll start to get more severe second degree burns, and while normally survivable, in a situation with diminished medical care available, these would be life threatening.  However, if you are inside, you can safely be closer, because the walls of the structure will insulate you from the heat and flash.

So, to summarize, with a 1 MT bomb, you’ll die from either burns or radiation or blast if you are within 5 miles of the blast.  If you’re not sheltered from the direct heat flash, you’ll die from burns if you’re within about 13 miles of the blast.

If you are indoors, then your structure may collapse around you (and on top of you) if it is within 5 miles of the blast, and if it is constructed from flammable materials (ie wood in particular), it might catch fire if within 7 miles.

There is one more immediate risk to be considered.  The blast is going to transform all sorts of things into dangerous flying objects.  You might survive the initial blast itself, only to be skewered by a flying telegraph pole a minute later, or be cut and bleed out from splinters of flying glass.

Here’s the thing – the blast wave travels more slowly than the initial flash.  So if you perceive an enormous flash, you should urgently take cover away from windows or weaker external structures, and wait several minutes until the hail of debris has subsided before venturing out.

Lastly for this part, here’s an interesting web program that shows the estimated ranges of the various effects of a nuclear explosion.  You can choose the power of bomb and where it is detonated, and see its coverage effects accordingly.

In our opinion, the ranges it shows are slightly over-estimated and fail to consider topography and other real-world factors, but it is probably acceptably accurate for the purposes it was created for, and on the basis of ‘better safe than sorry’ it does no harm to consider its results carefully.

Read More in Part Two

This first part of our two-part article has covered the immediate dangerous effects of a nuclear explosion that will occur within the first five minutes or so of a bomb blast.

But unlike a conventional bomb, don’t think that if you survive the first five minutes, then you’re safe.  There’s much more to consider, starting from perhaps about thirty minutes after the blast first occurred.  Please now turn to the second part to learn about the secondary and longer term effects of a nuclear explosion.