A:

Half of all the energy released by nuclear explosions is in the form of blast and shock and about 35% is in the form of heat. The following four drawings show what level of blast damage (at different psi overpressure) and fire ignition from the thermal pulse might be expected for different strength nuclear explosions (both ground and air bursts) at different distances from ground zero. Take note of the damage range distances from GZ - ground zero. (Courtesy of Nuclear Attack Environment Handbook, FEMA - August, 1990)

Obviously, the bigger the weapon yield the larger the area of overpressure damage from the blast wave. But, notice that the damage range does not increase in a linear fashion with the more powerful explosions. For instance, comparing the 200 KT air burst with the five times more powerful 1 MT air burst, the range of moderate damage and initial fires increased from only 4.3 miles to 7.3 miles. This is because the reach of blast and fire effects varies as the cube root of the weapon yield ratio and the cube root of 5 is 1.71. So, instead of a five-fold increase or 500% we have only about a 70% increase in this comparison.

A readily portable terrorist nuclear bomb would likely be only a fraction as powerful as the examples above, but for reference, the Hiroshima nuclear bomb was only a 15KT air burst. (The RA-115 backpack nukes reported missing from Russian stockpiles are one kiloton yield each, and they would most likely be surface exploded.)

As noted above, blast effects drop off quickly with distance. At Hiroshima a brick building survived only 640 feet from ground zero. And less than a mile away a trolley car remained intact and on its tracks.

For concerns of a future Soviet attack, the current thinking is that with the continuing trend towards more accurate MIRV'ed (multiple, independently targetable, re-entry vehicles) nuclear weapons, they are now mostly smaller than in the past, averaging on the order of 500 KT or less and for submarines only 200 KT. Of course, there are now more warheads per missile (4-10) and they are substantially more accurate than during the height of the cold war. Also, any targeted military installations can expect to receive multiple hits.

Again, we are exploring here only the initial direct effects of a nuclear explosion, and specifically, the shock wave and blast effect. (Thermal Pulse effects will be covered below.)

All buildings will suffer light damage from the shock wave at even 1 psi peak overpressure--shattered windows, doors damaged or blown off hinges and interior partitions cracked. The maximum wind velocity would be only about 35 miles per hour. As the overpressure increases, so does the blast wind--exceeding hurricane velocities above about 2 psi.

It, of course, depends on where you are when it comes charging through, but from a 500 KT blast, 2.2 miles away, it'll be arriving about 8 seconds after the detonation flash. (An even larger 1 MT blast, but 5 miles away, would give you about 20 seconds.) Like surviving an imminent tornado, utilizing those essential seconds after the initial flash to 'duck & cover' could be the difference between life & death for many. Both the overpressure in the blast shock wave and the blast wind are important causes of casualties and damage.

For the man-in-the-open example above, that's 2.2 miles from the detonation of a 500 KT air burst where the shock wave would arrive about 8 seconds after the detonation flash, this sharp body slap would produce a 10-psi overpressure over his body that might perforate his eardrums. Additionally, though, he would experience a blast of wind of about 295 mph for about three seconds that would launch him careening into a probably fatal impact and he would also likely suffer injuries from flying missile fragments of glass and debris. See the following chart from A. Longinow People Survivability in a Direct Effects Environment and Related Topics:

Again, though, as in a tornado, prompt protective actions can make a great difference in ones survivability. For example, it requires about eight times the blast wind force to move a person who is lying down compared to a standing person. Diving into a ditch, depression, basement or anywhere else normally thought of for tornado protection will improve your odds greatly. You are also much less a target for glass shards and debris missiles. This simple change in vulnerability, but of this magnitude, can save many lives.

Regarding the Thermal Pulse that accompanies the thousand suns brighter flash, that represents 35% of the energy expended in a nuclear explosion, burns caused by this heat energy of the fireball can produce the most far reaching consequence of the immediate weapons effects. For our example above of the man-in-the-open, 2.2 miles from a 500 KT air detonation, fatal blast injuries would have served in most cases to put him out of his misery. The thermal pulse, travelling at the speed of light, would have already delivered lethal burns and his clothing would have burst into fire if truly exposed in the open. In fact, about 50% of those fully exposed to the fireball anywhere in the 2 psi or greater range would eventually die from the severity of their burns.

However, if there is fog or haze or any kind of opaque material or structure between people and the location of the fireball the effects of the thermal pulse can be greatly reduced. With medium haze it can be cut by 50% and with heavy fog down to even just 10%. So, smog in the big cities could actually be partly protective for once. Also, while it arrives at the speed of light and delivers most of it's energy within the first second, the larger the bomb the longer it'll take to deliver its full compliment of thermal energy, perhaps even several seconds. Quickly diving behind anything creating a shadow could be lifesaving.

In most places however, besides fog, smog, haze or clouds, there are buildings, trees, hills and other objects that would also block and reduce some portion of the thermal pulse. In fact, the more densely built up an area is then the less likely the inhabitants would be exposed to suffer the full impact of the thermal pulse. Of course, though, they may still have to deal with the resultant fires created by the thermal pulse and from any blast damage.

Bottom Line: The majority of Americans, even in a full-scale all-out nuclear war, would survive the initial blast and thermal effects of nuclear explosions. Even with a large 1 MT explosion and being as few as 8-10 miles away from ground zero, you would likely find that you had survived the initial thermal, blast and shock wave. With any kind of prompt protective action your odds of surviving at even half that distance are quite high. Also, increasing your odds, is that our military installations would be the primary targets and a multitude of thousands of purely civilian concentrations (cities & towns) would be of much less importance strategically to have wasted a nuke on in a first strike. (With the exception of our nations capital and militarily important targets in or adjacent to cities.) In the next Q&A section entitled: What Areas are Likely Targets? we'll explore fully if you are likely residing at a probable 'ground zero'. Also, it needs to be mentioned here, with the much smaller yield and resulting blast damage area of a likely terrorist nuclear weapon, your odds of being in the wrong place at the wrong time are even more remote.

But, remember, too, we've only been talking here, so far, about the initial blast and thermal damage...