occurs. It is proper to attempt evasive action to prevent burns if one is caught outside a shelter or in an exposed position. Like the heat from the sun, the heat from a nuclear explosion can be attenuated by an opaque substance which will cast a shadow. Adequate shelter can provide satisfactory protection.

The interiors of shelters with open doors near points of detonation may become hot enough in high pressure regions to cause burns. In Japan burns were received by sheltered persons who could not be seen by the fireball. The phenomenon was also observed in biomedical shelter tests at the Nevada test site. The mechanism is not completely understood.

BLAST

The light, nuclear radiation, and thermal radiation travel with velocities comparable to the speed of light, that is 186,000 miles per second. The blast, the last of the immediate effects, travels at lower speeds. Upon leaving the surface of the fireball, the blast wave is moving at many times the speed of sound, 750 miles per hour or 1,100 feet per second, but it is slowed down as it progresses and soon degrades to approximately the speed of sound. A part of the energy released in the form of blast may be transmitted to the earth and produce ground shock. Its damage potential in the earth is much less than the damage potential of the blast in air.

EFFECTS ON UNDERGROUND SHELTERS

Mr. HOLIFIELD. I would like to question you on that.

In the event of a nuclear explosion at ground zero, how much shock is emitted from, let us say, the perimeter of the cavity which is caused, how much shock is transmitted through the ground?

Let us take a 1-megaton explosion and say it makes a cavity a half a mile wide and maybe 100 feet deep. Now, would shelters outside of the perimeter of that cavity that were underground be able to withstand the earth tremor?

Mr. CORSBIE. We estimate the damage zone from the lip of the crater to extend about three crater radii, and under less than megaton weapons, structures have survived very close to that region, or roughly in the region of the fireball.

Mr. HOLIFIELD. That is above ground?

Mr. CORSBIE. Above ground.

Mr. HOLIFIELD. I am talking about the so-called 3 feet below ground radiation shelter, which is commonly accepted.

Mr. CORSBIE. It would survive more easily below ground than above ground. The transmittal of energy through the ground would accelerate the structure. I think it would not be too difficult to design a structure strong enough to withstand the forces. One would have to give careful attention to objects inside the shelter to prevent them being blown about with damage to both equipment and potential hazard to occupants.

Mr. HOLIFELD. But you would start gaining lives from a short distance outside the lip of the crater if people were underground? Mr. CORSBIE. Yes, sir.

Mr. HOLIFIELD. In other words, the flash burn would not affect them, because it would not go through 3 feet of earth.

The rolling ground shockwave would naturally seek the point of least resistance, which would be the atmosphere. And, therefore, while there might be some disturbance or shock wave felt in the ground for some distance, it would not be sufficient to destroy a concrete shelter underground, or even a quonset hut-type of shelter underground, would it?

Mr. CORSBIE. It would be very violent. But a structure underground is very resistant, essentially it moves with the earth, accelerated both upward and downward, and in the direction to and fro of the blast.

Mr. HOLIFIELD. Proceed.

BLAST

Mr. CORSBIE. The blast wave or shock is the source of physical damage from nuclear explosions. As would be expected, its destructive range is increased as the yield is increased; however it does not increase directly in proportion to the yield. Ás will be discussed later, blast is a producer of casualties through several mechanisms. Supplementing the damage caused by the overpressure and the blast winds, the blast is a source of secondary fires caused by damage to stoves and furnaces, breaks in electrical circuits and disruption of chemical and manufacturing processes.

COMBINED THERMAL, BLAST, AND RADIATION EFFECTS

It is particularly instructive to view the variations in the immediate effects which appear as the yield of the weapon varies. A chart has been prepared on which relationships are shown for nuclear explosions having yields of 1 kiloton, 20 kilotons, 1 megaton, 10 megatons, and 20 megatons.

The three parameters selected for this description are the following: For the effects of the thermal radiation-the heat-emitted by the fireball we use the range at which second degree burns will occur to the bare skin of exposed persons. Second degree burns are burns with blisters, requiring medical attention.

For the blast effects we shall use the range at which an overpressure of 5 pounds per square inch occurs. An overpressure of 5 pounds per square inch is sufficient to destroy conventional wooden and brick homes and inflict moderate to severe damage on brick apartment buildings. Blast induced injuries will occur in profusion at this pressure. I shall have more to say on the effects on personnel later.

For the initial ionizing radiation, we shall use the range at which a dose of 700 rems will be delivered. A dose of 700 rems to the whole body can produce fatal radiation injury.

On the chart the circles represent the relative maximum sizes of the fireballs of the weapons. (See fig. C-1.)

Mr. CORSBIE. The fireballs are drawn to scale. The diameters range from a little less than 0.1 mile for the 1-kiloton airburst to about 4.6 miles for the 20-megaton airburst.

The bars depict the ranges to the second-degree burn line, the 5pounds-per-square-inch line and the 700-rem line.

For the 1-kiloton airburst, you will observe that the three levels appear at almost the same distance from ground zero.

For the 20-kiloton airburst the levels no longer appear at about the same distance. The second-degree burn line extends to 1.7 miles, while 5 pounds per square inch is at about 1.1 miles and the 700-rem is found at 0.7 mile.

For the 1-megaton explosion, the second-degree burn range is at 9 miles, the 5-pounds-per square-inch range is nearly 4 miles and the 700-rem is at just under 111⁄2 miles. The serious burn area is many times larger than the serious blast area. The area subjected to heavy doses of initial ionizing radiation is relatively small.

For the 10-megaton explosion, second-degree burns can appear at a range of almost 24 miles; 5-pounds-per-square-inch at about 92 miles and the 700-rem range is about 21/4 miles.

For the 20-megaton explosion, second-degree burns can occur at about 32 miles, while the 5 pounds-per-square-inch range is 11 miles, and the 700-rem range is about 2 miles. Over 3,000 square miles may be subjected to the hazard of at least second-degree burns in the event of a 20-megaton explosion.

These are all the immediate effects. In a matter of a few seconds and at the most a few minutes they will have done their primary damage. The secondary fires may develop in from minutes to hours,

as in Hiroshima and Nagasaki, and add to the difficulties of the situation. For the examples air bursts were used and no important local fallout would be anticipated from airbursts. For surface bursts the ranges at which the effects would appear are decreased somewhat, but actually not enough to greatly ease the situation. The surface burst is the source of the early fallout which is the principal delayed effect against which we must defend in nuclear war.

FALLOUT

The fission reaction in nuclear weapons results in the creation of intensely radioactive elements which are called fission products. The radioactive material from megaton weapons used in war may contaminate hundreds and thousands of square miles to the point where the radiation may become a hazard to life.

The mechanism by which the radioactive fallout is produced and reaches the ground has been described in detail in reports, public booklets, films, the press, and other media. Because of the amount of time and space devoted to the subject, fallout in nuclear war should be the nuclear hazard best understood by officials and the public.

In brief, a nuclear weapon, when detonated so that the fireball comes in contact with the surface, draws up a vast amount of earth. The earth is melted and mixed with the fission products in the process; and when the temperature drops, radioactive substances condense with the particles of earth. In a matter of minutes the particles will commence to fall toward the surface and be deposited in a rough circle around the point of detonation under the mushroom cloud and then downwind.

EARLY FALLOUT

In the discussion of fallout it is convenient to consider it in two parts: early fallout and delayed fallout. Early fallout is defined as that which reaches the ground during the first 24 hours following a nuclear explosion. It is the early fallout from surface, subsurface, and low air bursts that is capable of producing radioactive contamination over large areas with an intensity great enough to represent an immediate biological hazard.

The early fallout may be influenced by fractionation, which is a change in composition of the fission product mixture. The occurrence of the phenomenon is shown by the fact that the larger particles deposited near the point of detonation have radiological, physical, and chemical properties different from those of the fine particles which leave the radioactive cloud later and are deposited farther downwind.

DELAYED FALLOUT

Delayed fallout, that arriving after the first 24 hours, consists of very fine, invisible particles that settle in low concentrations over a considerable portion of the earth's surface. The radiation from delayed fallout is greatly reduced in intensity as a result of radioactive decay during the relatively long time that the particles remain suspended in the atmosphere. Delayed fallout radiation generally poses no immediate danger to health, although there may be a long-term hazard.

FALLOUT PATTERNS

As an illustrative device we are accustomed to discussing fallout patterns as rather smooth elliptical figures. In the Pacific and in Nevada the actual fallout patterns have been_characteristically irregular and variable in radiation distribution. For planning purposes we can make use of predictions, but for local, operational purposes there is no satisfactory substitute for measurements.

We should expect overlapping of fallout patterns from two or more detonations in a nuclear attack. There is every probability that an area contaminated by one explosion will receive more fallout from other explosions leading to a more serious situation. In preparing planning assumptions, conditions such as this must be treated; that is, conditions which occur because of the combination of effects from two or more weapons.

INDUCED RADIOACTIVITY

The second delayed effect is that of radiation from neutron-induced radioactivity. Some of the neutrons liberated by the reaction which produces the explosion escape from the environment of the bomb.

The neutrons have a capability of producing radioisotopes just as neutrons do in a reactor. There are present in the air and sea and in the ground elements which are susceptible to neutrons. Nitrogen and sodium are important elements, in this category; other examples are aluminum, silicon, and manganese. Their radioisotopes have short half lives but can cause high initial activity; and hazardous dose rates may exist. Because of the short half lives, the decay progresses rapidly, more rapidly than in fallout.

VARIATIONS DUE TO HEIGHT OF BURST

In a war situation all the effects may not appear at a point in the order in which they have been discussed today; that is, the order in which we would normally expect them at a test. An obvious departure would be the appearance of fallout only whether from one or more detonations. Another would be the arrival of fallout from a distant explosion followed sometime later by the immediate effects, and possibly fallout, from a nearby explosion. But it is proper to remember that there are no known fallout casualties among the 220,000 Japanese hurt and killed in Hiroshima and Nagasaki. The conditions of burst influence the effects quantitatively. There is no single condition of burst which maximizes the severity or range of all the effects. An analysis has been prepared to show the variations which may be expected from different conditions of burst for weapons of similar energy yield. It deals with the light, heat, initial nuclear radiation, ground shock, air blast, and early fallout. (See fig. C-2.)