CHAPTER X. The Radioactive Cloud

The Radioactive Cloud

In February 1954 preparations were made on Bikini Atoll for the explosion of a hydrogen bomb. March 1 was the “ready” date. It did not seem probable that the shot would actually be fired on that date because the shot could be fired only under quite favorable wind conditions. Large amounts of radioactivity, especially fission products, were expected from the explosion. The shot could be fired only if no inhabited places lay in the downwind direction.

Bikini is an oval-shaped coral reef, an atoll. It is one of several such atolls belonging to the group called the Marshall Islands. If you look at the map, you will see that west of Bikini at a distance of 200 miles lies Eniwetok, on which our people were making preparations for further tests.

To the east of Bikini, a hundred miles or so, is Rongelap Atoll. At that time 64 people were living there. They lived primitively in palm houses on the southern part of the atoll. The northern part was uninhabited.

On nearby Ailinginae Atoll 18 of the Marshallese islanders were on a fishing expedition, while farther to the east on Rongerik 28 American servicemen were stationed. The servicemen lived and worked in aluminum huts. Their main job was to collect weather data.

Much farther to the east, 300 miles from Bikini, is Utirik. One hundred and fifty-seven Marshallese people lived on this atoll.

Early on the morning of March 1, a Japanese fishing boat lay somewhere to the north of Rongelap. Her name was Fukuryu Maru, which means in English the Fortunate Dragon. There were 23 men on board. Actually she was in a patrolled zone but had not been sighted by the patrol aircraft.

Operations for the test were being directed from ships of Joint Task Force 7. For several days prior to the morning of March 1, the weathermen had been mapping the winds. A wind to the west would be bad for Eniwetok. A wind to the east might hurt Rongelap and Rongerik. A wind to the south could affect Kwajalein. The ideal direction would have been due north, but this probably would not happen for months. On “shot” morning the wind was blowing to the northeast. The meteorologists gave their “O.K.” It was at dawn, the first of March, 1954.

The firing crew of nine people led by a man of considerable experience, Jack Clark, were responsible for the final arrangements. They were in a blockhouse on the south side of the atoll 20 miles from the bomb. Others, more than 1000 people, watched from shipboard under the direction of Al Graves, who was responsible for the technical phases of the operation. The ships lay south and a little east of Bikini.

The firing mechanism was set into operation in the blockhouse. One after another signals indicated that the various experiments and observations were set to work. Finally a red light went off and a green light appeared on the panel. This meant that the bomb had been detonated.

The men on shipboard watched the enormous fireball through darkened glasses. The firing crew, sealed off in the blockhouse, saw nothing. A couple of long seconds and Graves’ voice announced over their radio: “It was a good shot.” A quick estimate indicated 15 megatons.

Some more slow seconds and the expected ground shock arrived. It was like a big earthquake. A bad moment passed. The blockhouse rocked but held.

Another minute or so and the air shock passed over. One could hear the hinges groan—but this was no longer frightening.

Would the water wave pour over the blockhouse? Everything was watertight. After fifteen minutes a porthole was opened—no water came in. The men in the blockhouse emerged to look at the drifting atomic cloud.

While they watched, Jack Clark’s radiation instrument began to show a reading. The firing crew was called back into the blockhouse. There, in the lowest corner shielded by a considerable amount of sand, they were safe. Outside, the evaporated and condensing coral came down in pellets carrying more and more radioactivity.

In the meantime there was fallout on the ships too. The wind had definitely veered after shot time. Quickly the activity was washed down. No one got a dangerous exposure. But it was wiser to sail away. A message was sent to the blockhouse: “We will come back for you in the evening.”

After a little more than an hour the activity around the blockhouse started slowly to decrease. The firing crew waited patiently inside without communication, without light for the rest of the day.

Finally the ships came back. At sundown a helicopter went out to the island using the last of daylight and allowing as much time as possible for the activity to decay. Clark and his friends rushed out of the blockhouse wrapped in sheets to stop the beta rays and keep off the radioactive dust. They moved as fast as possible to avoid unnecessary exposure.

It was a hard experience but they got no more than two roentgens—no more reason to worry than if they had had a medical X-ray. Toward the east, however, some people were in real trouble.

Six or seven hours after the shot the American servicemen on Rongerik noticed a mistlike fallout of highly radioactive dust. The wind had veered enough to carry the atomic cloud over the occupied islands of Ailinginae, Rongelap, and Rongerik. In the anxious hours which followed no one could say how much damage had been done.

The Americans on Rongerik had had some education in the dangers of radioactivity. They washed themselves, put on extra clothes, and remained inside of the aluminum huts as much as possible. These actions helped to protect them against beta ray burns on the skin. The Marshallese on Rongelap and Ailinginae knew nothing of the danger and took no precautions. Many of them suffered quite severe skin burns.

All of the exposed persons were evacuated to Kwajalein as soon as the Task Force facilities would permit. But it was not until a week or so after the explosion that arrangements could be made for men with radiation measuring instruments to tour the atolls and determine what the levels of exposure had been.

On the southern tip of Rongerik they measured the activity and calculated that the American servicemen had received approximately 78 roentgens. This was good news because a dosage of 50 to 100 roentgens is not lethal and only in rare cases leads to any sickness. In any event full recovery could be expected within a few days.

As they prowled around Rongerik atoll, the measuring crew found places where the radiation levels had been much higher. At the northern end a person would have received more than 200 roentgens.

On Ailinginae the measured values were comparable to those on Rongerik. The estimated dosage to the Ailinginae people was 69 roentgens.

On Rongelap the situation was much worse. Measurements in the southern part of the atoll showed that the Rongelap people had gotten a dose of about 175 roentgens. Such a dose would not be fatal, but at least some of the people would probably be sick.

The crew then went on to explore the rest of the atoll. As they moved north, the dose levels rose higher and higher. In the middle of the atoll, only ten or fifteen miles from the inhabited part, a person would have received 400 roentgens of radiation. At this level he would have a fifty-fifty chance of surviving.

On the northern tip of the atoll, about thirty miles away, the dose would have been over a thousand roentgens. Such a dose means certain death in less than a month.

The following table contains a summary of what happened:

Number Time of Time of Dose of fallout evacuation (roentgens) persons after after shot shot (hours) (hours)

Rongelap 64 4 to 6 51 175 Ailinginae 18 4 to 6 58 69 Rongerik 28 7 32 78 Utirik 157 22 65 14 Fortunate Dragon 23 4 200

On Kwajalein the Marshallese were cared for and underwent medical observation. As soon as possible their skin and hair were scrubbed with soap and water. The coconut oil in their hair made decontamination difficult.

During all this time the presence of the Japanese fishing boat in the area was not even suspected. Not until two weeks after the explosion, when the little boat returned to Yaizu harbor, did the world find out. By this time the 23 fishermen were pretty sick. We do not know precisely what dose the fishermen received, but the best guess is about 200 roentgens. Unhappily, one of the fishermen died, presumably from complications associated with the exposure to radiation. The other 22, however, are in good health and back at work.

Our medical information on the Marshallese islanders is complete. After staying three months on Kwajalein they were removed to Majuro atoll, where homes were built for them and where they have been cared for and under continuous surveillance since the incident. Frequent and thorough medical examinations have been conducted, handicapped somewhat by the problem of communicating through an interpreter.

In the first twenty-four hours some of the victims complained of nausea, fever, and stomach-ache. But these symptoms abated promptly in every case without treatment. There was also some complaint of skin itching and a burning sensation, but these symptoms also lasted only a couple of days. Then followed a week or so of comfort and no complaint. After that skin lesions and loss of hair began to occur.

Fifty to eighty per cent of the beta rays during the exposure period had an average energy of 0.3 million electron-volts. Much of this energy was stopped in the outer layer of skin, which is two thousandths of an inch thick. The remainder of the beta rays had an average energy of 0.6 million electron-volts; these beta rays could easily penetrate into the deeper layer of live skin. The most important fact, however, was that clothing of any kind, even a thin cotton fabric, provided protection against all the beta rays. Lesions developed only on the exposed parts of the body and in a few other places such as the armpits and the creases of the neck where material tends to accumulate. Bare feet were especially bad. During the acute period some of the people walked on their heels.

At the end of six months lost hair had grown out again unaltered in texture and color, and the skin lesions had healed. Everyone appeared healthy and normal with no apparent after effects.

There had been four pregnancies amongst Rongelap women at the time of the exposure. One baby was born dead, but the other three were quite normal. There was no evidence that the stillbirth had been due to radiation effects. In fact the percentage of stillbirths amongst the Rongelapese is normally high. Statistically, one in four is not an unusual ratio.

Today, more than three years since the accident, all of the Marshallese and American victims seem to be fully recovered. No malignancies or leukemias have shown up, but these long-term effects are still being carefully watched for by an AEC medical group.

All in all some serious but limited harm has been done. It was a close shave. To see how close, one only needs to glance at the map below, which shows the roentgen dosage for 48 hours of exposure. At the southern tip of Rongelap, where the inhabitants lived, the dosage was 175 roentgens. But at the northern tip, less than thirty miles away, the dosage was more than a thousand roentgens. If the wind had veered just a little bit farther to the south, probably all of the people on Ailinginae, Rongelap, and Rongerik would have been killed.

This shot proved what had been argued for many years: that radioactivity is not just an incidental part of an atomic explosion. The people on Rongelap were far outside the area of danger from blast and thermal effects. But they received a sizeable dose of radiation. In fact, a person could have stood unprotected at a distance of thirty miles from the explosion and been perfectly safe from the blast and thermal radiation. But at that same distance in a downwind direction he would have accumulated a lethal dose of radiation within a matter of minutes after the fallout began.

Because of the radioactive fallout, the test sites must be located in remote parts of the world. It would be desirable if sites could be found which are so remote from populous areas that the tests could be conducted without regard to the direction of the winds. Unfortunately the bombs are too big and the planet is too small.

As a result the winds must be watched before every test; and the tests must be delayed until the winds are favorable. What happened to the Marshallese was an accident which might have been avoided if the winds had been blowing more directly toward the north at shot time. Since this accident the wind requirements for the tests have become far more stringent, our knowledge of the danger has increased, and the rules of safety have in all respects improved. Many large yield weapons have been tested since March 1, 1954, but no other accidents have occurred. We can be confident that accidents of this kind are now very improbable.

At the U. S. test site in Nevada there has been no instance of a major fallout on a populated area. Probably the most worrisome situation which has occurred there was in the spring of 1953 during the Upshot-Knothole test series. After the ninth shot of the series the cloud drifted eastward over St. George, Utah, a town of about 5000 people. Some fallout occurred shortly before nine o’clock in the morning. About nine-thirty AEC officials issued a warning advising the residents to stay indoors. By noon the warning was withdrawn and people were allowed to continue with their normal affairs. The incident left everyone a little bit scared, but no one had received a radiation dose greater than two or three roentgens.

We have been talking about the local fallout which occurs within a few hundred miles of the test site. Not all the radioactivity which is made in the explosion goes into this fallout. Some of it travels for really long distances, not hundreds but actually thousands of miles from ground zero. This part of the radioactivity is disseminated world-wide and completely escapes the control of man. To be sure, by the time this radioactivity is distributed over a large fraction of the earth’s surface, the dosage levels of radiation are very tiny, less than a ten thousandth of a roentgen for a megaton explosion. There is no danger whatever that a person would die or even become mildly sick from this amount of radiation. There is, however, the possibility of long-range effects such as bone cancer, leukemia, and genetic mutation.

The world-wide danger is, of course, primarily due to the big bombs. The little ones, such as are tested in Nevada, release about ten kilotons (TNT equivalent) of fission energy. Some of the big ones in the Pacific release a few megatons of fission energy. Since the amount of radioactivity is proportional to the fission energy released, one big bomb is equivalent to several hundred or possibly a thousand little ones. Altogether in Nevada, to date, there have been only sixty or seventy shots. It may be desirable to minimize the world-wide fallout from the big shots in the Pacific. But for the little shots in Nevada, it is probably more important to minimize the local fallout. How much radioactivity goes into the local fallout, how much into the world-wide, and how these relative amounts can be controlled, are the main topics for the remainder of this chapter.

Not all the radioactivity which is made in the explosion contributes to the fallout, either local or world-wide. Some of the radioactive fission fragments (gamma emitters) have such short half-lives that they actually disintegrate before the bomb has disassembled. A great many others disintegrate in the first few minutes while the atomic cloud is rising. The energetic beta and gamma rays released in these early, rapid disintegrations are stopped in short distances and merely add to the havoc at the scene of the explosion.

1. A shallow underground explosion. The radioactivity and the ground dirt are thoroughly mixed.]

2. An atomic test tower—five hundred feet high.]

3. A tower shot. Ground dirt rises along the stem, but very little actually mixes with the fireball.]

4. An air shot—3,500 feet above ground. No dirt.]

From a chapter by Vaughan, Tutt, and Kidman in the book Biological Hazards of Atomic Energy, edited by Haddow, published by Oxford University Press, 1952]

From an article, “The Late Effects of Internally Deposited Radioactive Materials in Man,” by Aub et al., in Medicine—a professional journal, Vol. 31, No. 3, September, 1952]

7. Capsules of cobalt⁶⁰, shielded in a water tank. One hundred and thirty million dollars’ worth of radium, twice the world’s present supply, would be needed to equal the rays from this powerful gamma source.]

8. Cobalt irradiation.]

9. The smoke-ring cloud from the air-defense atomic weapon.]

10.]

11. The streaks are condensation trails produced by charged particles in a Wilson Cloud Chamber. They appear bright because the chamber is illuminated and the condensation trails reflect light just as an ordinary cloud does.]

12. Another picture in the Wilson Cloud Chamber. A large number of closely-spaced tracks form a cloud. (The tracks are curved because of the presence of a magnetic field.)]

13. Cutaway section of a nuclear reactor. The heart of the reactor is a small region at the center where the fission energy is generated. Most of the weight and volume are needed for cooling apparatus and shielding material to keep in nuclear radiation.]

For the radioactivity to affect areas at a large distance from the point of the explosion, considerable time must elapse while the atomic cloud rises and drifts in the horizontal winds. During this time more disintegrations occur, due mainly to the short-lived nuclei. The rate at which they occur keeps diminishing as the short-lived nuclei disappear. Roughly speaking, the rate diminishes simply in proportion to the time. More precisely, the rate drops somewhat faster, decreasing by a factor of ten when the time increases by a factor of seven. A minute after the explosion the activity is less than one per cent of what it is at a second. After an hour it is less than one per cent of its value at a minute. This law for the decrease in activity of fission products is, of course, quite different from the simple law of radioactive decay. The latter law applies to a single radioactive species. The fission products consist at any instant of many different radioactive species. Each one obeys the simple law of radioactive decay, but the totality follows a different law.

It should be kept in mind that the product nucleus of a radioactive disintegration may itself be radioactive with a different half-life. For example, there is strontium⁹⁰. Only a small amount of this isotope is made directly in the fission process. The fission process yields large quantities of krypton⁹⁰, which decays with a half-life of one-half minute into rubidium⁹⁰. The latter has a half-life of three minutes and decays into strontium⁹⁰. This is how practically all of the strontium⁹⁰ is made in the explosion. Thus both the intensity and the nature of the radioactivity keep changing with time.

These facts are important because they determine the magnitude and the character of the danger when the radioactivity finally falls out of the cloud and is deposited on the surface of the earth. Those radioactive particles which disintegrate while still in the cloud need not worry us since this radiation can have no effect on living organisms that may be underneath. Provided that the cloud is more than a few hundred feet above the ground, the beta and gamma rays released in these disintegrations merely dissipate their energy in ionizing the air.

The time which the radioactive debris spends in the cloud depends most critically on one factor: the proximity of the explosion to the ground surface. The nature of the surface, whether it is soil or water, also plays a role. If the explosion has taken place right on the ground, on a soil surface, a lot of big, heavy dirt particles become incorporated into the fireball and begin to fall under the action of gravity even before the cloud stops rising. This fallout continues for a period of several hours to perhaps a half day. At the same time some of the radioactive fission products which have adhered to these dirt particles also fall out. This is the origin of the so-called close-in or local fallout, which extends for a distance downwind of the explosion of a few miles to a few hundred miles, according to the energy of the bomb and the strength of the winds. Approximately eighty per cent or so of all the fission products are accounted for by this close-in fallout in the case of a surface explosion. The shot on March 1, 1954 was of this variety.

There are several possibilities for influencing the amount of close-in fallout. One is to explode the bomb over deep water. In this case the close-in fallout amounts to between thirty and fifty per cent. This is because many of the water drops to which radioactive particles have adhered evaporate before they hit the ground. Over shallow water, however, if the fireball actually touches the bottom, the close-in fallout resembles the case of a land explosion and is again about eighty per cent or so. The close-in fallout for underground or underwater explosions will be even higher than for the surface explosions. In fact a really deep underground or underwater explosion would be completely contained and no activity would be spread around.

Another possibility for reducing the close-in fallout is to detonate the bomb on a tower so tall that the fireball cannot touch the surface. In this case the amount of close-in fallout is reduced from eighty per cent to approximately five per cent. Of course, it is not feasible to build towers for really big bombs whose fireballs may be a mile or so in diameter. In this case the bomb might be dropped from an airplane to produce the same effect. The Hiroshima explosion was an example of an air burst of a small bomb. The close-in fallout in that case was very small. Such radiation sickness as occurred there was due to the direct gamma rays and neutrons released in the explosion itself.

In the case of a near-surface explosion, where the fireball almost touches the ground, the close-in fallout is also only about five per cent. This is a somewhat surprising fact since in this case photographs show large quantities of surface material being sucked up into the cloud, just as they are in a true surface explosion.

This material certainly consists of large, heavy dirt particles which subsequently fall out of the cloud. Yet most of them somehow fail to come in contact with the radioactive fission products.

This peculiar phenomenon can be understood by looking at the details of how the fireball rises. At first the central part of the fireball is much hotter than the outer part and thus rises more rapidly. As it rises, however, it cools and falls back around the outer part, creating in this way a doughnut-shaped structure. The whole process is analogous to the formation of an ordinary smoke ring. In most of the photographs one sees, the doughnut is obscured by the cloud of water that forms, but sometimes when the weather is particularly dry, it becomes perfectly visible. During the rather orderly circulation of air through the hole, the bomb debris and the dirt that has been sucked up remain separated. (See pictures 1-4.)

The close-in fallout accounts for only a portion of the radioactivity, ranging from less than a per cent for a high altitude shot to almost complete deposition for some ground shots. For the world-wide fallout we are interested in what happens to the remainder. This depends on how the atomic cloud is carried by the upper winds for long distances. In this connection it is important to distinguish between a big bomb and a little bomb. It is also important to distinguish between the lower and higher portions of the atmosphere called, respectively, the troposphere and the stratosphere.

The atmosphere is heated by the sun in an indirect way. The sun’s rays pass through air without warming it. They heat up instead the bottom of the atmosphere, that is, the solid ground. The atmosphere is heated in the same manner in which a boiling pot is heated on the kitchen range. The heat is delivered from below and is carried in rising currents to the top.

Only in the case of the atmosphere there is no sharp upper limit. The currents rise to an altitude of thirty to fifty thousand feet, then turn and descend. This boiling part of the atmosphere is called the troposphere or region of heat. Above it there is less vertical motion. The upper region is called the stratosphere or stratified region.

For a little bomb the atomic cloud stops rising before it reaches the stratosphere. For a big bomb, above about a megaton of energy (a million tons of TNT equivalent), the cloud pokes right into the stratosphere and keeps going to a height of a hundred thousand feet or so.

The most important fact about the stratosphere is this: It has very little weather. Most of the weather phenomena such as clouds, rain, snow, fog, mist, etc., are confined to the lower portion of the atmosphere, the troposphere. The stratosphere, however, contains practically no water.

Now suppose a little bomb whose cloud will remain in the troposphere has been exploded at one of the United States test sites. The Nevada test site is at a latitude of 37°N and the Pacific test site at 12°N. In these middle latitudes, in the troposphere, the winds blow mainly from west to east with an average speed of approximately 20 miles an hour. There will be a slight southerly or northerly motion on top of this. But by and large the radioactive cloud will stay in a pretty narrow band around the latitude at which the explosion took place.

After the first few hours, when the close-in fallout has dwindled, the radioactive particles remaining in the cloud are too light and too fine to fall any more under the action of gravity. At this point the weather becomes important. Rain, fog, or mist captures the radioactive particles, and returns them to the ground in the rainfall. This results in the so-called tropospheric fallout. The average time for this fallout to occur is approximately two weeks to a month. During this time, while staying more or less in the latitude of the explosion, the radioactive particles may actually have encircled the earth.

The clouds of the big bombs rise high into the stratosphere. The winds in the stratosphere do not blow so predominantly in a latitudinal direction. What is more important, they stay in the stratosphere for years, in which time the radioactivity is distributed to all areas of the globe. The fallout from the big bombs is thus really world-wide.

The tropospheric fallout takes about a month. The stratospheric fallout takes 5 to 10 years. The reason for this difference is the weather, or rather the lack of it. In the stratosphere there is no rain or fog to catch the radioactive particles and hence no effective mechanism for producing the fallout. In fact, since the radioactive particles are too fine to fall by gravity, they must simply wait until some turbulent motions impel them downward back into the troposphere. This process requires a long time.

That rainfall is the most important mechanism for producing the world-wide fallout has been shown by examining the fallout in certain dry regions of southern California and South America. In every case the fallout was found to be considerably sub-normal. In one place in Chile, where there is never any rain, the fallout was found to be only one per cent of what might be expected on the basis of the average fallout at the same latitude.

In regions having at least a few inches of rain per year, the fallout tends to be proportional to the rainfall on the average. However, the proportionality to rainfall depends on the nature of the weather so that, say, twenty inches of rain in one part of the world may not give as much fallout as the same amount of rain in other weather zones. We are rapidly learning about this.

Having said what the age is of the various kinds of fallout, we are in a position to say which radioactive species are still present when the radioactivity is deposited on the ground. The close-in fallout, being only a few hours old, still includes many short-lived isotopes, which disintegrate before there is a possibility of ingestion or inhalation into the body. Consequently the danger from the close-in fallout results from external exposure, mainly to gamma radiation on the whole body, and to a lesser extent to energetic beta rays on the skin. Clothes and ordinary housing provide relatively little shielding against gamma rays. Special protective shelters are needed. During a war if the enemy were to bomb our cities with super-megaton weapons surface-burst, the close-in fallout would be a far greater agent of destruction against an unsheltered populace than either blast or thermal radiation.

In the stratospheric world-wide fallout, however, all of the short-lived radioactivity has disappeared, since a period of many years has elapsed since the explosion. After a year or so the only gamma emitter which is left in appreciable quantity is cesium¹³⁷, with a half-life of 30 years. Its gamma ray, however, is not very penetrating. In spite of this fact cesium¹³⁷ is considered to be the second most important hazard for the long term fallout. The first is strontium⁹⁰, which is a beta emitter with a half-life of 28 years. This is long enough so that most of these nuclei will still be present even after spending a long time in the stratosphere. Since strontium is chemically similar to calcium, it contaminates our foodstuffs and is easily incorporated into our bodies. Once inside it stays for long periods of time, deposited in our bones. We shall see in a later chapter how serious this danger may be.

The tropospheric fallout, and to a lesser extent, the stratospheric, includes some other radioactive species besides cesium¹³⁷ and strontium⁹⁰, and we shall discuss these in the next chapter. But by and large they are of little consequence (with the possible exception of iodine¹³¹) either because they are not easily absorbed in the body or else because their radiation is not very energetic. The world-wide hazard is thus narrowed down to just two isotopes, an internal beta emitter and a weak gamma emitter.