by Isaac Asimov
The first Nautilus reactor core lasted for 62,500 miles; included among those miles was a dramatic demonstration. The Nautilus made an underwater crossing of the Arctic Ocean in 1958. This trip demonstrated that the ocean depth at the North Pole was 13,410 feet (2½ miles), far deeper than had been thought previously. A second, larger nuclear submarine, the U.S.S. Triton, circumnavigated the globe underwater along Magellan’s route in eighty-four days, between February and May of 1960.
The Soviet Union also possesses nuclear submarines and, in December 1957, launched the first nuclear-powered surface vessel, the Lenin, an icebreaker. Shortly before, the United States had laid the keel for a nuclearpowered surface vessel; and in July 1959, the U.S.S. Long Beach (a cruiser) and the Savannah (a merchant ship) were launched. The Long Beach is powered by two nuclear reactors.
Less than ten years after the launching of the first nuclear vessels, the United States had four nuclear surface ships operating, being built, or authorized for future building. And yet, except for submarines, enthusiasm for nuclear propulsion also waned. In 1967, the Savannah was retired after two years of life. It took 3 million dollars a year to run, and was considered too expensive.
NUCLEAR REACTORS FOR ELECTRIC POWER
But it is not the military alone who must be served. The first nuclear reactor built for the production of electric power for civilian use was put into action in the Soviet Union in June of 1954. It was a small one, with a capacity of not more than 5,000 kilowatts. By October 1956, Great Britain had its Calder Hall plant in operation, with a capacity of more than 50,000 kilowatts. The United States was third in the field. On 26 May 1958, Westinghouse completed a small nuclear reactor for the production of civilian electric power at Shippingport, Pennsylvania, with a capacity of 60,000 kilowatts. Other reactors quickly followed both in the United States and elsewhere.
Within little more than a decade, there were nuclear reactors in a dozen countries, and nearly half the supply of civilian electricity in the United States was being supplied by fissioning nuclei. Even outer space was invaded, for a satellite powered by a small reactor was launched on 3 April 1965. And yet the problem of radioactive contamination is a serious one. When the 1970s opened, public opposition to the continued proliferation of nuclear power plants was becoming louder.
Then, on 28 March 1979, on Three Mile Island in the Susquehanna River near Harrisburg, there was the most serious nuclear accident in American history. Actually, there was no broadcasting of any significant quantity of radioactivity, and no danger to human life, even though there was near panic for a few days. The reactor was, however, put out of action indefinitely, and any cleanup was going to be very long and expensive.
The chief casualty was the nuclear-energy industry. A wave of antinuclear sentiment swept the United States and various other nations, too. The chances of new nuclear reactors being set into operation in the United States have dimmed drastically.
The accident, by bringing home to Americans the terrors of even the possibility of radioactive contamination, seemed also to strengthen public opinion worldwide against the production (let alone the use) of nuclear bombs, and this, to any rational person, would seem to be a good result.
And yet nuclear energy in its peaceful aspect cannot be easily abandoned. The human need for energy is overpowering; and, as I pointed out earlier in the chapter, it may be that we cannot rely on fossil fuels for long or expect a massive replacement by solar energy for some time. Nuclear energy, on the other hand, is here, and there are not lacking many voices who point out that, with the proper safeguards, it is not more dangerous than the fossil fuels but less dangerous. (Even in the particular case of radioactive contamination, it should be remembered that coal contains tiny quantities of radioactive impurities, and that coal burning releases more radioactivity into the air than nuclear reactors do—or so it is argued.)
BREEDER REACTORS
In that case, suppose we consider nuclear fission as an energy source. For how long a period could we count on it? Not very long, if we have to depend entirely on the scarce fissionable material uranium 235. But, fortunately, other fissionable fuels can be created with uranium 235 as a starter.
We have seen that plutonium is one of these man-made fuels. Suppose we build a small reactor with enriched uranium fuel and omit the moderator, so that fast neutrons will stream into a surrounding jacket of natural uranium. These neutrons will convert uranium 238 in the jacket into plutonium. If we arrange things so that few neutrons are wasted, from each fission of a uranium-235 atom in the core we may get more than one plutonium atom manufactured in the jacket. In other words, we will breed more fuel than we consume.
The first such breeder reactor was built under the guidance of the Canadian-American physicist Walter Henry Zinn at Arco, Idaho, in 1951. It was called EBR-1 (Experimental Breeder Reactor-1), Besides proving the workability of the breeding principle, it produced electricity. It was retired as obsolescent (so fast is progress in this field) in 1964.
Breeding could multiply the fuel supply from uranium many times, because all of the common isotope of uranium, uranium 238, would become potential fuel.
The element thorium, made up entirely of thorium 232, is another potential fissionable fuel. Upon absorbing fast neutrons, it is changed to the artificial isotope thorium 233, which soon decays to uranium 233. Now uranium 233 is fissionable by slow neutrons and will maintain a self-sustaining chain reaction. Thus, thorium can be added to the fuel supply, and thorium appears to be about five times as abundant as uranium in the earth. In fact, it has been estimated that the top hundred yards of the earth’s crust contains an average of 12,000 tons of uranium a~d thorium per square mile. Naturally, not all of this material is easily available.
All in all, the total amount of power conceivably available from the uranium and thorium supplies of the earth is about twenty times that available from the coal and oil we have left.
And yet the same concerns that cause people to fear ordinary reactors are redoubled where breeder reactors are concerned. Plutonium is much more dangerous than uranium, and there are some who maintain it is the most poisonous material in the world that has the chance of being produced in massive quantities, and that if some of it were to find its way into the environment, that would be a catastrophe that could not be reversed. There is also the fear that plutonium intended for peaceful reactors can be hijacked or purloined and used to build a nuclear bomb (as India did) that could then be used for criminal blackmail.
These fears are perhaps exaggerated, but they are reasonable; and not only accident and theft gives cause for fear. Even if nuclear reactors work without hint of accident, there will remain danger. To see the reason, let us consider radioactivity and the energetic radiation to which it gives rise.
THE DANGERS OF RADIATION
To be sure, life on Earth has always been exposed to natural radioactivity and cosmic rays. However, the production of X rays in the laboratory and the concentration of naturally radioactive substances, such as radium, which ordinarily exist as greatly diluted traces in the earth’s crust, vastly compounded the danger. Some early workers with X rays and radium even received lethal doses: both Marie Curie and her daughter Irène Jeliot-Curie died of leukemia from their exposures, and there is the famous case of the watchdial painters in the 1920s who died as the result of pointing their radium-tipped brushes with their lips.
The fact that the general incidence of leukemia has increased substantially in recent decades may be due, partly, to the increasing use of X rays for numerous purposes. The incidence of leukemia in doctors, who are likely to be so exposed, is twice that of the general public. In radiologists, who are medical specialists in the use of X rays, the incidence is ten times greater. It is no wonder that attempts are being made to substitute for X rays other techniques, such as those making use of ultrasonic sound. The coming of fissionadded new force to the danger. Whether in bombs or in power reactors, it unleashes radioactivity on a scale that could ma
ke the entire atmosphere, the oceans, and everything we eat, drink, or breathe increasingly dangerous to human life. Fission has introduced a form of pollution that will tax man’s ingenuity to control.
When the uranium or plutonium atom splits, its fission products take various forms. The fragments may include isotopes of barium, or technetium, or any of a number of other possibilities. All told, some 200 different radioactive fission products have been identified. These are troublesome in nuclear technology, for some strongly absorb neutrons and place a damper on the fission reaction. For this reason, the fuel in a reactor must be removed and purified every once in a while.
In addition, these fission fragments are all dangerous to life in varying degrees, depending on the energy and nature of the radiation. Alpha particles taken into the body, for instance, are more dangerous than beta particles. The rate of decay also is important: a nuclide that breaks down rapidly will bombard the receiver with more radiation per second or per hour than one that breaks down slowly.
The rate of breakdown of a radioactive nuclide is something that can be spoken of only when large numbers of the nuclide are involved. An individual nucleus may break down at any time—the next instant or a billion years hence or any time in between—and there is no way of predicting when it will. Each radioactive species, however, has an average rate of breakdown, so if a large number of atoms is involved, it is possible to predict with great accuracy what proportion of them will break down in any unit of time. For instance, let us say that experiment shows that, in a given sample of an atom we shall call X, the atoms are breaking down at the rate of lout of 2 per year. At the end of a year, 500 of every 1,000 original X atoms in the sample would be left as X atoms; at the end of two years, 250; at the end of three years, 125; and so on. The time it takes for half of the original atoms to break down is called that particular atom’s half-life (an expression introduced by Rutherford in 1904); consequently, the half-life of atom X is one year. Every radioactive nuclide has its own characteristic half-life, which never changes under ordinary conditions. (The only kind of outside influence that can change it is bombardment of the nucleus with a particle or the extremely high temperature in the interior of a star—in other words, a violent event capable of attacking the nucleus per se.)
The half-life of uranium 238 is 4.5 billion years. It is not surprising, therefore, that there is still uranium 238 left on Earth, despite the decay of uranium atoms. A simple calculation will show that it will take a period more than six times as long as the half-life to reduce a particular quantity of a radioactive nuclide to 1 percent of its original quantity. Even 30 billion years from now, there will still be two pounds of uranium left from each ton of it now in the earth’s crust.
Although the isotopes of an element are practically identical chemically, they may differ greatly in their nuclear properties. Uranium 235, for instance, breaks down six times as fast as uranium 238; its half-life is only 710 million years. It can be reasoned, therefore, that in eons gone by, uranium was much richer in uranium 235 than it is today. Six billion years ago, for instance, uranium 235 would have made up about 70 percent of natural uranium. Humanity is not, however, just catching the tail end of the uranium 235. Even if we had been delayed another million years in discovering fission, the earth would still have 99.9 percent as much uranium 235 then as it has now.
Clearly any nuclide with a half-life of less than 100 million years would have declined to the vanishing point in the long lifetime of the universe. Hence, we cannot find more than traces of plutonium today. The longest-lived plutonium isotope, plutonium 244, has a half-life of only 70 million years.
The uranium, thorium, and other long-lived radioactive elements thinly spread through the rocks and soil produce small quantities of radiation, which is always present in the air about us. We humans are even slightly radioactive ourselves, for all living tissue contains traces of a comparatively rare, unstable isotope of potassium (potassium 40), which has a half-life of 1.3 billion years. (Potassium 40, as it breaks down, produces some argon 40 and probably accounts for the fact that argon 40 is by far the most common inert-gas nuclide existing on earth. Potassium-argon ratios have been used to test the age of meteorites.)
There is also a radioactive isotope of carbon, carbon 14, which would not ordinarily be expected to occur on Earth since its half-life is only 5,770 years. However, carbon 14 is continually being formed by the impact of cosmic-ray particles on nitrogen atoms of our atmosphere. The result is that there are always traces of carbon 14 present, so that some is constantly being incorporated into the carbon dioxide of the atmosphere. Because it is present in the carbon dioxide, it is incorporated by plants into their tissues and from there spreads to animal life, including ourselves.
The carbon 14 always present in the human body is far smaller in concentration than that of potassium 40; but carbon 14, having the shorter half-life by far, breaks down much more frequently. The total number of carbon-l4 breakdowns may be about one-sixth that of potassiumAO breakdowns. However, a certain percentage of carbon 14 is contained in the human genes; and as these break down, profound changes may result in individual cells-changes that would not result in the case of potassium-40 breakdowns.
For this reason, it might well be argued that carbon 14 is the most significant radioactive atom to be found naturally in the human body. This likelihood was pointed out by the Russian-American biochemist Isaac Asimov as early as 1955.
The various naturally occurring radioactive nuclides and energetic radiations (such as cosmic rays and gamma rays) make up the background radiation. The constant exposure to natural radiation probably has played a part in evolution by producing mutations and may be partly responsible for the affliction of cancer. But living organisms have lived with it for billions of years. Nuclear radiation has become a serious hazard only in our own time, first as we began to experiment with radium, and then with the coming of fission and nuclear reactors.
By the time the Manhattan Project began, physicists had learned from painful experience the dangers of nuclear radiation. The workers in the project were therefore surrounded with elaborate safety precautions. The “hot” fission products and other radioactive materials were placed behind thick shielding walls and looked at only through lead glass. Instruments were devised to handle the materials by remote control. Each person was required to wear strips of photographic film or other detecting devices to “monitor” his or her accumulated exposure. Extensive animal experiments were carried out to estimate the maximum permissible exposure. (Mammals are more sensitive to radiation than are other forms of life; but as mammals, we have average resistance.)
Despite everything, accidents happened, and a few nuclear physicists died of radiation sickness from massive doses. Yet there are risks in every occupation, even the safest; the nuclear-energy workers have actually fared better than most, thanks to increasing knowledge of the hazards and care in avoiding them.
But a world full of nuclear power reactors, spawning fission products by the ton and the thousands of tons, will be a different story. How will all that deadly material be disposed of?
A great deal of it is short-lived radioactivity which fades away to harmlessness within a matter of weeks or months; it can be stored for that time and then dumped. Most dangerous are the nuclides with half-lives of one to thirty years. They are short-lived enough to produce intense radiation, yet long-lived enough to be hazardous for generations. A nuclide with a thirty-year half-life will take two centuries to lose 99 percent of its activity.
USING FISSION PRODUCTS
Fission products can be put to good use. As sources of energy, they can power small devices or instruments. The particles emitted by the radioactive isotope are absorbed, and their energy converted to heat, thus in turn producing electricity in thermocouples. Batteries that produce electricity in this fashion are radioisotope power generators, usually referred to as SNAP (Systems for Nuclear Auxiliary Power) or, more dramatically, as atomic batteries.
They can be as light as 4 pounds, generate up to 60 watts, and last for years. SNAP batteries have been used in satellites—in Transit 4A and Transit 4B, for instance, which were put in orbit by the United States in 1961 to serve, ultimately, as navigational aids.
The isotope most commonly used in SNAP batteries is strontium 90, which will soon be mentioned in another connection. Isotopes of plutonium and curium are also used in some varieties.
The astronauts who landed on the moon placed such nuclear-powered generators on the surface to power a number of lunar experiments and radio transmission equipment. These continued working faultlessly for years.
Fission products might also have large potential uses in medicine (as in treatment of cancer), in killing bacteria and preserving food, and in many fields of industry, including chemical manufacturing. For instance, the Hercules Powder Company has designed a reactor to use radiation in the production of the antifreeze ethylene glycol.
Yet when all is said and done, no conceivable uses could employ more than a small part of the vast quantities of fission products that power reactors will discharge. This represents an important difficulty in connection with nuclear power generally. It is estimated that every 200,000 kilowatts of nuclear-produced electricity will involve the production of 1½ pounds of fission products per day. What to do with it? Already the United States has stored millions of gallons of radioactive liquid underground; and it is estimated that, by 2000 A.D., as much as half a million gallons of radioactive liquid will require disposal each day! Both the United States and Great Britain have dumped concrete containers of fission products at sea. There have been proposals to drop the radioactive wastes in oceanic abysses, to store them in old salt mines, to incarcerate them in molten glass, and bury the solidified material. But there is always the nervous thought that in one way or another the radioactivity will escape in time and contaminate the soil or the seas. One particularly haunting nightmare is the possibility that a nuclear-powered ship might be wrecked and spill its accumulated fission products into the ocean. The sinking of the American nuclear submarine U.S.S. Thresher in the North Atlantic on 10 April 1963, lent new substance to this fear, although in this case such contamination apparently did not take place.
