Is Fusion Power the Answer?
An atomic scientist frankly analyzes the hurdles that must be cleared before fusion power can be used to fill energy needs
WITHOUT QUESTION, the challenge of controlling fusion (a combining process) is a tantalizing one. If we could carry out even one of the various fusion reactions, say the one involving two deuterium atoms (No. 4 in the chart on page 20), we could tap an endless supply of fuel. One out of every 3,000 water molecules in the whole world, including the great oceans, contains an atom of deuterium. Think of it! One pint of water has the potential to supply 400 kilowatt-hours, a month’s supply of electricity for your household. And we would be rid of the mounting piles of radioactive fission products from the present nuclear plants. Is this not a promising solution to the energy problem?
The apparatus called a cyclotron is useful to study these reactions, but not to produce the energy in usable form. It takes a lot of energy to make millions of particles move fast enough to react, but only a few of them strike other atoms and pay off in energy; all the rest give off their energy in small portions and are wasted. Far more energy is poured into the experiment than can be recovered.
The secret of the sun’s superiority is that its interior is so hot that the particles maintain their high velocity from one collision to another, until they finally react. So you can see why it is so difficult to achieve a useful fusion process on earth. Somehow we must duplicate a bit of the sun’s interior. But how can a batch of hydrogen be heated to millions of degrees and held together until it reacts? No known material would hold it. Substances most resistant to high temperatures melt and vaporize at a few thousand degrees.
True, scientists have demonstrated the power of fusion on earth, but only in the explosion of the fearsome hydrogen bomb. Of course, everything in and around the bomb is vaporized and blasted away in the merest fraction of a second. How could one possibly tame such a ferocious monster and harness its power?
Fusion in Magnetic Containment
Impossible as it seems, there is a way by which the seemingly insurmountable problem may be surmounted. It is by the use of magnetic thermal insulation. Here is how it works. The hydrogen is heated by electric discharge to such a high temperature that it is completely converted into particles called ions. It then consists only of positive nuclei and negative electrons. This is the state of matter called a plasma. If a plasma is surrounded by a strong magnetic field, the charged particles or ions cannot move away in straight lines, but are forced into tight corkscrew paths. If the magnetic field is shaped properly, these spiral paths will be reflected from the two ends of the container, which becomes a “magnetic bottle.”
In another design, the paths are bent into a circle, in a doughnut-shaped field called a torus (a bulge). In such devices, the protons and electrons cannot come in contact with the walls of the metal container, and they can be heated to millions of degrees while the container stays cool. The most successful device of this kind was named a tokamak by the Russian scientists who invented it.
No matter how the plasma is confined by the magnetic field, it must meet three conditions to get fusion to start and continue. These conditions specify the temperature, the density, and the time.
First, the plasma must be heated to the ignition temperature. The reaction of the atoms of deuterium and tritium ignites at the lowest temperature, about 46,000,000 degrees C (82,800,000° F). The plasma can be heated by inducing an electric current in it, or by injecting a beam of high-energy atoms. But always working against the fusion reaction is the loss of energy from glancing collisions. These produce X rays, which readily escape through the magnetic field, thus carrying heat out of the plasma. The plasma must be hot enough that the energy produced by fusion overcomes this loss, in order to attain the threshold for a self-sustaining reaction.
Second, the plasma must be compressed to crowd the particles together at a very high density, 100 trillion (1014) or more in every cubic centimeter. And, finally, these conditions must be maintained for a time interval long enough for a minimum number of collisions to occur. The product of the density multiplied by the time in seconds must reach at least 60 trillion (60 x 1012). This number is mathematically called the confinement parameter. It tells us that if the maximum density can be held for one tenth of a second, for example, that density must be at least 600 x 1012 to attain self-sustaining fusion between deuterium and tritium.
The plasma can be compressed by rapidly strengthening the magnetic field. At the same time that this increases the density, it further heats the plasma. Then, if the magnetic field is rightly designed and able to keep the plasma together long enough, fusion will result. Disappointingly, it has proved very difficult to do this. The plasma is exasperatingly elusive stuff. It finds a weak spot in the magnetic field and squeezes into it to make a pouch through which it quickly blows out. It acts like a bare innertube being overinflated without the support of a tire casing.
Many years and millions of dollars have been spent in frustrating efforts to overcome the instabilities. Only in the past two years have some experiments given hope that the herculean efforts to tame the capricious plasma may finally succeed. At the Massachusetts Institute of Technology a tokamak, called “Alcator,” attained a confinement parameter of 30 trillion. But the temperature fell far short, only about 10 million degrees. In a later test at Princeton, their Large Torus (“bulge”) reached a temperature of 75 million degrees, high enough, for the first time, to kindle the deuterium-tritium reaction. But here the confinement parameter did not exceed one trillion. So the fusion flame again flickered and went out before it was really lit.
These near approaches to the threshold of breaking even on energy have spurred hopes that the next generation of tokamaks, bigger and more expensive, will bring success. In the next two or three years one is to be built at Princeton in the U.S., and one in Europe at Culham, England. Each will cost about $300 million. If these machines successfully demonstrate controlled fusion, then nuclear physicists will be ready to face other obstacles remaining on the path to a commercial fusion reactor.
One problem looming up ahead is the accumulation of impurities in the plasma that poison it. The X-ray losses mentioned above become much greater as the atomic number increases. Even the gaseous element helium causes eight times as great a loss as does hydrogen. Oxygen is 500 times worse. This means that the plasma will have to be kept extraordinarily clean to turn out useful fusion power.
If all these problems can be solved, what might a fusion power plant look like? A design drawn up at the University of Wisconsin, based on the most optimistic data so far available, gives us an idea. The torus, or doughnut-shaped vessel, would be 27 m (89 feet) tall and 44 m (144 feet) in diameter. It would be made in 12 pie-shaped sections, each weighing 3,500 tons. The building housing it would be 102 m (335 feet) high and 120 m (394 feet) in diameter, roughly the size of the Houston Astrodome. These huge sections would have to be fabricated to meet the most rigorous standards for high vacuum. The gigantic magnets enclosing them would be cooled with liquid helium to within four degrees of absolute zero (−273° C; −460° F).
When the plant is operating, with its charge of deuterium and tritium circulating in the torus at fusion temperatures, it will generate 1,400 megawatts. But every 90 minutes, this whole monstrous plant will have to be shut down to pump out the impurities and replace the fuel. Alternate power must be supplied to the electric network for six minutes during this periodic shutdown, 15 times a day. Little wonder that utility managers are not eager to take over such a fitful giant!
Laser Fusion—An Inertial Method
Another possible way to get fusion under control was developed in secrecy and was revealed recently. It is called the inertial method. A device of this kind has a number of laser beams focused symmetrically from all sides so as to intersect at a common point. A microscopically tiny glass balloon containing a mixture of deuterium and tritium is dropped through the point of convergence. When it is exactly in position, the laser beams are fired. They all strike the sphere simultaneously, and heat the pellet with a power of millions of kilowatts for a fraction of a billionth of a second. The sudden heat vaporizes the pellet, and as the outer glass shell explodes, it pushes the gas in, in an implosion. This instantly heats the fuel to an estimated 10 million degrees, and compresses the gas to a density 200 times normal. Although the temperature is considerably less than the ignition temperature, it is high enough to cause some fusion. In some tests, as many as 10 million neutrons have been formed. Almost immediately the mass blows apart, since there is nothing to hold it together. Fusion continues only as long as the inertia of the mass holds the hydrogen atoms together; as soon as the intense pressure blows it apart, it stops.
This method is in some ways more promising for early development than is the magnetic confinement approach. But the present stage of success is no more than a demonstration that the idea is scientifically sound. It takes thousands of times more energy to power the laser beam than is produced in the experiments. With more powerful lasers, a higher temperature can be reached and fusion will become more efficient. Lasers 10 to 100 times as powerful as today’s best will be required to reach the point where as much energy can be produced as is required to operate them.
But breaking even on energy is a long way from breaking even on cost. Even if lasers with the needed power can be made, only a little energy can be gotten out of a single pellet. To achieve useful power would require firing the laser hundreds or thousands of times a minute, while pellets fall in equal numbers through the target point. It will take a major effort to extend the useful lifetime of laser generators and to manufacture the microspheres by the millions at reasonable cost.
Fusion: Clean or Not So Clean?
A problem that blights both methods of fusion is the radioactive pollution. This is true notwithstanding the claims sometimes made that fusion power will avoid this curse of fission power. Some fusion reactions (Nos. 4 and 5) involve tritium, the radioactive isotope of hydrogen. These reactions also produce neutrons, which escape into the surrounding materials and render them radioactive. Looking at the table of fusion reactions, we see that the reactions in the sun are “clean.” They do not involve any radioactivity. But the only other reaction of which this is true is the one (No. 6) between deuterium and helium-3. Unfortunately, these clean reactions all require a very high ignition temperature.
Because the deuterium-tritium reaction (No. 5) has the lowest ignition temperature, it is the only one used in current research, and it is the one that will be used in the first fusion power plants. This reaction produces copious neutrons, far more per unit of energy than does uranium fission. They will radioactivate strongly everything in and around the reactor. So it will be a dangerous task to handle and dispose of reactor parts when they need repair or replacement.
More than the activation, there is the damage done to the metal shell around the reactor, because the neutrons knock the very atoms out of place. This weakens the material, so the doughnut sections of the magnetic reactor, for example, will probably last no more than two to five years. The task of moving these colossal radioactive structures, weighing 3,500 tons and standing nine stories high, out of the plant and disposing of them presents an appalling challenge. The bulk of radioactive waste from a fusion power plant may turn out to be greater than that from present nuclear plants.
Another point often overlooked is that tritium itself is radioactive. Tritium is found in traces in the atmosphere, being produced by cosmic-ray reactions. As for unit quantity for unit quantity (curie), tritium is not nearly as hazardous as such fission products as iodine and strontium, but the quantity required in inventory for a fusion plant would be hundreds of millions of curies. Some leakage is inevitable; routinely, it might be kept as low as 10 curies a day. But an accidental release—after all, hydrogen mixed with air is explosive—would be quickly combined in the form of water and disseminated irrecoverably world wide. The loss of tritium from just one plant could increase the atmospheric concentration globally by 1000 percent.
Periodically we hear optimistic news reports in the U.S. about a new breakthrough on the way to fusion power. These usually seem to happen just about the time for the annual request to Congress for more money to expand the research. But the cold facts are that economic fusion power is a long time in the future, even if all the now recognized hurdles can be cleared. Edward Teller has said that useful power from laser fusion may be two generations yet in the future.
Unlimited Energy from Fusion Power
Really, if one were mentally to construct an ideal fusion energy plant, it would be something like this: First, take enough hydrogen to hold itself together by gravity; that solves all the problems of containment. Gravitational compression of this ball of hydrogen would increase its temperature and density enough to ignite the fusion reaction. The balance between gravity and internal pressure would automatically set the speed of the reaction, so it would neither burn too low nor run out of control.
Instead of building elaborate shields to keep the radiation inside, we would reduce it to a safe level simply by placing this nuclear reactor a tolerable distance away, say 100 million miles. Rather than build power lines to carry the energy to us, we could just have it delivered in the form of radiant energy, heat and light. And, finally, to protect ourselves from any stray protons or neutrons from the reactor, we need only wrap around us a weak magnetic field to deflect and a layer of air to absorb those particles.
The reader will, of course, recognize that this kind of fusion reactor is just what our Creator has provided for us, in the sun. How thankful we should be that an unfailing, unlimited source of energy has been given to all earth’s inhabitants by the wise Maker and Source of all energy. And this comes to us just for the taking. It is not followed by a monthly utility bill.
[Blurb on page 19]
‘If a fusion plant were operating, the whole monstrous thing would have to be shut down every 90 minutes to pump out the impurities and replace the fuel.’
[Blurb on page 21]
“The cold facts are that economic fusion power is a long time in the future, even if all the now recognized hurdles can be cleared.”