By-products of Nuclear Reactors
Nuclear reactors generate energy with the help of nuclear fission. Every
time a fission occurs we are left with radioactive by-products. It is
most important to prevent the uncontrolled escape of these fission
products from the reactor. Fortunately the dangerous products can be
retained in the reactor—if the machine has been constructed and operated
with reasonable care.
In the end, however, the burnt or partly burnt uranium charge will have
to be removed from the reactor and fresh charge, fresh fuel will have to
be added. What will become of the fission products at this time?
During protracted operation of a reactor most of the short-lived fission
products decay. Those with longer lives accumulate. The discharge of the
reactor is strongly radioactive, and it will remain radioactive for many
years. One certainly must not dispose of this radioactive waste in a
careless manner. There are, however, many ways in which one can store
such waste with reasonable safety.
One can deposit the radioactive material in well-built underground
tanks. One can concentrate the activity, imprison it in concrete blocks,
and deposit it at the bottom of the ocean. If one is very much worried
he might even put the radioactivity in rockets and let it decay
harmlessly in outer space. These procedures will cost money and will add
to the expense of nuclear energy.
It would be far better if we could find a way in which the radioactive
by-products could be made to serve a useful and safe purpose. Some of
the by-products can be used and have been used. These uses are connected
with some hazards. Furthermore, only a small fraction of the fission
products have found good employment up to the present. But the
importance of fission products is growing.
We are using them in research. A radioactive isotope imitates the
behavior of its non-active brother in all chemical reactions and in all
the intricate processes in which matter changes its form inside a living
body. Furthermore a radioactive substance can be detected with the
greatest ease. It can be found in a concentration which is less than a
millionth of a safe radiation dose. What the microscope has been in the
exploration of the structure of organisms, the radioactive elements may
become in the understanding of the chemical functioning of living
matter.
With better understanding there comes the possibility of using
radioactive by-products for diagnosis. As with the medical use of X-rays
the possible small damage due to radiation exposure should be regarded
as the price for the help we can get from early and correct recognition
of diseases.
In the treatment of patients, particularly in the case of persons
stricken by cancer, radioactive destruction of the diseased tissue is
often preferable to the use of the surgeon’s knife. Such radioactive
treatment is new. There is much room for improvement. Appropriate use of
radioactive substances for this purpose may become a far more powerful
tool and much more widespread than it is at present.
But all these applications will use up only a vanishing fraction of the
fission products. Moreover, most of the biologically important elements
are not produced in the fission of uranium. Many useful activities can
be produced by neutron absorption in reactors. But among the fragments
of uranium perhaps only radio-iodine has been put so far to direct
physiological use.
Industry deals with less sensitive objects than living tissue. Therefore
greater amounts of radioactive materials can be used here. And indeed
radioactivity has done a great variety of jobs. The penetrating power of
X-rays has been used to control the thickness of sheets in an easy and
automatic manner. Radioactivity has been incorporated into surfaces
which are exposed to mechanical wear or corrosion, to check the rate at
which the surface is worn away by the appearance of activity in the
lubricant or other fluids which have been in contact with the surface.
By such methods industry has accumulated savings which are rapidly
approaching the billion dollar mark. These savings will increase as
people learn how to use the new materials. But in all these cases it is
important to make sure that the activity will not hurt anyone while it
is used and after it has served its purpose.
Possibly the greatest amount of radioactivity will be needed in food
sterilization and preservation. One may incorporate the activities into
rods which will safely retain the materials but which will allow a
considerable fraction of the penetrating gamma rays to escape.
To sterilize food means to destroy all microorganisms. Many of these are
radiation-resistant and may have to be exposed to 50,000 or more
roentgens—that is one hundred times as much as would kill a mammal.[17]
Such massive irradiation begins to affect the foodstuff itself. In some
cases sterilization by irradiation changes the food more than would be
the case by boiling it or freezing it. In other cases irradiation
produces less undesirable side effects than any other methods.
Another way to use radiation is the preservation of agricultural
products. This need not be done by the difficult procedure of
sterilization. It is enough to control pests and to prevent germination
of the seeds which one is trying to preserve. Thus we need here
approximately one per cent of the radiation that would be required for
sterilization. By so little radiation the food is not altered to a
noticeable extent. It is precisely in such processes, where great
amounts of materials will have to be irradiated, that a substantial
fraction of the fission products might find employment.
In all applications care has to be exercised lest radioactive materials
should inadvertently be scattered around. Where great amounts are needed
as in food sterilization and preservation, caution has to be redoubled.
That trouble may arise has been illustrated by an occurrence in Houston,
Texas.
Radioactive iridium¹⁹², which is a beta and a gamma emitter, was being
used by a certain industrial concern to take X-ray pictures of metal
parts. A shipment of this radioactive material in the form of powder
pellets was being opened by remote control when compressed gas in the
container exploded and scattered some radioactivity around. The area was
shielded but some of the radioactive dust escaped to the rest of the
building. The two men who were operating the remote control apparatus
became contaminated. They washed themselves and cleaned up the area but
did not report the incident.
A few weeks later a standard radiation check showed that the plant was
still radioactive. Company officials became worried and called in
experts. At this late stage the plant was thoroughly decontaminated. The
homes of the two men were also examined and were found to be slightly
radioactive. The men and their families were temporarily moved out while
their homes were being cleaned up. When they returned, neighbors and
friends shunned them. The four year old son of one of the men lost his
playmates. People were afraid to enter the houses. One of the houses was
put up for sale but no one wanted to buy it.
The fact that the houses had been checked by radiation meters and found
to be clean, and the fact that the half-life of iridium¹⁹² is only 75
days so that any trace of activity would disappear in a reasonably short
time, did not dispel people’s fears.
It is fortunate that no one was seriously hurt in this incident. But
there is an important lesson we can learn from it: Ignorance may hurt
more than radioactivity. That a house should lose its value in spite of
the fact that its radioactive contamination has been removed, that a
little boy should be shunned as though radioactivity were infectious
like the plague—these are examples of suffering caused by one of the
greatest sources of human misery: unreasoning fear.
The greatest potentialities of fission products for the future might lie
in still a different direction. Radioactivity can induce mutations. To
what extent this is a danger we have discussed in an earlier chapter. In
the hands of a breeder who tries to bring about changes in animals or
plants radioactivity could become exceedingly useful.
Of course it is true that most mutations are harmful. It is also true
that artificial mutations have been produced for many decades. But now
it is possible to place simple and cheap tools in the hands of many more
people. Therefore the chances will increase to find among the many wrong
mutations the few and decisive changes which lead to improvement.
Do we dare to place dangerous materials in so many hands? We should not
do so without making certain that only competent and responsible
individuals will get radioactive materials. This can be done. Druggists
have dispensed poison; doctors and biologists have bred in their
laboratories the multiplying menace of germs. All this was done and is
being done with safety and to the great benefit of all people.
The use of radioactivity should be even more safe because this material
is easy to detect. If poisons or germs become lost, they may be hard to
find. Radioactive materials, however, give unmistakable evidence of
their presence. It is, of course, never easy to find a needle in a
haystack. But the chance to find it is much better if it is a
radioactive needle.
Radioactive by-products need not remain what they seem to be today: dirt
and danger to be disposed of and hidden. But in the immediate future we
shall incur some expense to keep radioactivity in a safe place.
Some gaseous by-products like the long-lived krypton⁸⁵ (half-life: 10.4
years) might continue to give rise to real difficulties and to
considerable expense. The trouble is, of course, that a noble gas like
krypton will not be bound to any material by strong bonds. It may be
inadvisable to let long-lived gases escape. On the other hand, their
adsorption or their storage at low temperature or high pressure may
prove to cost a considerable amount of money.
We have been talking about the problem of handling the by-products of
nuclear power. This problem will not appear in proper proportion unless
we also give some thought to the by-products of the kind of power we are
using at present.
That we do not like smoke and smog is obvious. To what extent these
residues of incomplete burning can cause cancer or other damage we do
not know. Chemistry is more tricky than radiation. Our lack of knowledge
about the slow biological effects of chemicals is much greater than our
remaining uncertainties about radiation.
In addition to the obvious annoyance and worry caused by the products of
incomplete combustion there exists an interesting question connected
with the result of complete combustion. The carbon that has been
deposited through the geologic ages as coal and as oil is being used up
gradually and converted to a colorless, odorless, harmless gas—carbon
dioxide. There is always some carbon dioxide in our atmosphere. The
amount is approximately 300 parts per million of common air. All the
carbon that has been burned since the beginning of the industrial
revolution could have increased the carbon dioxide in the atmosphere by
ten per cent to the value of 330 parts per million.
This increase could be significant. Carbon dioxide acts like a blanket
or a valve for some kinds of radiation. In the daytime we receive energy
in the form of visible light from the sun. This form of radiation has no
difficulty in penetrating the carbon dioxide gas. However, the incoming
radiation is balanced by invisible heat radiation, which flows out from
the earth into space day and night. This infrared radiation is quite
similar in nature to light, only our eyes are not sensitive to it. Now
the carbon dioxide gas acts like a barrier, though only a partially
effective barrier, to this outgoing heat radiation. If the carbon
dioxide content of our atmosphere were to increase too greatly, it would
act like the glass in a greenhouse and our climate would grow warmer.
A ten per cent increase in the carbon dioxide content of the atmosphere
should have produced an observable rise in temperature. Such a
temperature rise has not, in fact, been observed. The reason is that not
all the carbon dioxide which has been generated in the processes of
combustion has actually remained in the atmosphere. Most of it has found
its way into the great reservoir of our oceans. Some of it is deposited
as lime at the bottom of the oceans. However, some time is required for
the carbon dioxide to be removed from the atmosphere and to reach the
oceans. One would expect, therefore, that there would have been at least
a slight increase in the carbon dioxide content of the atmosphere.
Measurements show that this is the case and that the increase is about
two per cent—which is too small to have changed our climate.
If we continue to consume fuel at an increasing rate, however, it
appears probable that the carbon dioxide content of the atmosphere will
become high enough to raise the average temperature of the earth by a
few degrees. If this were to happen, the ice caps would melt and the
general level of the oceans would rise. Coastal cities like New York and
Seattle might be inundated.
Thus the industrial revolution using ordinary chemical fuel could be
forced to end before the advantages of civilization have spread all over
the earth. However, it might still be possible to use nuclear fuel. With
nuclear fuel the industrial revolution and its countless benefits for
man could continue to every part of the globe. The by-products of the
nuclear age are less bulky and therefore are more easily handled than
the by-products of our coal- and oil-economy. The main advantage of
nuclear energy may yet turn out to be this: With proper care nuclear
energy may turn out to be the cleanest among the available sources of
power.