Safety of Nuclear Reactors
At the beginning of the scientific and industrial revolution two old
ambitions were found to be impossible dreams. One was the transmutation
of elements, the other the machine of perpetual motion.
Modern nuclear physicists had to retract one of these statements:
elements can be transmuted. But the product is expensive, for the time
being much more expensive than gold.
The perpetual motion machine remains impossible in principle but the
problem may be considered solved in practice. It can be proved, of
course, that a machine can do useful work only if it burns up some fuel.
But the price of fuel is quite often less than the cost to operate and
maintain the machine.
Nuclear fuel even today is no more expensive than conventional fuel in
many parts of the United States. Nuclear fuel is neither heavy nor bulky
and can be therefore transported easily. In those parts of the world
where ordinary fuel is expensive, nuclear energy will soon become of
great importance. Furthermore, we shall learn to use most of the energy
in uranium rather than just the part contained in its rare and valuable
isotope, U²³⁵.
One only has to add a neutron to common U²³⁸ to get radioactive U²³⁹. In
the course of time this decays into plutonium. This element can be used
like U²³⁵: It produces fission, a great amount of energy and enough
neutrons to keep the process going. We shall also learn to extract
energy from other nuclear fuels. Thorium acts like uranium, while
deuterium can give energy by building up bigger nuclei rather than
breaking them into smaller pieces. Therefore the source of energy will
be universally available and quite inexpensive. This really means that
we are as well off as though we had a machine of perpetual motion.
But, of course, all this does not mean that the machine will do its job
free of charge. Even a perpetual motion machine would need servicing and
maintenance. Unfortunately our nuclear machines need a lot of such
servicing and therefore for the time being, nuclear energy is not the
cheapest.
The main reason why a nuclear energy source, or a nuclear reactor is
difficult and expensive to run is that the reactor after a short time of
operation becomes strongly radioactive. Therefore it cannot be
approached and it has to be handled by remote control. We can hardly
expect that energy will be free like air or water. But when we learn how
to handle inexpensively our nuclear machines, we shall be able to obtain
energy for a reasonable price at any place on the earth. Sooner or later
conventional fuel will become scarce. But nuclear energy will allow the
industrial revolution to continue and to expand into every corner of the
earth.
There can be little doubt that during the next decades nuclear reactors
will greatly multiply and by the beginning of the next century they will
be found everywhere. It is therefore of the greatest importance that
these reactors should be operated safely. On the face of it, a nuclear
reactor is a sluggish instrument which can be made to run itself. But
the ease of operation is deceptive. (See picture 13.)
One need not fear that a nuclear reactor might explode like an atomic
bomb. Nuclear explosives are very carefully constructed so that they can
release a lot of energy in a short time. Nuclear reactors on the other
hand are put together so as to make it possible that energy will be
released only at a moderate rate. Some reactors if improperly handled
may explode, but the violence of the explosion cannot greatly exceed
that of a similar weight of high explosive.
Nevertheless a reactor accident could become exceedingly dangerous. The
reactor is charged with radioactive fission products and some other
radioactive substances produced by neutron absorption. Any accident
which will allow even a portion of these products to escape into the air
will endanger people at a considerable distance in the downwind
direction. One reason why reactors can be dangerous is that in
protracted operation of the reactor, fission products which have longer
lives accumulate. It is precisely these longer-lived products which are
more dangerous because they have a better chance to find their way into
the human body.
Reactors are now planned which will produce 300,000 kilowatts of
electricity. If such a reactor operates for half a year and then
explodes and releases its radioactive content into the atmosphere, its
radioactivity will be comparable to that of a hydrogen bomb. In one
important respect such an accident would be worse than a hydrogen
explosion. The nuclear explosive lifts most of its radioactive products
to a high altitude and the poisonous activity gets dispersed and diluted
before it descends. The activity from a reactor on the other hand will
remain close to the ground and might endanger the lives of the people in
an area of hundreds of square miles. It will contaminate an even greater
territory.
In the extensive operation of many reactors in the United States no one
has yet been killed by the radioactivity. This has been due to extremely
careful operation and also to good luck. We must be prepared that sooner
or later accidents will occur. On the other hand we must try to take
sufficient precautions to avoid the kind of catastrophic accident which
we have mentioned above. With great care such accidents can indeed be
avoided.
In thinking of all kinds of man-made machines we find some which move
fast and seem dangerous like, for instance, airplanes; others which are
stationary and apparently harmless, like the bath tub. Yet more
accidents happen in bath tubs than in air travel. The most dangerous
element in all operations is the human element. We ourselves constitute
the greatest safety hazard. This is a situation no different in nuclear
technology than in any other kind of technology. What is new in nuclear
technology is that a reactor is usually very safe but may become
extremely dangerous when something unexpected happens to it. Also we
dare not use the method of trial and error. An error in the reactor
business could exact a far heavier toll of lives than an error in the
testing of H-bombs. We cannot wait to learn by experience; we must
forestall accidents.
An especially difficult safety problem is connected with the use of
reactors in small countries. A serious accident could endanger the lives
of people in adjacent countries. Thus modern technology may force
cooperation across national boundaries.
There is only one way to avoid traffic accidents and that is care
exercised by everyone, particularly the drivers. Similarly reactor
safety will depend on the people who operate the reactors. At the same
time a lot of help can be obtained by careful construction and scrutiny
of each new reactor.
One of the first acts of the Atomic Energy Commission was to establish a
Committee for Reactor Safeguards. With the passing of years this
committee had to take on more heavy responsibilities. At first it had to
operate under secrecy. With the wider and more public use of reactors
the safety considerations are becoming more available to the public. The
question of safe operation of a machine cannot be separated from a
thorough understanding of the working of the machine. We cannot attempt
to give an adequate description of a reactor or of the safety rules. A
few general statements have to suffice.
A working reactor is full of neutrons. In a small fraction of a second
these neutrons produce fission and a new generation of neutrons comes
into being. In slow reactors which contain lots of light elements like
hydrogen or carbon, the neutrons move with speeds little greater than
that of sound and a generation may last as long as a millisecond (one
thousandth of a second). In fast reactors which contain almost
exclusively heavier elements like uranium or iron, neutrons move with a
great speed which is about three per cent of the speed of light. In this
case one generation replaces another in less than a microsecond (one
millionth of a second).
Fortunately not all the neutrons get reproduced so rapidly. Some
fissions produce delayed neutrons which are emitted usually with a delay
of several seconds. In a steadily working reactor each generation should
have the same number of neutrons as the previous one. If each succeeding
generation has even a slight surplus, the reactor will become hot and
may explode in a small fraction of a second. The main reason why safe
operation is possible is the fact that fast multiplication can occur
only if each generation becomes more populous _even when one does not
count the delayed neutrons_. A slightly overactive reactor is easily
governed, but there comes a point when the dormant dragon begins to
stir. This happens when there are enough neutrons produced so that
multiplication can occur without waiting for the delayed neutrons. At
that point a well behaved dragon will perform a harmless action. For
instance it may blow a fuse. But a vicious dragon will spit radioactive
fire.
It is not easy to predict whether the dragon will be always well
behaved. But with careful analysis one can make such a prediction. For
instance one must look into the question of whether the reactor is
stable. If it gets hotter, does this make the reactor proceed even
faster so that the rate of heating increases and the reactor runs away?
In a stable reactor excess heat should tend to stop the energy
production and thus the reactor cools and returns to its normal
operating temperature.
But too great a stability may also be dangerous. Heating may be
overcompensated by the cooling mechanism; after the reactor has become
too cold it may then heat up too fast and overshoot again. We must guard
not only against a simple run-away, but also against increasing
oscillations.
In many reactors unusual chemical compounds are used. A reactor accident
may start with nothing worse than an ordinary chemical reaction between
strange compounds under strange conditions. But if this chemical
reaction destroys the reactor sufficiently to allow some fission
products to escape, then such a chemical accident can be as bad as one
of nuclear origin.
In the interior of the reactor materials are exposed to unusually strong
radiation. Under this effect some materials can change their chemical
properties so that what has been inert as a construction material may
become dangerous during the operation of the reactor.
Perhaps the most important single item is the arrangement of mechanical
controls. The reactor is adjusted by a system of sheets or rods made of
a material which absorbs neutrons. This arrangement must be so
constructed that the control rods can be withdrawn only at a very slow
rate. But it must be possible to put them back quite fast. Any danger
signal should shove the absorbers in at maximum speed. The technical
expression is “scram.”
The main point, however, is that all the dangers and safety devices can
be studied and after careful study a nuclear accident can be avoided.
Some reactors are now so thoroughly understood that they can be safely
used for training of future nuclear engineers. Other reactors which are
more powerful or less well studied have to be used more carefully. Some
reactors should be, and are being, enclosed in gas-tight containers. If
an explosion occurs the fission products will be harmlessly confined
inside the container. Of course, one must be quite sure that the reactor
is of such a type that it cannot produce an explosion great enough to
burst the container and what is even more important one should be quite
sure that the container is closed except when the reactor is shut down
and completely safe. Often it may be best to build the reactor
underground.
The safety of a reactor, of course, depends to a great extent on the use
to which the reactor is put. In general a power station is less likely
to give trouble than a moving power source. It is not probable that
nuclear locomotives will ever be safe. In nuclear ships more room is
available and more room permits more safety measures. But even so the
safety of nuclear motors in ships will have to be considered
particularly carefully because ships will have accidents in harbors.
Between the urgent need for progress and the absolute necessity of
safety it is difficult to keep a sense of balance and one can easily
make the mistake of being unnecessarily cautious. Such unnecessary
caution was probably exercised when the Committee on Reactor Safeguards
considered the earthquake hazard of the Brookhaven reactor on Long
Island. A seismologist, who is a Jesuit Father, was asked to tell the
committee[15] of the possibilities and probabilities of an earthquake on
Long Island. The chairman[16] of the committee subjected the expert to a
long and detailed questioning. After half an hour the Committee on
Reactor Safeguards ran out of questions. But the Jesuit Father had not
given any signs of running out of answers. The session being at an end
the expert, looking the chairman of the committee firmly in the eye and
in a more authoritative voice than he had yet used, said, “Mr. Chairman,
I can assure you on the highest authority that there will be no major
earthquakes on Long Island in the next fifty years.”