X Международная студенческая научная конференция Студенческий научный форум - 2018

«When the world had recovered from the shock of this unimaginable horror, people everywhere asked the scientists how soon they could apply the immense power of the fissioned nucleus to peaceful purposes. But this took much longer. It was considerably easier to use the nuclear chain reaction for distruction than for theproduction of usable energy for homes and factories–to control it and release it insmall doses. Many problems had to be solved; the main one was that of 'braking' thereleased neutrons efficiently so that the chain reaction would not get out of hand.

The first atomic reactor, as the apparatus for the utilization of atomic

energy is now called, had been Set up by Enrico Fermi on the football ground of the University of Chicago in 1942. It was a somewhat crude assembly, whose main purpose was to get experimental proof for the theory of chain reaction. Fermi scattered rods of uranium through a stack of graphite blocks, which acted as a brake for the neutrons – a 'moderator', to use the technical term. Fermi used natural uranium, which is a mixture of the stable U-238 and the unstable U-235 in a proportion of 140 : 1. Thus there was only slight radioactivity, i. e. breaking-up of nuclei. In order to control it, Fermi inserted some cadmium rods into the pile; this metal absorbs neutrons very readily, and by pushing the rods completely into the pile he could stop the chain reaction altogether.

Fermi's assembly is still the basic blueprint of today's nuclear reactors. Their main parts are the fuel, the moderator, the control rods, and the cooling system. But the scientists and technicians have since developed a great many different types of reactors – some already in everyday use, others running experimentally in atomic research establishments or being built for special jobs and purposes of all kinds, from producing nuclear explosives for weapons to 'cooking' stable elements' so that they become unstable isotopes for use in medicine, industry, agriculture, and research.

Why do we speak of the atomic age as a new chapter in the history of civilization, and why have the technologists made such great efforts to utilize the energy of the split nucleus? For a long time the shadow of a future without sufficient fuel loomed over mankind. Coal has been mined at a steadily increasing pace which set in with the industrial Revolution, and some experts predicted that in Britain, for instance, an acute shortage of cheaply mined coal would set in after 1980. Oil is still to be found in plenty, but consumption has been increasing in leaps and bounds all over the world.

Atomic energy is produced in a different way. It is not generated by the chemical process of combustion. It is released when nuclei undergo fission, and although here» too, matter is used up, the amounts are small compared with the energy produced. A few pounds of uranium 235 can be made to supply a medium-

sized town with all the electricity it needs during a whole year. True, our reserves of uranium are limited. But there is one reactor type, which in fact produces more nuclear fuel than it uses! This type has a 'blanket' of thorium, one of the most com- mon elements on earth, which is turned into the artificial radio-active element plutonium when bombarded by neutrons. And there is good reason to hope that before long1 we shall be able to produce energy from ordinary sea-water by another nuclear reaction called 'fusion'.

So there is little doubt that mankind's energy problems will be solved in the near future, if they have not been solved already in principle. All we have to do is build nuclear reactors and supply them with atomic fuel. But how do we turn it into usable energy?

The 'classical' solution of this question, although it may soon be regarded as an old-fashioned one, is to conduct the heat generated by the fission process out of the reactor, make it boil water, and let the resulting steam drive turbines which, in their turn, drive electric generators. It is a roundabout way, but it works well, although it is still rather expensive.

Britain's first two nuclear power stations were Calder Hall (opened in 1956) and Chapelcross (1959), both of the same type. The reactor 'vessel' a giant steel cylinder, contains a pile of pure graphite, the material from which pencil leads are made. It has many hundreds of vertical or vertical and horizontal channels; in some of them the fuel – rods of uranium metal in magnesium alloy sheaths – are stacked, in the rest there are the control rods made of boron or cadmium; these can be pushed in and pulled out. A very thick concrete or steel wall around the reactor vessel–the 'biological shield' – prevents the escape of radio-activity.

As soon as the control rods are pulled out the chain reaction begins; uranium nuclei split up under neutron bombardment, and release more neutrons. These neutrons bounce off the graphite atoms so that they shoot to and fro through the reactor until they hit and split more uranium nuclei: the graphite acts as the moderator in this process, helping to keep the chain reaction going and preventing the 'capture' of fast neutrons by the nuclei through slowing them down. The uranium rods get hot (up to 400° C, in Calder Halland Chapelcross), and this heat is removed by the 'coolant', carbon dioxide gas under pressure. It circulates through the reactor vessel in tubes entering at the bottom at 140° С and leaving it at the top at about 340° С The coolant gas, after leaving the 'core' of the reactor, is conducted to the heat exchangers. They are basically ordinary boilers in which water is turned into steam. The water is contained in steel pipes around which the hot coolant gas is blown. The resulting steam is directed into the turbines which rotate the electric generators. Calder Hall and Chapelcross have eight of them each, generating 180,000 and 140,000 kW respectively of electricity, which is fed into the national grid.

If the chain reaction gets too fast and the reactor becomes too hot, the control rods are lowered into the core automatically, thus slowing down the process; if pushed in completely they will stop it altogether.

Uranium as the fuel, graphite as the moderator, and carbon dioxide gas as the coolant are only one possible combination.

Some nuclear engineers believe that organic substances can be used as moderators and coolant fluids, others that the fuel should be given the form of a ceramic. A good deal of research work is done with various types of homogeneous reactors, in which fuel, moderator, and coolant circulate as a single, fluid mixture.

Nuclear power is still in roughly the same early phase as steam power was at the beginning of the nineteenth century, and it may not reach maturity until the end of our own century. By that time, however, we shall not only have fission but also fusion as a basic energy-producing nuclear process.

The theory of nuclear fusion was discovered in the early 1930's – years before that of fission – by John Cockeroft at the Cavendish Laboratory, Cambridge, where he worked under Lord Rutherford. Here they built a simple machine, which looked more like a couple of stove-pipes than an atom-smashing tool, for shooting electrically speed-up protons at the nuclei of light elements, such as lithium. The result was that the lithium nuclei turned into nuclei of helium. This was strange; for helium is heavier than lithium. Somehow the helium atoms must have been formed

not only by splitting but by subsequent accumulation of protons and neutrons. It was only later that it dawned on the physicists that some such process is responsible for the way in which the stars, including our own sun, produce their tremendous energy.

Today we know that in the sun light elements – mainly hydrogen –are turned into heavier ones, such as helium. This 'thermo-nuclear' process of fusion, as it is called, takes place at fantastically high temperatures (in the centre of the sun the temperature is believed to be about 15 million degrees Centrigrade). The heat fuses the nuclei, which would normally repel each other because they have the same (positive) electrical charge; heat means violent movement of particles, in other words: energy. Thus the hydrogen nuclei bump into each other and combine to form helium nuclei, with a simultaneous release of energy. As in nuclear fission, some mass is converted into energy in the fusion process, but the sun can keep up its rate of loss of mass – five million tons per second – for some thousands of million years.

This process is only possible where light elements are concerned; hydrogen, the lightest of them, has the smallest electrical charge, and therefore the repellent force of its nuclei can be more easily overcome than that of heavier elements. If there was any chance at all of producing nuclear energy from fusion – this was a point about which scientists agreed – it could only be done by using hydrogen: in short, by emulating on earth the process that makes the sun shine.

Again, as in the case of atomic fission, this was first achieved in the form of a weapon, the hydrogen bomb. Even the testing of this weapon has proved to be highly dangerous because it contaminates the atmosphere all over the world with radio- active 'fall-out' isotopes which can produce cancer of the bone and blood. No one doubts that a nuclear war fought with fission and fusion bombs would mean the suicide of mankind.

As these lines are being written many scientists in at least half a dozen countries are busy trying to find a system to tame the energy of the H-bomb for peaceful use, but no decisive 'break-through' has been achieved. It may, however, come at any moment. In August 1957 British physicists working with their thermo- nuclear device called '2eta' believed they had succeeded, but this turned out to be a mistake. Still, the scientists' efforts towards that goal are all based on the same basic principle, and some time somewhere another Zeta will achieve the 'break-through'.

In these experiments the heavy hydrogen isotope deuterium – which has an extra neutron in its nucleus – plays the decisive part. At very high temperatures the protons are detached from the electrons revolving around them, and the neutrons fly off at great speed, thus providing extra energy, i. e. heat, as the protons melt together to form new nuclei. There are many difficult problems to overcome before the thermo-nuclear power station based on this process can become a reality, but that of fuel supply is the least of them: the oceans of the earth are practically inexhaustible source of deuterium, and its extraction from sea water is neither complicated nor expensive. One gallon of sea water may be sufficient to yield as much energy as 100 gallons of petrol, and a bucketful containing one-fifth of a gram of deuterium could keep a five-room house warm for a whole year.

The real trouble starts when we attempt to produce the very high temperatures

required to achieve thermo-nuclear fusion. Up to 1950, the highest temperature ever produced in a laboratory was 30,000° Centigrade. All the Zeta-type assemblies, therefore, are machines designed to reach temperatures of many millions of degrees for heating deuterium gas. This is done electrically. When an electric current is passed through a gas it sets up an electric discharge in it, with a corresponding rise in temperature. A hollow vessel – either ring-shaped or tube-shaped, and usually made of aluminium – is partly encircled by a huge electromagnet which produces the field that heats up the deuterium inside. But if the hot gas touched the walls of the vessel they would melt, and the gas would cool down; therefore, it must be kept in the centre. This is done by another intense magnetic field around the gas, usually by winding an electrically charged cable around the vessel. In this way the gas, which tries to resist that 'pinch effect', is prevented from behaving about like an angry snake as soon as the current is switched on and the temperature rises.

Zeta, the British assembly which was originally built at the atomic research establishment, had a ring-shaped form; the 'pyrotfon1, set up at the University of California, was designed as a linear tube with a special 'mirror' effect: the magnetic field was made much stronger at either end so that the 'plasma', as the gas in the machine is usually called, assumed the shape of a sausage – thick in the middle and pinched at the ends. This arrangement had the effect of a magnetic mirror; the particles racing around in the plasma were reflected back from both ends into the centre, which increased the temperature and also the probability of the particles bumping into each other to achieve fusion. Another American fusion research instrument did away with the magnetic coils, and used a layer of accelerated electrons instead for the production of the necessary magnetic field.

When one of the scientists' teams working with these machines achieves genuine fusion – a temperature of up to 500 million degrees Centigrade may be needed to start a thermonuclear process which can maintain itself–the question of how a thermo-nuclear power station could work will become topical. As in a conventional power station, coal-fired or atomic, the heat could be used to produce steam for the turbo-generators. But by that time there may be a better and more direct way of turning heat or radio-activity into electricity.

There are several basic systems of doing this. One, called the 'thermionic converter', uses the principle of the cathode-ray tube in which electrons, particles of negative electricity, are given off by a hot strip of metal, the cathode, in a vacuum. The heat necessary to produce this effect could be that generated in a nuclear reactor; the greater the temperature difference between the cathode, or 'emitter', and the anode, or 'collector', the greater will be the yield of electrons and therefore of electric current. There is, at least theoretically, no reason why a nuclear power station should not be operating on this principle once the technological problems have been solved.

Atomic as well as conventional power stations may be made much more efficient by the gas-blast system of generating electricity. It is based on the fact that a blast of very hot gas (at least 2,000° Centigrade), which could be produced by a fission or fusion reactor, becomes an electrical conductor and generates current when moving through the poles of a powerful magnet. American and British research laboratories are working on this scheme, but the principal problem is that of finding materials which can withstand such temperatures for any length of time.

Another system – which might be better suited for smaller, mobile electricity producing units – is based on a discovery recorded already by the Curies around 1900, but neglected by scientists for nearly half a century. That was the observation that radio-activity could produce electricity directly in certain materials. When, after the Second World War, cheap radio-active sources–isotopes–became available the idea was taken up at last. The first, somewhat crude 'atomic battery', as it was called, was produced in 1954 by a research team in the laboratories of the Radio Corporation of America: a little box containing a thin wafer of the isotope, strontium 90 – one of the dangerous elements in radio-active 'fall-out' after H-bomb tests; it bombarded with its particles a semi-conductor crystal, an adaptation of the transistor. The current generated in the crystal by the radio-active emanation of the strontium was strong enough to produce a buzzing noise in an earphone.

Isotopes for direct generation of electricity will be available in growing quantities as the utilization of atomic energy spreads to more and more countries. One of the major problems connected with nuclear power stations is the safe disposal of radio-active waste; burying it, or dumping it into the sea, is not everywhere the best means of getting rid of it. But when devices such as atomic batteries are mass produced they will require great quantities of radio-active 'waste' products; they must, of course, be made absolutely safe for everyday use.

How can we tell if we are the target of radio-active emanation? It is invisible and inaudible, and we cannot feel it – unless and until we have received too much of it and become ill. But there is a vital tool in our nuclear age, the Geiger counter in its manifold forms, which measures radio-activity accurately. Invented by Hans Geiger, a German physicist and one of Lord Rutherford's close collaborators, in the 1920's, it is an ingenious instrument which can make any type of radiation, whether in the form of particles or of electro-magnetic waves, visible and audible.

The Geiger counter consists of a metal cylinder filled with gas at low pressure; two electrodes – one being the cylinder itself, the other a fine wire stretched along its centre – are maintained at a large potential difference, usually about 1,000– 1,500 volts, but no spark is allowed to pass between them. Only when some subatomic particle or unit of electro-magnetic radiation pierces the thin metal of the cylinder and produces ionization (i. e. when the gas atoms become electrically charged), there is a sudden discharge between the electrodes, and the potential drops for the fraction of a second. This can be made either visible on a dial, or audible in a pair of headphones. Frequently, simple counting devices such as telephone counters are attached to the tube to register the number of incoming particles.

Geiger counters are being made and adapted for all kinds of purposes–light ones for uranium prospecting; built-in types for atomic power stations and research establishments; counters with warning signals for factory workers who have to handle radio-active matter and whose hands and clothes have to be checked; counters which can test human breath for traces of radon gas, and so on.

Finally, a new source of energy could be created by 'depositing' the heat of a nuclear explosion deep underground and using it – just as volcanic heat is used in some parts of the world – for the production of power. It has been estimated that an atomic blast 3,000 feet underground in a suitable geological formation would produce about 8,000 million kilowatt hours of electrical energy at a cost of 0.04 d. (less than-ycent) per kilowatt. In short, the peaceful uses of atomic energy are vast and tempting– but we must stop squandering it on weapons of mass annihilation.»

Questions on the article

Кто и когда установил первый атомный реактор?

The first atomic reactor, as the apparatus for the utilization of atomic energy is now called, had been Set up by Enrico Fermi on the football ground of theUniversity of Chicago in 1942.

Как Ферми управлял ядерной реакцией?

Fermi inserted some cadmium rods into the pile; this metal absorbs neutrons very readily, and by pushing the rods completely into the pile he could stop the chain reaction altogether.

Какие основные части ядерного реактора?

The main parts are the fuel, the modera tor, the control rods, and the cooling system. The reactor 'vessel' a giant steel cylinder, contains a pile of pure graphite, the material from which pencil leads are made. It has many hundreds of vertical or vertical and horizontal channels; in some of them the fuel – rods of uranium metal in magnesium alloy sheaths – are stacked, in the rest there are the control rods made of boron or cadmium; these can be pushed in and pulled out. A very thick concrete or steel wall around the reactor vessel–the 'biological shield' – prevents the escape of radio-activity.

Как работает ядерный реактор?

As soon as the control rods are pulled out the chain reaction begins; uranium nuclei split up under neutron bombardment, and release more neutrons. These neutrons bounce off the graphite atoms so that they shoot to and fro through the reactor until they hit and split more uranium nuclei: the graphite acts as the moderator in this process, helping to keep the chain reaction going and preventing the 'capture' of fast neutrons by the nuclei through slowing them down. The uranium rods get hot, and this heat is removed by the 'coolant', carbon dioxide gas under pressure. It circulates through the reactor vessel in tubes entering at the bottom at 140° С and leaving it at the top at about 340° С. The coolant gas, after leaving the 'core' of the reactor, is conducted to the heat exchangers. They are basically ordinary boilers in which water is turned into steam. The water is contained in steel pipes around which the hot coolant gas is blown. The resulting steam is directed into the turbines which rotate the electric generators. Calder Hall and Chapelcross have eight of them each, generating 180,000 and 140,000 kW.

Какая реакция называется термоядерной?

The hydrogen nuclei bump into each other and combine to form helium nuclei, with a simultaneous release of energy.

Кем и когда создана теория термоядерного синтеза?

The theory of nuclear fusion was discovered in the early 1930's – years before that of fission – by John Cockeroft at the Cavendish Laboratory, Cambridge, where

he worked under Lord Rutherford.

В какой форме пока возможна термоядерная реакция?

A thermonuclear reaction is possible inform a nuclear bomb.

Почему в термоядерных реакциях дейтерий играет решающую роль?

At very high temperatures the protons are detached from the electrons revolving around them, and the neutrons fly off at great speed, thus providing extra energy, i. e. heat, as the protons melt together to form new nuclei. There are many difficult problems to overcome before the thermo-nuclear power station based on this process can become a reality, but that of fuel supply is the least of them: the oceans of the earth are practically inexhaustible source of deuterium, and its extraction from sea water is neither complicated nor expensive. One gallon of sea water may be sufficient to yield as much energy as 100 gallons of petrol, and a bucketful containing one-fifth of a gram of deuterium could keep a five-room house warm for a whole year.

В чем проблема использования энергии термоядерного синтеза?

The principal problem is that of finding materials which can withstand such temperatures in 2000 degrees for any length of time.

В чем состоит одна из главных проблем атомных станций?

One of the major problems connected with nuclear power stations is the safe disposal of radio-active waste.

В чем опасность радиоактивного излучения?

It is invisible and inaudible, and we cannot feel it – unless and until we have received too much of it and become ill.

Как называется прибор для определения уровня радиации?

It is the Geiger counter.

Из чего состоит счетчик Гейгера?

The Geiger counter consists of a metal cylinder filled with gas at low pressure;

two electrodes – one being the cylinder itself, the other a fine wire stretched along its centre – are maintained at a large potential difference, usually about 1,000– 1,500 volts.

Каков принцип работы счетчика Гейгера?

Only when some subatomic particle or unit of electro-magnetic radiation pierces the thin metal of the cylinder and produces ionization (i. e. when the gas atoms become electrically charged), there is a sudden discharge between the electrodes, and the potential drops for the fraction of a second. This can be made either visible on a dial, or audible in a pair of headphones. Frequently, simple counting devices such as telephone counters are attached to the tube to register the number of incoming particles.

В каких случаях можно использовать счетчики Гейгера?

Geiger counters are being made and adapted for all kinds of purposes–light ones for uranium prospecting; built-in types for atomic power stations and research

establishments; counters with warning signals for factory workers who have to handle radio-active matter and whose hands and clothes have to be checked; counters which can test human breath for traces of radon gas.

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