T H E FUEL OF T H E T W E N T I E T H C E N T U R Y

S. V e n e t s k i i

It is difficult to say what name the German scientist Martin Klapprot would have thought of giving to the e le- ment he discovered back in 1789, had i t not been for an event which occurred only just before that yearand which exci ted scientif ic circles throughout the world, In 1781, the British astronomer Wil l iam Herschel, observing the ce les t ia l vault through a h o m e - m a d e telescope, discovered a luminous cloud which he first took for a comet, but was later to become convinced that what we had spotted was a new, previously unknown, planet, the seventh in the solar

system. Herschel dubbed this planet Uranus, in honor of the ancient Greek sky god. Sti l l under the influence of that event, Klapprot assigned the name of the new planet to the newly discovered element.

About a half -century later , in 1843, the French chemist Eugene Peligaud succeeded in isolating me ta l l i c ura- nium for the first t ime. The industrial world showed no interest in this heavy, but corhparat ively soft, m e t a l Its mechan ica l and chemica l properties left both metallurgists and machine designers cold. Only the glassblowers of Bohemia andthe Saxon porcela in craftsmen reacted warmly to uranium oxide, which imparted a beautiful yellowish green colorat ion to their wares, or which could be used to make ve lve ty black decorat ive patterns.

Even scientists of the t ime had no more than a f leet ing and nodding acquaintance with the element. They had at their disposal only rather meager , and as often as not, s imply incorrect , information. It was assumed, for in- stance, that the a tomic weight of uranium was in the neighborhood of 120. When D. M. Mendeleev devised the periodic table of the elements, this figure caused him no l i t t le concern; uranium simply did not want to occupy

that box in the tab le which was "reserved" for an e lement with that assigned a tomic weight. And then the scientist went against a l l established opinion in deciding to assign a new value to the a tomic weight of ttranium: 240, which put the e lement a l l the way to the very end of the per iodic table. Life confirmed the correctness of the great c h e m - ist: the a tomic weight of uranium was found to be 238. 03.

In 1896, the French physicist Henri Becquerel, while conducting experiments on one of the uranium salts, made a discovery which by rights ranks among the greatest scient i f ic discoveries in the history of mankind. Here is how it happened. Becquerel had long been interested in the phenomenon of fluorescence (i .e . , spontaneous radiation) inher- ent in cer ta in substances. At one t ime, the scientist decided to make use of the properties of a uranium salt, to which

chemists had given the name potassium uranyI double sulfate. He

placed a patterned figure cut from meta l and coated with a layer

of the uranium salt on a photographic plate wrapped in l ight - t ight b lack paper, and placed the package in bright sunshine. After four

hours, Becquerel developed the paper and saw the c leareut silhou- et te of the meta l l i c figure. He repeated his experiments over and over again: the results were the same each t ime. And then on February 24, 1896, the scientist reported at the meet ing of the

Paris Academy of Sciences that, when a fluorescent mate r ia l such as potassium uranyl double sulfate is exposed to light, there is to be observed an invisible radiat ion which passes through black

opaque paper and reduces the silver salts on the photographic plate.

Two days later , Becquerel decided to repeat the experiment , but as bad luck would have it the sky was cloudy from the morning

Translated from Metallurg, No. 3, pp. 43-45, March, 1970.

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on, and what kind of fluorescence can one count on with no sun? Regretting the inconvenience of the pc~or Wea,:hero

the scientist put away his slides combined with samples of uranium salts, which had been prepare d for the experi~ ment, but not exposed to sunlight, in a box in his desk, where they lay undisturbed for several days. F!:naliy, at night on March I, wind cleared the sky of clouds, and the next morning saw sunlight streaming and spark!lug over paris~

Becquerel had been impatiently waiting for that break in the weather, and rushed to this laboratory and removed the

slides from the box in order to expose them to the sum. But being a very pedantic and fastidious experimenter, he

stil l decided to have a try at developing the slides, even though it seemed perfectly �9 obvious that n0thing, could have possibly transpired with them in the past few days: for they were enclosed in a dark box, and there is no substance capable of fluorescing without Iight. The scientist did not suspect that in several hours these quite ordinary photo- graphic plates costing a few francs would become a priceless treasure, but the day of March 1, 1896 was marked down forever in the history of world science.

What Becquerel saw on the developed plates l i te ra l ly astonished him: the b lack silhouettes of the specimens stood out c lear ly and sharply on the l ight-sensi t ive layer. That meant that fluorescence was irrelevant here, BU~ then what sort of rays was the uranium salt emi t t ing? The scientist conducted s imilar experiments wit h other ura- nium compounds, including those which had no abi l i ty to fluoresce or which had lain for years in a d a r k place, and each t ime the image showed up on the plates. Becquerel began to entertain the thought that perhapslurani~a was

the "first example of a meta l exhibit ing a property similar to that of invisible fluorescence." This property, invOlv- ing a spontaneous fission of the nuclei of atoms, was given the name radioact ivi ty .

Becquerel 's discovery ushered in a new era in physics, the era of the transmutation of elements; From now on

the atom could no longer be viewed as a single indivisible entity; the pathway into the depths of ~his "build~ng block" of the mater ia l world lay open before science.

In the Thirties of the present century, uranium became a serious object of interest to the "boys,~i as the group of young ta lented physicists working under the supervision of Enrico Fermi at the University of Rome Came ~ : b e known, The special interest of these scientists was neutron physics, which harbored many novel and Unsuspected

secrets.

It was found that when nuclei of one e lement were i rradiated by neutrons, the nuclei as a rule became trans- muted into the nuclei of some other e lement occupying the next box in the periodic table. Now What would happen

if neutrons were used to bombard the last e lement , the 92rid e lement , uranium ? Then,of ' = course,~cnc e lement ia the

9ard box would turn up, an e lement which Nature i tself had not been ab!e to produce I

The idea appeated to the "boys." How could there not be curiosity in finding out what this a r t i f ic ia l ly made

e lement would be l ike, how i t would appear, how it would behave? And so, the uranim~q was irradiated, But what happened? Not just one radioact ive e lement resulted, as expected, but at least ten of them, Clear!y there was some unexplained r iddle in the behavior of uranium. Enrico Fermi published a report on this in one of the scientif ic periodicals. It is possible, he felt, that the 93rd e lement in the table might have been formed, but precfse proof of this was lacking. On the other hand, there was proof that several other e lements were present i n t h e i r radia ted urn ~

nium. But which ones?

An a t tempt to answer that question was undertaken by Irene Jol iot-Curie, daughter of the famou~ physicists Pierre Curie and Marie Curie-Skladowska. She repeated Fermi 's experiments and carefully investigated the chemi - cal composition of the uranium after exposure to neutron bombardment. The result was more than unexpected: the e lement lanthanum, situated more of Iess in the middle of Mendeleev 's table, i .e . , quite remote :from the positron of uranium in the table, was discovered in the bombarded specimen.

When the same experiments were conducted by the German scientists Otto Hahn and Friedrich Strassmann; they found in the uranium not only lanthanum, but barium as w e l l One riddle on top of another, Hahn and Str~assman reported on the experiments they had conducted to their friend, the famous physicist Lisa Mei tne r . There were now~ several leading scientists a t tempt ing to solve the uranium puzzle. Frederic Joi iot-Curie and Lisa Meitner somewhat later came to the same conclusion: when a neutron hits a uranium nucleus, the la t ter breaks: up into parts, This is what accounts for the unexpected appearance of lanthanum and barium, elements with a tomic weights about One~

half that of uranium.

But Jol iot-Curie pointed out another fact of crucial importance: the decay of the uranium nucleus is of ex- plosive character, with the fragments forming flying out in different directions at enormous speed~. As tong as 0niy individual nuclei had been split, the energy of the fragments would only heat up a tiny lump of uranium. If ~he number of fission events were increased, a prodigious quantity of energy would be l iberated.

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But where could one lay ho!d of the quantity of neutrons needed to simultaneously bombard a large number of

uranium nuclei? Nature itself came forth with an answer~ 5oliot-Curie discovered that when a uranium nucleus

underwent fission several neutrons were released from the nucleus. When these nuclei impinged on the nuclei of

neighboring uranium nuclei, they gave rise to a new disintegration event, thus starting what is known as a chain re-

action. And since these processes take only millionths of a second, a colossal amount of energy is liberated virtually

instantaneously, and an explosion is inevitable. This is obvious enough, it would seem at this point. But lumps of uranium had been irradiated by neutrons on repeated occasion, and they had not exploded, i .e. , no chain reaction

ensued" Clearly, some additional conditions were required to bring off the chain reaction. What might these condi- tions be? Frederic Joliot-Curie was unable to provide the answer to the question at that time.

The answer was forthcoming nonetheless. In that same year 1939, the young Soviet scientists Ya. B. Zel 'dovich

and Yu. B. Khariton found it. They established the existence of two pathways for the development of a nuclear chain reaction. The first was by increasing the size of the lumps of uranium irradiated, since when a small lump is irra- diated many of the newly liberated neutrons can escape from the lump without impinging on a single nucleus block-

ing their escape path. As the mass of uranium is increased, the probability that the neutrons will hit the target is naturally increased.

And there is another way: by enriching the uranium with isotope 235. The problem is that natural uranium

has two isotopes, the atomic weights of which are 238 and 235. The nucleus of the first contains three extra neu-

trons. The neutron-poor uranium-235 absorbs these neutrons much more readily than its more "well- to-do" brother,

and on some occasions, upon absorbing a neutron, it does not even split up into parts, but rather transmutes into a

different element. The property of the isotope was later utilized by scientists in order to produce and isolate art if i-

cial ly made transuranium elements. The indifference shown by uranium-238 to neutrons proved to be fatal to the chain reaction however: the process dies out, without having had t ime to build up its forces. But the more the ura-

nium is enriched with "neutron-greedy" atoms of isotope 235, the more energetically the reaction proceeds.

To get the process started, however, we still need the first neutron, the "seed" or the "match" to touch off the

atomic conflagration. Or course, the ordinary neutron sources which scientists had been using in earlier research work could be used for that purpose. But were there any more suitable "matches" available.*

There were indeed. They were found by two other Soviet scientists, K. A. Petrzhak and O. N Flerov. Investi-

gating the behavior of uranium in 1939-1940, they reached the conclusion that the nuclei of that d e m e n t are cap- able of decaying spontaneously.

K. A. Petrzhak and G. N. Flerov wrote the concluding page in that part of the biography or uranium, which pre- ceded the staging of the first nuclear chain reaction in the world. This feat was realized on December 2, 1942, by Enrico Fermi.

In the late Thirties, Fermi, as well as many other leading scientists, were forced to emigrate to America in order to escape the Hitlerite plague. There they intended to continue their most important experiments. But these

experiments required ample funds. They had to convince the American government that the scientists' experiments would result in a powerful atomic weapon, which could be used in the struggle against fascism. This mission was undertaken by the great physicist Albert Einstein. He wrote a letter to the president of the USA, Franklin Roosevelt,

in which he called upon him to begin financing research work on uranium. Taking the enormous authority of Einstein into account and the seriousness of the international situation, Roosevelt gave his approval.

Lanthanum

207

In those years, when the events described here were taa~ng piace,

the chain reaction was regarded above all as a step on the road to achiev-

ing the atomic bomb. It was precisely in that direction that the research of the atomic scientists was directed in America.

Work on producing an atomic bomb, into which two bii l ion dollars

had been invested, was completed at about the middle of the ,.,,ear !945, and on August 6 the gigantic fireball mushroom arose over the Japanese

city of Hiroshima, c la iming hundreds of thousands of tires. This date became a black day in the history of civiIization, The greatest achieve- ment of science became the cause of mankind's greatest tragedy~

The question confronting scientists throughout the world was: where do we go from here? Continue perfecting

nuclear weapons, making even more horrible means of destruction of humans?

No! From now on the colossal energy locked in the nuclei of the atoms must be in the service of mankind,

The first step on this road was made by Soviet scientists under the supervision of the outstanding physicist Academi-

cian I. V. Kurchatov. On June 27, 1954, Moscow radio broadcast a news item of exceptional importance: "At the

present time, through the efforts of Soviet scientists and engineers, work on the design and construction of the world's

first full-scale industrial electric power generating station working on nuclear power, with an effective power output

rating of 5000 kilowatts, has been completed in the Soviet Union." For the first time, current-carrying energy born

in the depths of the uranium atom was transmitted along power lines.

The commissioning of the world's first nuclear electric power station laid down the beginnings of the develop ~

merit of a new branch of industry, the nuclear power industry. Uranium became the fuel of peace of the ~wentieth

century.

Another five years elapsed, and from the launching ways of Soviet shipyards came the world's first nuclear-

powered icebreaker, the Lenin. In order to keep its engines working at full power, with an output of 44,000 horse-

power, several dozen grams of uranium at most have to be "burned."

Fabulous perspectives have been opened up before mankind by uranium, which could right!y be termed the

most remarkable metal in nature !

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