Uranium transmutation

Rutherford s work has made him known as the father of nuclear physics with his research on radioactivity (alpha and beta particles and protons, which he named), and he was the first to describe the concepts of half-life and decay constant. He showed that elements such as uranium transmute (become different elements) through radioactive decay, and he was the first to observe nuclear reactions (split the atom in 1917). In 1908 he received the Nobel Prize in chemistry for his investigations into the disintegration of the elements, and the chemistry of radioactive substances. He was president of the Royal Society (1926-30) and of the Institute of Physics (1931-33) and was decorated with the Order of Merit (1925). He became Lord Rutherford in 1931. 

The new elements neptunium and plutonium have been produced in quantity by neutron bombardment of uranium. Subsequently many isotopes have been obtained by transmutation and synthetic isotopes of elements such as Ac and Pa are more easily obtained than the naturally occurring species. Synthetic species of lighter elements, e.g. Tc and Pm are also prepared. 

The transformations of the radioactive elements, whereby, e.g. uranium ultimately becomes lead, are not usually regarded as instances of transmutation because the processes are spontaneous, and cannot be controlled by the experimenter. 

Each of the elements has a number of isotopes (2,4), all radioactive and some of which can be obtained in isotopicaHy pure form. More than 200 in number and mosdy synthetic in origin, they are produced by neutron or charged-particle induced transmutations (2,4). The known radioactive isotopes are distributed among the 15 elements approximately as follows actinium and thorium, 25 each protactinium, 20 uranium, neptunium, plutonium, americium, curium, californium, einsteinium, and fermium, 15 each herkelium, mendelevium, nobehum, and lawrencium, 10 each. There is frequently a need for values to be assigned for the atomic weights of the actinide elements. Any precise experimental work would require a value for the isotope or isotopic mixture being used, but where there is a purely formal demand for atomic weights, mass numbers that are chosen on the basis of half-life and availabiUty have customarily been used. A Hst of these is provided in Table 1. 

Nuclear power reactors cause the transmutation of chemicals (uranium and plutonium) to fission products using neutrons as the catalyst to produce heat. Fossil furnaces use the chemical reaction of carbon and oxygen to produce CO2 and other wastes to produce heat. There is only one reaction and one purpose for nuclear power reactors there is one reaction but many puiposes for fossil-burning furnaces there are myriad chemical processes and purposes. 

Fig. 17.7), is therefore the nucleus of an atom of a different element. For example, when a radon-222 nucleus emits an a particle, a polonium-218 nucleus is formed. In this case, a nuclear transmutation, the conversion of one element into another, has taken place. Another important difference between nuclear and chemical reactions is that energy changes are very much greater for nuclear reactions than for chemical reactions. For example, the combustion of 1.0 g of methane produces about 52 kj of energy as heat. In contrast, a nuclear reaction of 1.0 g of uranium-235 produces about 8.2 X 10 kj of energy, more than a million times as much. 

Nuclear fuel reprocessing was first undertaken with the sole purpose of recovering plutonium, for weapons use, from uranium irradiated in nuclear reactors. These reactors, called the production reactors, were dedicated to transmuting as much of the uranium as possible to plutonium. From its original scope of recovering exclusively plutonium, with no attempts to either recover or recycle uranium, nuclear fuel reprocessing has since grown into a much more sophisticated and complex operation with expanded scope. It is now called upon to separate uranium and plutonium from the fission products, and to purify these elements to levels at which these fissile materials can be conveniently recycled for reuse. The present scope also extends to fission products separation and concentration. 

Using the elements mentioned in Section 22-13, induced radiation and the artificial transmutation of elements occur with both light elements, like the nonmetals 3H, 12C and 170 as well has heavier elements, like 97Tc, mFr, 210At and 239U, which can be metals, metalloids or nonmetals. Transuranium elements, i.e. the elements with atomic numbers greater than 92 (uranium), must be prepared by nuclear bombardment of other elements. 

In the year 2000, 15% of the world s electric power was produced by 433 nuclear power reactors 169 located in Europe, 120 in the United States, and 90 in the Far East. These reactors consumed 6,400 tons of fresh enriched uranium that was obtained through the production of 35,000 tons of pure natural uranium in 23 different nations the main purification step was solvent extraction. In the reactors, the nuclear transmutation process yielded fission products and actinides (about 1000 tons of Pu) equivalent to the amount of uranium consumed, and heat that powered steam-driven turbines to produce 2,400 TWh of electricity in 2000. 

Radon is the heaviest of the noble gases and is the only one that is radioactive. It is the decay product of radium, thorium, and uranium ores and rocks found underground. As it decays, it emits alpha particles (hehum nuclei) and is then transmuted to polonium and finally lead. The Earth s atmosphere is just 0.0000000000000000001% radon, but because radon is 7.5 times heavier than air, it can collect in basements and low places in buildings and homes. 

Radon s source is a step in the transmutation of several elements uranium —> thorium — radium —> radon —> polonium —> lead. (There are a number of intermediate decay products and steps involved in this process.) Radon-222 forms and collects just a few inches below the surface of the ground and is often found in trapped pockets of air. It escapes through porous soils and crevices. 

Promethium is not found in nature. Therefore, it is by far the least abundant on Earth none exists on the Earth. AH of it is man-made in nuclear reactors. It is found only in the transmuted decay by-products ( ashes ) from the fission of radioactive uranium. 

Plutonium exists in trace amounts in nature. Most of it isotopes are radioactive and manmade or produced by the natural decay of uranium. Plutonium-239 is produced in nuclear reactors by bombarding uranium-238 with deuterons (nuclei of deuterium, or heavy hydrogen). The transmutation process is as follows + deuterons—> 2 nuclei + Np + p— ... 

Today, physical chemistry has accomplished its great task of elucidating the microcosmos. The existence, properties and combinatory rules for atoms have been firmly established. The problem now is to work out where they came from. Their source clearly lies outside the Earth, for spontaneous (cold) fusion does not occur on our planet, whereas radioactive transmutation (breakup or decay), e.g. the decay of uranium to lead, is well known to nuclear geologists. The task of nuclear astrophysics is to determine where and how each species of atomic nucleus (or isotope) is produced beyond the confines of the Earth. 

Herbert Newby McCoy, 1870-1945. American chemist who made outstanding contributions to radioactivity and the chemistry of the lare earths In 1904 he showed that indium is produced by spontaneous transmutation of uranium Three years later, m collaboration with W. H. Ross, he pointed out the identical chemical behavior of the compounds of certain elements which F Soddy later called isotopes. Dr McCoy also gave the first quantitative proof that the o-ray activity of uranium compounds is directly proportional to their uranium content (78). 

In 1904 B. B. Boltwood, H. N. McCoy, and R. J. Strutt proved independently that radium is produced by spontaneous transmutation of uranium (107). Three years later Boltwood discovered an element which he named ionium and which he found to be the parent substance of radium (39). Professor Boltwood had acquired a broad cosmopolitan education in Munich, Leipzig, Manchester, and New Haven, and was a skilled laboratory technician, a sympathetic teacher, and a polished gentleman with a certain courtliness of manner. He proved that there is a genetic relationship between uranium, ionium, and radium (13). Ionium was discovered independently at about the same time by Otto Hahn and by Willy Marckwald (14, 73, 77). 

Twentieth Century Demargay discovers europium. Rutherford and Soddy discover thorium X. B. B. Boltwood, H. N. McCoy, and J. W. Strutt prove independently that radium is produced by spontaneous transmutation of uranium. 

Particle accelerators are used to produce most isotopes by transmutation. All elements greater than uranium, known as the transuranium elements, have been produced in particle accelerators. For example. 

Transmutation The changing of one element into another by a nuclear reaction or series of reactions. Example the transmutation of uranium-238 into plutonium-239 by absorption of a neutron. 

III hen a radioactive nucleus emits an alpha or beta particle, the identity UU of the nucleus is changed because there is a change in atomic number. The changing of one element to another is called transmutation. Consider a uranium-238 nucleus, which contains 92 protons and 146 neutrons. When an alpha particle is ejected, the nucleus loses 2 protons and 2 neutrons. Because an element is defined by the number of protons in its nucleus, the 90 protons and 144 neutrons left behind are no longer identified as being uranium. What we have now is a nucleus of a different element—thorium. 

TRANSMUTATION. The natural or artificial transformation of atoms of one element into atoms of a different element as the result of a nuclear reaction. The reaction may be one in which two nuclei interact, as in the formation of oxygen from nitrogen and helium nuclei (/3-particles), or one in which a nucleus reacts widi an elementary particle such as a neutron or proton. Thus, a sodium atom and a proton form a magnesium atom. Radioactive decay, e.g., of uranium, can be regarded as a type of transmutation. The first transmutation was performed bv the English physicist Rutherford in 1919. 

V I those with atomic numbers higher than uranium, do not occur naturally but are produced by nuclear transmutation reactions, discussed in Section 22.7. 

Other nuclear transmutations can lead to the synthesis of entirely new elements never before seen on Earth. In fact, all the transuranium elements—those elements with atomic numbers greater than 92—have been produced by bombardment reactions. Plutonium, for example, can be made by bombarding uranium-238 with a particles ... 

After a few years of storage, the main radioactive heat emitters in HLW are 90Sr and 137Cs. In addition, extremely long-lived actinides—neptunium, plutonium, americium, and curium—should be collected for transmutation in the future. Therefore, different flowsheets can be proposed for waste processing. It is possible to extract each radionuclide in the special extraction (sorption) cycle, for example, uranium and plutonium in the PUREX process, and after that, minor actinides (MAs) by the TRUEX process,4 strontium by the SREX process,5,6 and cesium by sorption7 or extraction.8... 

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