Nuclear chemistry chain reactions

In nuclear chemistry, a fission reaction (see atomic energy) may be initiated by a neutron and may also result in the production of one or more neutrons, which if they reacted in like manner could start a chain reaction. Normally, moderators such as cadmium rods which absorb neutrons are placed In the reactor to control the rate of fission. 

HANN, OTTO (1879-1968). A German physical chemist who won the Nobel prize for chemistry in 1944 for his discovery of Ihe fission of heavy nuclei and the principle of the chain reaction, Well-known lor work on nuclear fission he discovered prnlactinium and iransuranium elements with atomic numbers 94. 95, and 96. After receiving his doctorate at the University of Munich, he worked in Canada before returning to Europe. 

The very first nuclear reactor built, where the main objective was to perform condensed matter research, was the High Flux Beam Reactor (HFBR) at Brookhaven National Laboratory, Upton, NY. The first self-sustaining chain reaction at the HFBR took place on Halloween, 1965. For over 30 years, the HFBR was one of the premier beam reactors in the world, matched only by the ILL reactor in Grenoble, France. These reactor-based sources have been a continuous and reliable source of thermal neutrons for research in a wide range of different scientific fields from physics, chemistry, materials science, and biology to engineering and isotope emichment. The instrumentation that is in place at these sources has seen steady improvement from the days when Nobel laureates, Brockhouse and Shull, performed their pioneering work at these facilities. 

Chain reactions are recursive reaction cycles that regenerate their intermediates. Such cycles occur in combustion, atmospheric chemistry, pyrolysis. photolysis, polymerization, nuclear fusion and fission, and catalysis. Typical steps in these systems include initiation, propagation, and termination. often accompanied by chain branching and various side reactions. Examples 2.2 to 2.5 describe simple chain reaction schemes. 

The discovery of nuclear fission in 1938 proved the next driver in the development of coordination chemistry. Uranium-235 and plutonium-239 both undergo fission with slow neutrons, and can support neutron chain reactions, making them suitable for weaponization in the context of the Manhattan project. This rapidly drove the development of large-scale separation chemistry, as methods were developed to separate and purify these elements. While the first recovery processes employed precipitation methods (e.g., the bismuth phosphate cycle for plutonium isolation). 

Fermi, Enrico. (1901-1954). An Italian physicist who later became a U.S. citizen. He developed a statistical approach to fundamental problems of physical chemistry based on Pauli s exclusion principle. He discovered induced or artificial radioactivity resulting from neutron impingement, as well as slow or thermal neutrons. He was professor of physics at Columbia (1939) and awarded the Nobel Prize in physics in 1938. He was the first to achieve a controlled nuclear chain reaction, directed the construction of the first nuclear reactor at the University of Chicago (1942), and worked on the atomic bomb at Los Alamos. He also carried on fundamental research on subatomic particles using sophisticated statistical techniques. Element 100 (fermium) is named after him. 

A great many reactions in physics and chemistry proceed via chain mechanisms. This large family of mechanisms includes free radical and ionic polymerization, Fischer Tropsch synthesis, gas phase pyrolysis of hydrocarbons, and catalytic cracking. Nuclear reactions, of both the power generating and the explosive kind, are also chain processes. Notice that chemical chain reactions can be catalytic or non-catalytic, homogeneous or heterogeneous. One is almost tempted to say that chain reactions are the preferred route of conversion in nature. 

Otto Hahn (Germany) for his discovery of the fission of heavy nuclei. Hahn s and his colleague s work discovered nuclear fission, and in particular that uremium could be split in a chain reaction by nuclear fission. This discovery was perhaps recognized as much for its importance as it was for its potential cbnger to society if not properly used and controlled, and Hahn himself was keenly aware of the potential for danger. Nonetheless, this discovery paved the way for much future research into nuclear chemistry, as well as for the development of modern nuclear reactors. 

Take a look at the equation for the fission of U-235 in the preceding section. Notice that one neutron was used, but three were produced. These three neutrons, if they encounter other U-235 atoms, can initiate other fissions, producing even more neutrons. It s the old domino effect. In terms of nuclear chemistry, it s a continuing cascade of nuclear fissions called a chain reaction. The chain reaction of U-235 is shown in Figure 5-3. 

The fate of actinide elements introduced into the environment is of course not merely a scientific issue. The disposal of the by-products of the nuclear power industry has become a matter of public concern. For each 1000 kg of uranium fuel irradiated in a typical nuclear reactor for a three-year period, about 50 kg of uranium are consumed. In addition to a large amount of energy evolved as heat, 35 kg of radioactive fission products and 15 kg of plutonium and transplutonium elements are produced. Many of the fission-product nuclides are stable, but others are highly radioactive. All of the fission products are isotopes of elements whose chemical properties are well-understood. The transuranium elements produced in the reactor by neutron capture, however, have unique chemical properties, which are reasonably well-understood but are not always easily inferred by extrapolation from the chemistry of the classical elements. Plutonium is fissile and can be recycled as a nuclear fuel in conventional or breeder reactors, but the transplutonium elements are not fissile to the extent of supporting a nuclear chain reaction, and in any event they are produced in amounts too small to be of interest for large-scale uses. The transplutonium elements must therefore be secured and stored. 

The Flerov Laboratory of Nuclear Reactions (FLNR) in Dubna, Russia, has recently announced the observation of relatively long-lived isotopes of elements 108, 110, 112, 114, and 116 [63-66] confirming the over 30 years old theoretical prediction of an island of stability of spherical superheavy elements. Due to the half-lives of the observed isotopes in the range of seconds to minutes, chemical investigations of these heaviest elements in the Periodic Table appear now to be feasible. The chemistry of these elements should be extremely interesting due to the predicted dramatic influence of relativistic effects [67], In addition, the chemical identification of the newly discovered superheavy elements is highly desirable as the observed decay chains [63-66] cannot be linked to known nuclides which has been heavily criticized [68,69],... 

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