Hafnium nuclear properties

The elements will be discussed in the order of increasing atomic number in the Periodic Table, i.e. nickel, zinc, technetium, ruthenium, silver, hafnium, tantalum, tungsten, rhem um, osmium, iridium, platinum, gold, and mercury. Full numerical data of the relevant nuclear properties are summarized, as for other elements, in Appendix 1. 

Zirconium ores contain a few percent of its sister element, hafnium. Hafnium has chemical and metallurgical properties similar to those of zirconium, although their nuclear properties are markedly different. Flafnium is a neutron absorber but zirconium is not. As a result, there are nuclear and nonnudear grades of zirconium and zirconium alloys. Some commerdally available grades of zirconium and its alloys are listed in Table 22.2. 

Because the element not only has a good absorption cross section for thermal neutrons (almost 600 times that of zirconium), but also excellent mechanical properties and is extremely corrosion-resistant, hafnium is used for reactor control rods. Such rods are used in nuclear submarines. 

There are many advantages of using metal chlorides as interprocess intermediates. One of the most important advantages is that the metal chlorides could be readily purified. In other words, co-occurring metals could be more readily separated from one another as chlorides. This is particularly important when the co-occurring metals have very different technological properties and the presence of one in another in the final product is detrimental to the intended commercial application. A famous example of such co-occurrence is that of zirconium and hafnium in the mineral zircon. Not more than 100 ppm hafnium should be present in the zirconium intended for use in the nuclear reactor core. The hafnium content of zircon is about 2.5%. 

The chemical and physical properties of Unq (or rutherfordium) are homologous with the element hafnium ( jHf), located just above it in group 4 (fVB) in the periodic table. It was first claimed to be produced artificially by the Joint Institute for Nuclear Research (JINR) located in Dubna, Russia. The Russian scientists used a cyclotron that smashed a target of plutonium-242 with very heavy ions of neon-22, resulting in the following reaction Pu-242 + jjjNe-22 —> jj, Unq-260 + 4 n-1 (alpha radiation). The Russians named Unq-260 kurcha-tovium (Ku-260) for the head of their center, Ivan Kurchatov. (See details in the next section, History. )... 

Solvent extraction has proved to be the most effective method for the separation of zirconium and hafnium, which invariably occur in nature in close association, owing to their almost identical chemical properties. These metals have found considerable use in the nuclear-power industry on account of their unusually high (hafnium) and low (zirconium) neutron-capture cross-sections. It is evident that the mutual separation of the two metals must be of a high degree to make them suitable for such applications. Two different solvent-extraction processes are known to be used on a commercial scale in one process, zirconium is selectively extracted from nitrate media into TBP in the second process, hafnium is selectively extracted from thiocyanate solutions into methyl isobutyl ketone (MIBK). 

A problem of obtaining zirconium with lowest possible contents of hafnium comes from construction requirements when using zirconium and its alloys in building nuclear reactors. The construction material must have good mechanical properties and must be resistant to corrosion in contact with heat carriers. Since reactor power is proportional to the quantity of neutrons, their absorption into construction materials should be as small as possible. Zirconium and its alloys are unique materials that satisfy these requirements. However, hafnium has approximately the same chemical characteristics as zirconium but it absorbs neutrons strongly. 

Because hafnium has a high absorption cross-section for thermal neutrons (almost 600 times that of zirconium), has excellent mechanical properties, and is extremely corrosion resistant, it is used to make the control rods of nuclear reactors. It is also applied in vacuum lines as a getter —a material that combines with and removes trace gases from vacuum tubes. Hafnium has been used as an alloying agent for iron, titanium, niobium, and other metals. Finely divided hafnium is pyrophoric and can ignite spontaneously in air. 

The physical property of greatest interest for hafnium is how it responds to neutrons. A neutron is a very small particle found in the nucleus (center) of an atom. Neutrons are used to make nuclear fission reactions occur. Nuclear fission reactions take place when a neutron strikes a large atom, such as an atom of uranium. The neutron makes the atom break apart. In the process, a large amount of energy is released. That energy can be converted to electricity. 

In order to make electricity from nuclear fission, the fission reaction must be carefully controlled. To do that, the number of neutrons must also be kept under close control. Hafnium has the ability to absorb ( soak up ) neutrons very easily. It is used in rods that control how fast a fission reaction takes place. This property is one of the few ways in which hafnium differs from zirconium. Although hafnium is very good at... 

Element 104, the first transactinide element, is expected to have chemical properties similar to those of hafnium. It would, for example, form a relatively volatile compound with chlorine (a tetrachloride). The Soviet scientists have performed experiments aimed at chemical identification, and have attempted to show that the 0.3-s activity is more volatile than that of the relatively nonvolatile actinide trichlorides. This experiment does not fulfill the test of chemically separating the new element from all others, but it provides important evidence for evaluation. New data, reportedly issued by Soviet scientists, have reduced the half-hfe of the isotope they worked with from 0.3 to 0.15 s. The Dubna scientists suggest the name kurchatovium and symbol Ku for Element 104, in honor of Igor Vasilevich Kurchatov (1903—1960), late Head of Soviet Nuclear Research. The Dubna Group also has proposed the name for Element 104. In 1969, Ghiorso, Nurmia, Harris, K. A. Y. Eskola, and P. L. Eskolaof the University of California... 

Because of good corrosion resistance in both acids and bases, zirconium alloys are widely used in chemical plants. Commercial zirconium, as used primarily for corrosion resistance in the chemical industry [4], contains up to 4.5% hafnium, which is difficult to separate because of the similar chemical properties of zirconium and hafnium. TTie presence or absence of hafnium has no effect on the corrosion resistance, which is controlled by a very stable oxide. At ambient temperature, this passive oxide is 2-5 nm thick [4]. The pure metal low in hafnium (0.02% max) has a low thermal neutron capture, making it useful for nuclear-power applications. 

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