Zirconium nuclear properties

The nuclear properties of fuel cladding material must also be satisfactory. For thermal reactors, it is important that the material have a reasonably small absorption cross section for neutrons. Only four elements and their alloys have low thermal-neutron absorption cross sections and reasonably high melting points aluminum, beryllium, magnesium, and zirconium. Of these, aluminum, magnesium, and zirconium are or have been utilized in fuel-element cladding. 

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. 

Table 5.20 Minimum mechanical properties of nuclear grade zirconium alloys...

Zirconium carbide (ZrC) is a refractory interstitial carbide with a high melting point. It is produced by CVD mostly on an experimental basis although it has some nuclear applications. Like TiC, cubic ZrC has a variable composition and forms solid solutions with oxygen and nitrogen over a wide range of composition. Its characteristics and properties are summarized in Table 9.10. 

Zirconium carbide is a highly refractory compound with excellent properties but, unlike titanium carbide, it has found only limited industrial importance except as coating for atomic-fuel particles (thoria and urania) for nuclear-fission power plants.l " ] This lack of applications may be due to its high price and difficulty in obtaining it free of impurities. 

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%. 

Kroll process, 13 84-85 15 337 17 140 in titanium manufacture, 24 851-853 Kroll zirconium reduction process, 26 631 KRW gasifier, 6 797-798, 828 Krypton (Kr), 17 344 commercial, 17 368t complex salts of, 17 333-334 doubly ionized, 14 685 hydroquinone clathrate of, 14 183 in light sources, 17 371-372 from nuclear power plants, 17 362 physical properties of, 17 350 Krypton-85, 17 375, 376 Krypton compounds, 17 333-334 Krypton derivatives, 17 334 Krypton difluoride, 17 333, 336 uses for, 17 336... 

Zirconium is used for structural parts in the core of water moderated nuclear reactors to this end Zr has several good properties and especially it has low thermal neutron cross-section. Hf, on the contrary, has a high thermal neutron absorption coefficient, so it is necessary to be able to prepare Hf-free zirconium. On the other hand, in some cases the Hf properties too may be useful in nuclear technology, in the control rods of submarine reactors. 

The most important applications of zirconium involve its alloys, Zircaloy. The aUoy offers excellent mechanical and heat-transfer properties and great resistance to corrosion and chemical attack. This, in conjunction with the fact that zirconium has a low neutron absorption cross section, makes this ahoy a suitable choice as a construction material for thermal nuclear reactors and nuclear power plants. Other uses are as an ingredient of explosive mixtures, as getter in vacuum tubes, and in making flash bulb, flash powder (historical), and lamp filaments, in rayon spinnerets, and in surgical appliances. 

In the search for substitutes, other considerations than just sulfide stability have to be considered. These include the possible interference of the newly introduced element with other steel porperties, the plasticity of the new sulfides, the physical alloyability of the additive and, of course, the cost effectiveness of the additive. Zirconium and titanium interfere with other properties of the steel because of the excessive stability of their nitrides. Figure 9, and carbides. Figure 10. Although considerable usage of these two elements has played a part in sulfide substitution — over 500 metric tons of nuclear zircalloy scrap were used in — it appears that their role will progressively fade away primarily because of poor low temperature impact properties of steels treated with Zr and Ti. 

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). 

Zirconium hydride, ZrH2, finds application as an antioxidant for rubber, in addition to being a moderator for nuclear fuel elements, and as a hydrogenation catalyst. Zirconium hydroxide, best formulated as Zr02 hH20 because of its variable water content, is used as a drying agent and absorbent and also has deodorant properties.19... 

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 low cross-section for absorption of neutrons and high-temperature (330-350°C) aqueous corrosion resistance as well as its good mechanical properties promote the use of zirconium alloys in the nuclear reactors. In the development of zirconium alloys care must be taken that the added minor elements do not posses high cross-sections for the absorption of neutrons and contribute to greater corrosion resistance and improved mechanical properties. The good corrosion resistance of the alloys in acids and bases favors the use of zirconium alloys in chemical plants. 

Zirconium is one of the few metals that retains its structural integrity upon exposure to radiation. For this reason, the fuel rods in most nuclear reactors are made of zirconium. Answer the following questions about the redox properties of zirconium based on the halfreaction... 

All other materials used in nuclear reactors for construction or as tubes should exhibit low neutron absorption, low activation, no change in the properties under the influence of the high neutron and y-ray fluxes and high corrosion resistance. These requirements are best met by zirconium which has found wide application in nuclear reactors. Al, Be and Mg have hmited applicabihty. Steel and other heavy metals are only applicable if their relatively high neutron absorption is acceptable. 

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... 

Properties Dense, silvery solid. D 19.0, mp 1132C, bp3818C, heat of fusion 4.7 kcal/mole, heat capacity 6.6 cal/mole/C. Strongly electropositive, ductile and malleable, poor conductor of electricity. Forms solid solutions (for nuclear reactors) with molybdenum, niobium, titanium, and zirconium. The metal reacts with nearly all nonmetals. It is attacked by water, acids, and peroxides, but is inert toward alkalies. Green tetravalent uranium and yellow uranyl ion (U()2") are the only species that are stable in solution. 

Chapters S, 6, and 7 take up uranium, thorium, and zirconium in that order. Each chapter discusses the physical and chemical properties of the element and its compounds, its natural occurrence, and the processes used to extract the element from its ores, purify it, and convert it to the forms most useful in nuclear technology. 

Pure zirconium metal is highly resistant to heat and corrosion, and it imparts these properties to its alloys. For these reasons it has become an important material in the aviation, aerospace, chemical, and surgical instrument industries, and in nuclear reactor technology. The ability of zirconium to reject neutrons is utilized for the protection of heating elements in pressurized water and hot water reactors (see also Rubel 1983, Deknudt 1988, Trueb 1990). 

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