Zirconium neutron-absorption cross section

Zirconium is used as a containment material for the uranium oxide fuel pellets in nuclear power reactors (see Nuclearreactors). Zirconium is particularly usehil for this appHcation because of its ready availabiUty, good ductiUty, resistance to radiation damage, low thermal-neutron absorption cross section 18 x 10 ° ra (0.18 bams), and excellent corrosion resistance in pressurized hot water up to 350°C. Zirconium is used as an alloy strengthening agent in aluminum and magnesium, and as the burning component in flash bulbs. It is employed as a corrosion-resistant metal in the chemical process industry, and as pressure-vessel material of constmction in the ASME Boiler and Pressure Vessel Codes. 

Zirconium and zirconium alloys are used in the nuclear industry, because of their low neutron absorption cross-section and resistance to hot water at high pressures. 

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. 

The zirconium tetrachloride product must then be purified before reduction to metal. In particular, hafnium must be removed to less than 100 ppm Hf Zr because of the high neutron absorption cross-section it exhibits, and phosphorus and aluminum must be removed to even lower specifications due to their deleterious metallurgical impact on the final zirconium alloys. The tetrachloride product is first dissolved in water under carefully controlled conditions to produce an acidic ZrOCl2 solution. This solution is complexed with ammonium thiocyanate, and contacted with methyl isobutyl ketone (MIBK) solvent in a series of solvent extraction columns. Advantage is taken of the relative solubilities of Zr, Hf, and Fe thiocyanate complexes to accomplish a high degree of separation of hafnium and iron from the zirconium. 

In its natural state, zirconium, which is an important material of construction for nuclear reactors, is associated with hafnium, which has an abnormally high neutron-absorption cross section and must be removed before the zirconium can be used. Refer to the accompanying flowsheet for a proposed liquid/liquid extraction process wherein tributyl phosphate (TBP) is used as a solvent for the separation of hafnium from zirconium. [R. P. Cox, H. C. Peterson, and C. H. Beyer, Ind. Eng. Chem., 50(2, 14 (1958).]... 

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. 

Because of its neuronic, mechanical, and physical properties, hafnium is an excellent control material for water-cooled, water-moderated reactors. It is found together with zirconium, and the process that produces pure zirconium produces hafnium as a by-product. Hafnium is resistant to corrosion by high-temperature water, has adequate mechanical strength, and can be readily fabricated. Hafnium consists of four isotopes, each of which has appreciable neutron absorption cross sections. The capture of neutrons by the isotope hafnium-177 leads to the formation of hafnium-178 the latter forms hafnium-179, which leads to hafnium-180. The first three have large resonance-capture cross sections, and hafnium-180 has a moderately large cross section. Thus, the element hafnium in its natural form has a long, useful lifetime as a neutron absorber. Because of the limited availability and high cost of hafnium, its use as a control material in civilian power reactors has been restricted. 

Because of its low neutron-absorption cross section and good corrosion resistance zirconium is used in water-moderated nuclear reactors. In tubes for sealing the fuel a hafnium-free zirconium, alloyed with 1.5% tin, is used. 

It is possible to increase the fuel efficiency by selecting the zirconium isotope which has the lowest neutron absorption cross section (0.08 bams see Table 7.2) for the fabrication of the zircaloy. Several attempts have been made since the high cost of an enrichment process is a major capital... 

Based on the data available in 1947 the neutron absorption cross-section for zirconium was no better than stainless steel. Iron, the principal element in stainless steel, had a microscopic cross-section of 2.55 b (Lamarsh and Baratta, 2001). The bam had been adopted by early physicists as a measure of area for the incredibly small values at the atomic levels. Each bam represented 10" cm. In a rare show of humor, the term bam arose from the expression of hitting the broad side of a bam. Zirconium had been found to have a cross-section nearly identical to iron, 2.5 b, thus seeming to offer little advantage. 

Further studies by researchers at MIT and ORNL rechecked the absorption cross-section of pure zirconium samples. They found the cross-section to be nearly a factor of 5x lower than initially reported. Its actual value was calculated as being between 0.4 and 0.5 b, well below the level of stainless steel. This new level meant that zirconium was significantiy better than stainless steel at allowing neutrons to pass through it without being absorbed. Even more recent data has zirconium listed with a neutron absorption cross-section of 0.185, over a tenfold reduction in absorption cross-section over the initially listed value (El-Wakil, 1962). 

Commercial-grade zirconium contains from 1 to 3% hafnium. Zirconium has a low absorption cross section for neutrons, and is therefore used for nuclear energy applications, such as for... 

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. 

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. 

Zirconium and hafnium have very similar chemical properties, invariably occur together in nature, and are difficult to separate. Yet their absorption cross sections for thermal neutrons are very different ... 

The thermal absorption cross section of zirconium is the lowest of all mechanically strong, high-melting, corrosion-resistant metals. For this reason, zirconium and zirconium-based alloys are the materials preferred for cladding and structural materials in water-cooled, thermal-neutron power reactors. 

One of its attractive features of rhenium is that it is a spectral shift absorber (SSA), which means that it has a low relative absorption cross section for fast neutrons while in the thermal spectrum its absorption cross section increases dramatically. This has safety applications for the reactor design in accident scenarios. Rhenium has an absorption cross section of in the fast spectrum, however the magnitude of the difference between the absorption cross section and the fast fission cross section of is low compared to the difference at a thermal spectrum. It also provides a barrier that protects Niobium 1% Zirconium from nitrogen attack and damage caused by other fission products that outgas from the fuel. Most of the other SSA materials have a relatively low melting point, making them less attractive. 

Because hafnium has an elevated absorption cross section for thermal neutrons (almost 565 times that of zirconium), it is extensively used for producing nuclear-reactor control rods. On the other hand, hafnium carbide is the most refractory binary composition known, and the nitride is the most refractory of all known metal nitrides m.p. 3310 C). To a lesser extent, hafnium is used in gas-filled and incandescent lamps as an efficient getter for scavenging oxygen and nitrogen, and alloying with iron, titanium, niobium, and other refractory metal alloys. 

Absorption and scattering cross sections for the elements of importance in reactor physics are summarized in Table A.4. For reasons to be discussed later, the values given are for a neutron energy of 0.0253 eV, corresponding to a neutron velocity of 2200 ms Attention may be drawn to some of the particularly low absorption cross sections, such as that of zirconium, which is consequently valuable as a structural material in reactors. The reason for zirconium, and also lead, having such a low cross section is that the main isotope in both cases has a nucleus with a magic number of neutrons or protons, or both (see Section 1.4). 

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