Neutron absorbers hafnium

Hafnium is obtained as a by-product of the production of hafnium-free nuclear-grade 2irconium (see Nuclear reactors Zirconiumand zirconium compounds). Hafnium s primary use is as a minor strengthening agent in high temperature nickel-base superakoys. Additionally, hafnium is used as a neutron-absorber material, primarily in the form of control rods in nuclear reactors. 

The fourth component is the set of control rods, which serve to adjust the power level and, when needed, to shut down the reactor. These are also viewed as safety rods. Control rods are composed of strong neutron absorbers such as boron, cadmium, silver, indium, or hafnium, or an alloy of two or more metals. 

The Model 412 PWR uses several control mechanisms. The first is the control cluster, consisting of a set of 25 hafnium metal rods coimected by a spider and inserted in the vacant spaces of 53 of the fuel assembhes (see Fig. 6). The clusters can be moved up and down, or released to shut down the reactor quickly. The rods are also used to (/) provide positive reactivity for the startup of the reactor from cold conditions, (2) make adjustments in power that fit the load demand on the system, (J) help shape the core power distribution to assure favorable fuel consumption and avoid hot spots on fuel cladding, and (4) compensate for the production and consumption of the strongly neutron-absorbing fission product xenon-135. Other PWRs use an alloy of cadmium, indium, and silver, all strong neutron absorbers, as control material. 

Control of the core is affected by movable control rods which contain neutron absorbers soluble neutron absorbers ia the coolant, called chemical shim fixed burnable neutron absorbers and the intrinsic feature of negative reactivity coefficients. Gross changes ia fission reaction rates, as well as start-up and shutdown of the fission reactions, are effected by the control rods. In a typical PWR, ca 90 control rods are used. These, iaserted from the top of the core, contain strong neutron absorbers such as boron, cadmium, or hafnium, and are made up of a cadmium—iadium—silver alloy, clad ia stainless steel. The movement of the control rods is governed remotely by an operator ia the control room. Safety circuitry automatically iaserts the rods ia the event of an abnormal power or reactivity transient. 

The effect of the lanthanide contraction on the metal and ionic radii of hafnium has already been mentioned. That these radii are virtually identical for zirconium and hafnium has the result that the ratio of their densities, like that of their atomic weights, is very close to Zr Hf = 1 2.0. Indeed, the densities, the transition temperatures and the neutron-absorbing abilities are the only common properties of these two elements which differ... 

A flow sheet like Fig. 4.4 has been used to separate uranium from neutron-absorbing impurities (Chap. 5), and zirconium from hafnium (Chap. 7), by fractional extraction of an aqueous nitrate solution with an organic solution of TBP in kerosene. 

Zirconium has a high corrosion resistance and low cross-section for neutron capture (see Section 2.4) and is used for cladding fuel rods in water-cooled nuclear reactors. For this application, Zr must be free of Hf, which is a very good neutron absorber. The main use of pure Hf is in nuclear reactor control rods. Zirconium and hafnium compounds possess similar lattice energies and solubilities, and their complexes have similar stabilities this means that... 

Zirconium s major use is as cladding for nuclear reactors. It is ideal for this use, as it has a hmited ability to capture neutrons, strength at elevated temperatures, considerable corrosion resistance, and satisfactory neutron damage resistance. Almost all ores of zirconium contain about 2 percent of zirconium s sister element, hafnium (Hf). Hafnium readily absorbs neutrons and therefore must be completely separated and removed from zirconium before either element can be used in nuclear reactors. A major task of the Manhattan Project was the separation of hafnium from zirconium. The elements are the two most chemically similar in the Periodic Table. The recovered hafnium metal is used to make the control rods of nuclear reactors, as the metal readily absorbs neutrons. 

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. 

Advantages Highly effective neutron absorber. Control effectiveness in water-moderated reactors is close to hafnium. Used in pressurized-water reactors. Easily fabricated and adequate strength... 

Hafnium-like boron is known to be a neutron absorber or neutron moderator element, and, therefore, composites of boron carbide, B4C, and hafnium diboride, HfB2, can be considered as nuclear materials. These boron compounds after sintering and °B/"B isotopic ratio adapting are found to be heterogeneous polyphone cermets useful for nuclear applications (Beauvy et al. 1999). Boron acid obtained from the °B enriched boron trifluoride also was used in nuclear reactors (Shalamberidze et al. 2005). Amorphous boron powders enriched both in °B and "B, boron carbide, and zirconium diboride (ZrB2) powders and pallets labeled with °B isotope And applications in nuclear engineering too. The °B enriched Fe-B and Ni-B alloys are useful for the production of casks for spent nuclear fuel transfer and storage. 

C. The element is found with zhco-nium and is extracted by formation of the chloride and reduction by the KroU process. It is used in tungsten alloys in filaments and electrodes and as a neutron absorber. The metal forms a passive oxide layer in air. Most of its compounds are hafnium(IV) complexes less stable hafnium(III) complexes also exist. The element was first reported by Urbain in 1911, and its existence was finally established by... 

Other metals such as beryllium, hafnium, niobium, vanadium, and zirconium are known to have nuclear and other properties which make them desirable materials of construction in various designs of nuclear reactor, but also they have, or may have in the future, important uses outside that field. All these metals except hafnium have been used or proposed for canning materials to clad and protect the nuclear fuel metals from corrosion by the reactor coolants or moderators, air, carbon dioxide, water, heavy water, graphite or molten sodium, etc. In some cases the specifications for neutron-absorbing impurities are of the same order as for the fuel metals uranium and thorium. Hafnium, however, with a high neutron-capture cross-section, is a useful material for reactor control rods and exhibits favourable metallurgical properties under irradiation. 

A group of four fuel assemblies, surrounding a cruciform control rod, makes up a core module unit. The control rod blades and control rod drives for the BWR 90 are of a well-proven design. The cruciform rod is based on solid steel blades that are welded together. Holes filled with B4C as neutron absorber are drilled horizontally in the blades. In the top of the rod, the absorber consists of Hafnium which makes the rod tip more "grey" and provides for a long service life. 

The tubes themselves must be fully characterized in order to ensure that the zirconium alloys, or magnesium alloys, do not contain impurities that may affect the performance of the fuel. For example, the presence of traces of neutron absorbers, like hafnium that always accompanies zirconium in nature, or elements that modify the corrosion resistance of zirconium, must be determined. The ASTM has outlined the specifications for seamless wrought zirconium alloy tubes that are used for nuclear fuel cladding (B811 2013). The exact technical details and analytical test procedures do not directly involve uranium and are beyond the scope of this book. 

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. 

The CCR core has 69 cmciform control rods. The control rods contain boron carbide or hafnium as a neutron absorber. 

From the above short review, it appears that most of the present nuclear reactors use a narrow sampling of neutron absorber materials. This, of course, first results from the neutron properties of the elements. This is also a consequence of the materials and elaboration processes availability. For example, AIC has been developed as a surrogate to hafnium and boron is mainly used as boron carbide. This is always a compromise regarding the previous examples, AIC has the lowest melting point of all the core materials and boron carbide is a brittle ceramic enduring premature cracking. 

Hafnium can no longer be used as a neutron absorber in fast neutron reactors, due to insufficient absorption efficiency. However, due to the strong moderation effect of hydrogen (concentration in metal hydrides is comparable to water), hafnium hydride has an initial absorption efficiency comparable to enriched boron carbide, with much slower decrease during operation (Fig. 15.18, [52]). 

B. Cheng, R.L. Yang, Hafnium Alloys as Neutron Absorbers, US Patent 5330589, 1994. 

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