Hafnium absorber material

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. 

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. 

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. 

Compared with most metals, the annual production of hafnium is low. Mainly produced in Ihe United Stales. France, and Russia, the comhined production is in the range of 100 metric tons annually, or less. Several uses have been found for hafnium (11 as a control material in water-cooled nuclear reactors. Also hafnium is an effective flux-depressor in a reactor for absorbing neutrons to decrease the peaks in neutron flux ... 

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. 

Hafnium is a powerful absorber (105 barns) of thermal neutrons. Therefore, it is used in the nuclear industry as a material for reactor control rods (e.g., in nuclear submarines) and for protective screens in reactors. The nuclear industry utilizes about 90% of the total hafnium output. 

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. 

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. 

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. 

The neutron absorption of hafnium leads to the formation of hafnium isotopes, themselves absorbers (Fig. 15.7) and then to isotopes of heavier elements Ta, and W (up to a few percent Lu is produced in very small amounts). Among these, Ta is the main source of the radiotoxicity of the material, but with a 115-day period, making it a shortlived waste. 

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