Hafnium corrosion resistant alloys

Hafnium neutron absorption capabilities have caused its alloys to be proposed as separator sheets to allow closer spacing of spent nuclear fuel rods in interim holding ponds. Hafnium is the preferred material of constmction for certain critical mass situations in spent fuel reprocessing plants where hafnium s excellent corrosion resistance to nitric acid is also important. 

Hafnium is used in control rods for nuclear reactors. It has high resistance to radiation and also very high corrosion resistance. Another major application is in alloys with other refractory metals, such as, tungsten, niobium and tantalum. 

Tantalum-hafnium and tantalum-zirconium alloys are less suitable for aggressive acid environments. Compared with tantalum-tungsten alloys, tantalum rhenium alloys are superior in corrosion resistance and to hydrogen embrittlement, but the major disadvantage is the high cost of rhenium. 

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. 

Heat generation takes place in the reactor core of a nuclear plant (Figure 23.15). The core contains the fuel rods, which consist of fuel enclosed in tubes of a corrosion-resistant zirconium alloy. The fuel is uranium (IV) oxide (UO2) that has been enriched from 0.7% the natural abundance of this fissionable isotope, to the 3% to 4% required to sustain a chain reaction. Sandwiched between the fuel rods are movable control rods made of cadmium or boron (or, in nuclear submarines, hafnium), substances that absorb neutrons very efficiently. 

Metallic materials with the exception of noble metals are also thermodynamically not stable in the acidic environment under the PEFC operating conditions and therefore subject to corrosion. Nevertheless, many different metals such as stainless steels, aluminum, aluminum composites, copper, nickel and nickel alloys, titanium alloys and even highly corrosion resistant materials used in chemical industry such as tantalum, hafnium, niobium or zirconium have been investigated with respect to applicability in PEFC with respect to corrosion resistance [68—71]. 

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. 

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, hafnium, and their alloys can be either highly reactive or highly corrosion-resistant. In an incompatible environment, corrosion may occur within a short period of time, i.e., a few hours to a few days. On the other hand, in a compatible environment, there may be little change in mass (gain or loss) for years. Consequently, the commonly suggested test duration in hours, which is 50 divided by the corrosion rate in mm/y or 2000 divided by the corrosion rate in mpy, may be excessive and impractical. For example, if the corrosion rate determined from a short-term test is... 

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. 

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