Zirconium for Nuclear Power

The current predominant use of metallic zirconium is for commercial and military nuclear power. The rapid development of nuclear power changed the situation for zirconium and resulted in a big increase in demand. The background to this is that in tubes for containing the fuel in the uranium reactors a material is needed that absorbs thermal neutrons only to a very small extent and at the same time has corrosion resistance sufficiently high to withstand the very corrosive environment in the reactor. Zirconium is such a metal. 

There were, however, two large problems. One was that no commercial manufacturing method was available at the time when the exploitation of atomic energy started. The American Atomic Energy Commission initially used the iodide method. Impure zirconium metal was refined until its properties matched those required for material for fuel-rod containment tubes. It was expensive, but the development of the KroU process solved that problem. 

The other big problem was the hafnium content of natural zirconium. In the actual tubes the hafnium content must be lower than 100 ppm, because hafnium absorbs neutrons 550 times more strongly than zirconium. Zircon sand, the most important ore, has a hafnium content of 1.5-2.596 (relative to zirconium). Thus it was necessary to find a separation method with which hafnium-free zirconium could be manufactured. The solution of this problem was liquid-liquid extraction. From an impure zirconium solution in hydrochloric acid, iron is first removed by solvent extraction. Then ammonium thiocyanate is added to the chloride solution containing Zr -i- Hf. Hafnium is extracted into an organic phase of methyl isobutyl ketone. The pure zirconium solution in the water phase is worked up for zirconium. Hafnium is recovered from the ketone solution by scrubbing with dilute sulfuric acid. 

The separation technique described and the Kroll process created the possibihty of manufacturing hafnium-free zirconium on an industrial scale for atomic energy 

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