A role for zirconium23 October 2019
Zirconium has an important role in the nuclear sector. Keven Harlow looks at the development of the material and its applications.
THE EARLY DEVELOPMENT OF ZIRCONIUM metallurgy was essentially due to the nuclear power industry.
Zirconium alloys are the proven structural material for nuclear fuel cladding in light water reactors. This is primarily because of the alloy’s combination of good corrosion resistance in the water chemistry at 300oC and low capture cross-section for thermal neutrons.
The decision to use a zirconium alloy in nuclear fuel cladding, made during the early 1950s, was due to Admiral Hyman Rickover of the US Navy. Charged with developing nuclear-propelled ships and submarines, Rickover realised that the reactor for use in vessels had to be compact and able to operate when the ship was rolling or pitching, or at an angle when the submarine was diving or surfacing.
A pressurised water reactor was envisaged, requiring a fuel cladding material that would withstand corrosion at high temperatures and over long periods of time, would maintain its integrity in intense radiation and would not absorb the neutrons required for a nuclear reaction.
Stainless steel, beryllium and aluminium were compared and considered unsuitable.
In fact, initial tests with zirconium showed that it absorbed the neutrons needed for the fission process. This was because the zirconium contained about 2% (by weight) of hafnium (Hf). Tests at Oak Ridge National Laboratory in Tennessee successfully separated Hf from Zr and showed that zirconium in its pure form absorbed very few neutrons.
As nuclear power generation became commercialised there was a clear need to reduce the production cost of zirconium. The high-purity zirconium that proved to be a good choice for the USA’s Shippingport reactor (in 1958) was extremely expensive — around $300 per pound. But at the same time as the price of high-purity zirconium gradually fell, to around $10 per pound by the 1970s, fuel loads for all light water reactors began to use variants of zirconium alloy (zircaloy) cladding.
In the very early days it was found that, contrary to expectations, the corrosion resistance of the purer zirconium material was lower than that of the impure. The addition of small amounts of iron and tin were found to improve corrosion resistance, and over the years the PWR fuel suppliers began developing proprietary zirconium alloys by varying the amounts of tin, iron, niobium and chromium added.
These changes, coupled with improvements to alloy annealing treatments, enhanced the in-reactor dimensional stability of the materials and minimised corrosion while operating at higher fuel duties, improving economics.
Zirconium alloys are now a mature product, contributing to improved fuel performance via higher fuel burnups and lower failure rates.
Overall, the nuclear power sector consumes only a small percentage of the market for zircon. As a strategic material, the zirconium market is commercially confidential and fluctuates with the supply and demand for various forms of nuclear-grade zirconium.
The fuel fabricators can take different approaches to the procurement of nuclear grade Zr metal:
- Vertically integrated nuclear companies may procure zircon or zirconium oxychloride and process through to finished components;
- Non-integrated nuclear companies can acquire from companies such as the above;
- Both vertically integrated and non-integrated companies can purchase from specialist metal producers.
Some new reactor designs do not use zirconium as a fuel cladding. However, the very long lead-time for alternative cladding materials, such as silicon carbide, means that the nuclear market for zirconium alloys is likely to remain stable for many years to come.
Author information: Keven Harlow, Executive director, Zircon Industry Association