The hidden powers of molybdenum8 March 2017
Imparting greatly increase corrosion resistance to stainless steel, molybdenum is making an ongoing positive contribution to radioactive waste management around the world. By Alan Hughes
Molybdenum is extracted from the mineral molybdenite and was discovered by Peter Jacob Hjelm in 1782. It remained a laboratory curiosity until the extraction of commercial quantities became practical and experiments in the early twentieth century demonstrated its potential to replace tungsten in many steel alloys. The end of World War II in 1945 brought increased research investment to develop new civilian applications and post-war reconstruction provided new markets for structural steels containing molybdenum.
Today, molybdenum is found in superalloys, lubricants, chemicals, electronics and many other applications, but its largest use remains in steel. When used as an alloying element, molybdenum increases the strength of steels without sacrificing toughness. It also imparts greatly increased corrosion resistance to stainless steel – the reason why molybdenum can contribute to the safety of spent fuel management throughout the world.
Long-term dry storage
Most reactors in operation today employ a wet discharge route for spent fuel. They use a purpose-built storage pond to dissipate the most intense heat and radiation, after which the fuel can be removed for reprocessing or long-term storage.
However, the lack of a permanent waste disposal strategy in many other countries has meant that fuel has stayed in discharge ponds for much longer than expected. This growing inventory of spent fuel in pond stores, with heightened security concerns and the need to address earthquake and tsunami resilience after the Fukushima Daiichi disaster, has encouraged the development of other solutions.
Dry cask storage is an increasingly popular option for nuclear plant operators, particularly in countries such as the US where there is no reprocessing route to deal with spent fuel. The US Nuclear Regulatory Commission (NRC) formally recognised onsite dry cask storage as safe for ‘short to indefinite’ timeframes in a rule (the Continuing Storage of Spent Nuclear Fuel Rule), published in August 2014.
The difference with duplex
Most casks are constructed from high- performance stainless steel and concrete and designed to last for many years. In recognition of the trend towards de facto long-term dry storage, Areva TN has developed a version of its Nuhoms® horizontal dry storage system that incorporates molybdenum-bearing duplex stainless steel canisters. For long-term storage, duplex has helped to resolve concerns about canister ageing and chloride-induced stress corrosion cracking in marine environments.
The two-phase microstructure of duplex stainless steel is ideal for long-term nuclear fuel storage, and is especially suited to environments near the coastline. It provides superior strength (compared to austenitic stainless steel) and better resistance to stress corrosion cracking, pitting and crevice corrosion, which are all common causes of deterioration in standard stainless steel grades when used in more aggressive marine environments.
Duplex stainless steel is being actively marketed as an important enhancement to the long-term reliability of dry storage, forming a significant layer of protection when placed inside the thick-walled, steel- reinforced concrete storage modules.
Reprocessing and waste management
Apart from storage and ultimate disposal, the only other spent fuel management route available to nuclear plant operators
is reprocessing. Here the mostly unburned uranium is chemically separated from the plutonium and fission products. In order to do this, the metal fuel casing is stripped away and sent for disposal. This is classified as intermediate level waste (ILW), as is certain other waste streams from reprocessing such as contaminated equipment.
Spent nuclear fuel has been reprocessed at the Sellafield nuclear site in Cumbria, UK, for more than 50 years. Much of the ILW from historic reprocessing has been stored, prior to a suitable waste treatment route becoming available. As the focus of the site shifted from commercial operation to waste remediation and hazard reduction in the 1990s, several waste management plants were constructed at Sellafield and the operator began to deal with the stored ILW.
The final wasteform produced had to be packaged in a material robust enough to provide adequate containment for further decades of on-site storage and ultimate geological disposal, in accordance with current UK policy. One of the biggest challenges to overcome in the safety case documentation approving the packaging was the risk of corrosion – not from the waste itself, but from chlorides in the atmosphere due to Sellafield’s coastal location. Stainless steel was considered to be the ideal material, with the addition of molybdenum to provide extra protection against chloride corrosion. Type 316L stainless steel typically containing 2.1% molybdenum was eventually selected.
Intermediate level waste is placed into drums made from Type 316L stainless steel and filled with cement grouting to make a solid, secure wasteform for above-ground storage in specially engineered facilities (as shown above).
On-site atmospheric testing
To qualify the selection and long-term use of stainless steel containing molybdenum, a programme of atmospheric corrosion testing was initiated in 1991. It used Type 316L S11 stainless steel, and a number of molybdenum- free grades. The samples were placed at test sites a short distance from the coastline. After 17 years of exposure, the samples were retrieved and analysed by radiography for signs of corrosion and other damage.
There was no evidence of corrosion or significant pitting damage in the molybdenum-containing sample (left), whereas stainless steel without molybdenum (409 grade was used in this example) showed significant staining (right). The test conditions are likely to be much more severe than any encountered in the engineered drum stores, therefore the results greatly increased confidence in the continued integrity of the drums during above ground storage.
Safe and effective management of radwaste from current and historic back-end fuel cycle operations is of growing importance.
Dry storage is an increasingly common mid- to long-term management option for many operators, and molybdenum is playing a role in increasing the long-term performance of stainless steel used in dry storage casks. Similarly, proving the viability of a long-term storage and disposal route for nuclear waste is essential to the clean-up programme underway at Sellafield and many other nuclear sites, and the extra degree of corrosion resistance imparted to waste drums through the addition of molybdenum is making an important contribution to reducing overall hazard.