Securing Stainless

Stringent safety standards in the nuclear industry require fabricators to source stainless steel according to tightly controlled specifications

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Stainless steel is an essential material for the nuclear industry. It offers good mechanical properties and corrosion resistance that enable nuclear engineers to build long-lasting and durable pipework, tanks, pool liners and structural elements. This process equipment is often used for critical roles, such as containing coolants and fluids at the heart of the reactor.

Nuclear authorities protect quality standards and prevent risk by setting lists of approved materials for engineering, procurement and construction (EPC) contractors and fabricators to follow. For example, the US ASME III nuclear code allows the use of a limited range of stainless steel 304 and 316 and their close variants.

At Outokumpu’s stainless steel production unit in Calvert, Ala., employees carefully inspect all material before it leaves the facility.

These alloys are among the most widely available metals on the market, so it may come as a surprise that there is a supply bottleneck for them when it comes to nuclear projects. The reason for this is are the tight additional requirements imposed by nuclear design engineers over these alloys – in particular relating to trace levels of cobalt. As a result, fabricators may find it challenging to source stainless steel for the industry. The following article, however, explains how to overcome sourcing challenges for stainless steel so that material will be available when it’s needed.

More radioactive

The issue with cobalt is that when it is exposed to radiation, it transmutes into a radioactive isotope. This means that pipework or other process equipment that carries primary coolant will become more radioactive over time. Understandably, nuclear authorities want to protect workers by preventing this. Therefore, they use specifications to limit the cobalt content of stainless steel.

It might seem counterintuitive to specify the content of cobalt in stainless steel – after all, stainless steel alloys typically don’t include cobalt. However, nickel is one of the most important alloying elements of stainless steel and cobalt occurs naturally alongside nickel in nature.

The underlying issue is that the two elements cannot easily be separated. This means that cobalt exists in most stainless steel, albeit in trace quantities.

Production based

According to the Nickel Institute, around two thirds of global nickel production is used to produce stainless steel. As an alloying element, it provides formability, weldability and ductility to stainless steel through control of the microstructure. The austenitic grades of stainless steel used in the nuclear industry typically contain eight to 10 percent nickel to give the steel the correct microstructure and mechanical properties.

Whether a nuclear fabricator needs plate, sheet, pipe or tubing, it’s important that it’s sourced from a mill that has the right experience and procedures in place to meet quality specifications.

Although most nickel reserves contain cobalt, some have naturally very low levels. Therefore, by sourcing ore from these mines, a stainless steel manufacturer can produce low-cobalt stainless steel to meet nuclear authorities’ specifications.

These requirements vary between different nuclear authorities, with the maximum cobalt content typically being in the range of 0.05 to 0.2 percent. For example, in France, the RCC-M code often specifies a maximum of 0.2 percent cobalt.

There may also be other requirements around the alloy. For example, there might also be limits on other trace elements, such as boron, tantalum and phosphorous.

For fabricators, this can represent a major challenge when sourcing stainless steel. The low-cobalt requirement is limited to the nuclear industry. As a result, stocks of material may not be readily available.

Other requirements

The other aspect of delivering equipment for nuclear projects is that contractors should only source material from mills with the right experience and procedures in place to meet quality specifications. Nuclear operators need to have confidence that their systems will provide the required level of service. For a material such as stainless steel, that means consistent alloying content as well as consistent mechanical dimensions and mechanical performance.

As well as sourcing low-cobalt ore, the steelmaker needs to follow strict quality assurance processes to prevent cross-contamination with other batches during steel melting and throughout subsequent processing steps. A high level of destructive and non-destructive testing is also carried out on the stainless steel as it goes through production.

Together, these steps ensure that the material has achieved the required properties and is fully traceable. However, they also require the steelmaker to take additional steps and incur extra costs. As a result, nuclear-quality stainless steel comes at a price premium.

Steelmakers’ dilemma

Ordering material becomes even more complex for fabricators as they need it in specific product forms, such as plate, sheet, pipe or tubing, and in different sizes.

As a full specification, these multiple requirements can make it tricky to source material for a project. Steelmakers might have large quantities of material in their stock yards, but the additional low-cobalt requirement may make the specification impossible to meet. Instead, the stainless steel producer may be faced with melting a special batch for the project.

Even though a steelmaker might have large quantities of standard stainless steel on hand, the low-cobalt material nuclear fabricators need is often produced with less frequency.

This represents a risk and a complex commercial decision for steelmakers as well as longer lead times for their customers to deal with.

The challenge is best explained with a typical project. Let’s say that a fabricator wants to source 5, 10 or 20 metric tons of stainless steel in grade 316L in the forms of plate, sheet and tube. The issue for the steel mill is that it produces melts of 50 to 100 metric tons so a 20-metric ton order will be just a fraction of a full melt.

This creates a dilemma: Should the steelmaker accept the order and schedule a full melt of grade 316L with 0.2 percent cobalt? If they do, they can keep the remaining 30 to 80 metric tons in the stockyard to meet future orders. However, there’s no guarantee of when (if ever) they will receive further orders for the same grade and cobalt content. This could leave their cashflow tied up in stock for a long period. Alternatively, they could face selling the high-value low-cobalt material at a loss to another industry.

Another complexity is that the next nuclear project may need a different specification, such as grade 304 stainless steel with no more than 0.1 percent cobalt. The buyer might be happy to receive a batch of higher alloyed 316L due to its high level of corrosion resistance, however, they may not have the freedom to negotiate when the cobalt content is higher than their standard will accept.

For this reason, nuclear projects represent a commercial challenge for steelmakers. In turn, EPC contractors and fabricators may find it hard to source materials, especially when working on smaller projects or producing equipment for maintenance and repair projects.

Transparent demand

The solution to overcome this challenge is for project managers at nuclear projects to release technical specifications as soon as possible. By sharing these details, fabricators can work with steelmakers early and provide them with greater certainty over future demand. This will enable steelmakers to see a business case that justifies production of specialist melts for nuclear projects.

Another approach is for the project to combine multiple requirements with similar specifications. In the previous example, the steelmaker could supply both projects with 316L stainless steel with up to 0.1 percent cobalt. This will share the cost of producing nuclear material across multiple needs – but, it requires an open and collaborative approach so that requirements can be aggregated into a single batch.

Transforming sourcing

Although the same challenge will always remain for large-scale nuclear reactors, the advent of small modular reactors (SMRs) will simplify sourcing.

Stainless steel is used in nuclear applications due to its mechanical properties and corrosion resistance that result in long-lasting and durable products.

Under the SMR principle, small reactors will be built on a serial production line. This will create more predictable demand for stainless steel and other materials, making it easier for steel mills to schedule production. This will improve availability as well as cutting costs and lead times.

The same classic grades of 304 and 316 stainless steel and their variants will continue to be important to the market. However, high-strength duplex stainless steels such as Forta LDX2101 could also have a role for low-maintenance structural components. For example, Forta LDX2101 has already been specified in steel-concrete sandwich panels for the AP1000 plant, which is designed and sold by Westinghouse Electric Co.

Until SMRs enter serial production, the nuclear industry will remain dominated by building and maintenance of large-scale reactors. When serving this market, fabricators need to plan ahead and work closely with stainless steel suppliers to secure availability of the materials they need.

Outokumpu

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