Dear Dr. Zoomie,
I read an article the other day about some new wonder-alloy that’s going to be used in new nuclear reactors. I’m afraid I can’t find the piece again to link to it, but it made me wonder how many different metals are used in nuclear reactors and what purpose they all serve. Isn’t metal, well, metal? What makes some better than others?
Boy – I had pretty much the same thought…and then we had a metallurgy class when I was in the Navy’s Nuclear Power School. Talk about an eye-opener! Turns out that most metals have very distinctive properties – cobalt (for example) is very hard, nickel, copper, and chromium are resistant to different forms of corrosion, iron is inexpensive, and so forth. Fully seventy of the 92 natural elements are metals – and, thankfully, only a relatively small number are used in nuclear reactors. Let’s take a look at some of the more important and/or interesting. Also – you’d mentioned alloys, which are mixtures of metals, each of which contributes something to the final metal. Think of alloys as being sort of like mixed drinks – or as a blended whiskey, perhaps – each component adds something unique that improves the final alloy.
Iron is fairly inexpensive, it’s quite common, and it’s got reasonable strength. And, while elemental iron isn’t tremendously hard, iron is also the basis for steel, which is harder and stronger and, with the addition of other metals, can take on any of a huge number of properties. In our blended whiskey analogy, iron would be the base for the final mixture.
Nickel is added to steel to make it tougher and to protect against corrosion. Toughness is a material’s ability to absorb energy by deforming before breaking – many materials that are quite hard are not very tough (glass, for example). Adding nickel to an alloy can make the metal tougher, especially at the high temperatures at which nuclear reactors operate. And, by forming a corrosion-resistant oxide layer on the alloy’s surface, nickel can also help protect against some forms of corrosion, an ever-present concern in the harsh environments in which these alloys must exist.
Chromium is a metal you might associate with the big chrome bumpers on the cars of the last century, as well as on flashy metal decorations on…well…just about anything. Chromium is more than decorative, though – it’s a fairly hard metal in its own right and, more importantly for our purposes, it’s also resistant to corrosion. Mix chromium with regular steel to make stainless steel; mix chromium with an alloy of iron and nickel and you have a superalloy called Inconel. Like nickel, chromium forms a passive oxide layer on the surface of the metal objects, protecting the underlying alloy from corrosion.
So why (you might wonder) is stainless steel a mere alloy while Inconel is a “superalloy” – what is it that warrants the “super”? Well…the three-metal cocktail that comprises Inconel really is more than what we’d expect from just looking at what each individual metal brings to the alloy. At room temperature, Inconel’s strength and toughness are impressive, but metals tend to get more ductile – more willing to deform – at high temperatures, just as a wax candle will start to sag on a hot summer day. Nickel’s added strength and toughness combined with chromium’s added hardness and strength help Inconel resist this tendency to higher temperatures; coupled with both metals’ corrosion resistance this produces a metal that seems almost ideally suited for the high-pressure, high-temperature environment of a nuclear reactor.
So – the basic ingredients of stainless steels are iron and chromium; Inconel, while it also contains iron and chromium, is primarily made of nickel. Both of these can be further alloyed with any of a number of other metals – molybdenum for example, adds both corrosion resistance and high-temperature strength, niobium helps to make alloys harder and less brittle; tantalum adds its very high melting temperature and still more corrosion resistance; manganese reduces brittleness and can improve strength and toughness; and so on. Each metal can add its own unique properties to the blend – but each also adds cost and complexity that might, or might not, be worth it. If, for example, Inconel alone takes you 90% of the way to the “perfect” alloy for a particular use, it might not make sense to keep tinkering with the blend, at ever-greater cost, when simply making a slightly thicker piece of basic Inconel might do the trick more cheaply.
So these are the major metals used in the reactor plants, but there are some others that, although used in smaller quantities, are just as important and (dare I say?) even more interesting!
Cobalt is one of these – stellite (a blend of cobalt, chromium, and maybe tungsten or molybdenum) is an exceptionally hard metal thanks to cobalt and it’s perfect for use on surfaces that take a lot of punishment – such as some of the surfaces of valves. One example of this is a check valve – a heavy metal disc that swings open when water is flowing in the desired direction and that swings shut if the water tries to reverse direction. When that happens the check valve will slam against a ring of metal to block the reverse water flow, blocking the flow path. When that would happen in a naval reactor plant we could hear (and sometimes feel) the check valve slam everywhere in the submarine. Only a tremendously hard and tough alloy like Stellite could stand up to that amount of stress for a few decades. Stellite was also used in other types of valves in which the valve disc slid into a slot to block the water flow – the part of the disc and the part of the valve seat were faced with a hard cobalt alloy to minimize wear and tear here as well. The only problem is that cobalt becomes radioactive when it’s bombarded with neutrons, so this particular alloy also served to increase radiation levels in the reactor compartment, which we had to enter for routine maintenance several times annually (not when the reactor was operating!). But the radiation levels were never dangerously high and neither I nor anyone else onboard received an unacceptably high level of exposure, so the tradeoff worked out.
Yet another metal alloy that found a fair amount of use in our reactor (and many non-military reactors as well) was based on zirconium. Zirconium has good physical properties, and it’s also tremendously resistant to corrosion, including seawater corrosion. And while one hopes that the internal parts of a reactor plant will never, ever be exposed to sea water, it’s happened (possibly) to two American submarines and a number of Soviet/Russian boats. On top of that, zirconium is not very eager to soak up neutrons so it won’t intercept any neutrons needed for producing fissions, making it a great metal to use as cladding for fuel pellets, fuel rods, and control rods. The biggest drawback to zirconium is that, at high temperatures, it can react with water to release hydrogen gas, which can be explosive. Luckily, this only happens during reactor accidents, which have thus far been few and far between.
Although there are many more metals used in nuclear power plants, I’m only going to discuss one more to avoid taxing your patience too much – copper. Copper seems sort of mundane – in the US we usually think of pennies when we think of copper. One of the first chemical elements to be discovered, copper is also one of only a few to be found in nature in its elemental form – it’s been used for millennia. So antique a metal doesn’t really seem very interesting – at least, that’s what I thought when we first started learning about it. Turns out I was wrong.
What copper has going for it (in addition to being fairly inexpensive, easy to work with, and so forth) is that it resists seawater corrosion quite well, and alloys that include copper in their composition are great to use in pipes and systems that come in frequent contact with seawater. A modern alloy that uses copper is monel – a blend of copper and nickel. But two ancient copper alloys – bronze and brass – remain in use to this day as well; bronze, in fact, is used to make the propellers that push so many ships and a huge fraction of our raw materials and finished goods around the world. And I think this is a fitting place to end – by noting that one of the most ancient metals is used in modern versions of one of the most ancient alloys to transfer the energy produced in nuclear reactors, steam turbines, and diesel engines into forward motion to help today’s massive cargo ships transfer modern electronics, automobiles, and so much else around the globe to keep our unimaginably complicated economies and supply chains humming along.
References
- Header Image Source: Wikipedia Commons – Metal samples Image
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