Steer clear of the green glow; how is nuclear fuel made?

Uranium. It glows green and is highly radioactive. If you dug some up you should be terribly concerned, right? Not exactly. Unfortunately, the Simpsons has mislead us about both its nature (it isn’t a glowing green element) and danger (I’m telling the truth I promise, just stay with me). So then what is uranium and how do we take it from the earth and process it into a form that our nuclear reactors can use to power our modern world? In this article I’m going to try and demystify exactly what “nuclear fuel” is and why it’s not quite as dangerous as you might think.

Uranium is present in the ground in several different ores (that’s just a name for rocks containing minerals) pitchblende, uranite or zeunerite for example. You might be surprised that the uranium contained in these ores is nowhere near a radioactive as you would think and definitely not like the fuel we use in (most) reactors! This is all because over 99% of uranium found in nature is the stable isotope U-238, while the highly radioactive U-235 is what is used as fuel in the majority reactors and makes up just 1% of natural uranium. More on the difference between those later. Uranium itself a silvery metal, sadly not a glowing green one.

Some Pitchblende. Notice it’s not glowing green and in fact looks rather unexciting in my opinion. Geologists may think differently. Taken from [1].

Over half of the worlds uranium ore deposits are found in just three countries: Australia (28%), Kazakhstan (16%) and Canada (12%), so it’s not unsurprising that these countries also have the largest production of uranium ore. There are some sizable deposits also in North and South America, Southern Africa and Russia. From World Nuclear [2] “The world’s present measured resources of uranium (6.1 Mt) … used only in conventional reactors, are enough to last for about 90 years. This represents a higher level of assured resources than is normal for most minerals.” This is in comparison to the amount of coal, oil and gas reserves we have left, which are all under 100 years (here’s something else I wrote on that) and note that there are also types of reactors that can use old fuel or different elements to uranium – thorium for example.

It is important to note though that not all nuclear fuel comes directly from mined uranium. Other sources of fuel include reprocessed uranium and plutonium from spent fuels as well as stockpiles of old nuclear weapons – supplying 15% of the total global demand in 2017. There are also several hundred thousand tonnes of stockpiled uranium in countries across the globe. This is all done to meet the 67,500 tonnes of uranium required by the worlds nuclear power plants each year [2].

Now to explain how exactly the uranium is extracted. The first thing we need is to obtain the ore itself. This is commonly through open pit mining, which is what you might envision when thinking of traditional mining: large dugout pits, blasting rocks with dynamite and deep tunnel networks into the earth (pretty cool in my opinion, but of course there’s drawbacks). This is a relatively simple method however it’s extremely destructive and risks contamination of the air and water if the uranium dust isn’t controlled properly. Before you start to shout that this is terrible remember that this is no different from any other mining operation.

An open pit uranium mine. Taken from [3].

Alternatively, we can leave the ore in the ground and just extract the minerals. This is done by a method called in-situ leaching. This is similar in a way to fracking. Several injection and recovery wells are sunk into the earth to the depth of a uranium ore deposit and a leaching fluid (some acid or alkali that will dissolve the minerals) is injected into the ore. The dissolved uranium and other minerals are then extracted in the fluid and sent for processing. In-situ leaching accounts for around 57% of the global uranium production [4]. This is a much cheaper and less destructive method of extraction compared to conventional open pit mining. There can be some degradation of groundwater deposits in the region, however these are monitored and almost always the site is revegetated and returned to its initial state.

In terms of which method is best, I can only say that there are advantages and disadvantages to both. We need uranium ore, so until we develop less harmful mining techniques, then it is apt to just choose the most appropriate method for the situation.

A schematic of how in-situ leaching works. Notice the wells sunk into the uranium deposit, with minimal surface damage to the area. Taken from [5].

Once we’ve acquired the ore, we must go through a process called milling to extract the minerals from the ore and to convert it into a form called urania U3O8, A.K.A yellowcake (although I imagine you would have a pretty horrible death if you actually ate a cake made from the stuff). Traditionally, we can take the ore from our open pit mine and crush and grind it into thin sand then mix it with water into a slurry. It is then mixed with acids to dissolve the uranium and separated from the other minerals in the ore. Several chemical processes are completed to purify and concentrate the solution and it is then dried to yield the yellowcake. Finally this is sealed in barrels and shipped to be enriched.

A lot of urania (yellowcake) has been manufactured here. This will be sent off to be enriched. Taken from [6].

Remember this process is only for the ore deposits, we could have gathered our uranium minerals from in-situ leaching and skipped the grinding and crushing stage of the milling process. There is also another method to extract the uranium minerals from the ore called heap leaching. This drops acid onto a crushed ore stack that seeps through the ore, dissolving and extracting uranium before collecting in the basin of a resilient liner. This is much cheaper than in-situ leaching but can take up to 90 days!

A schematic of heap-leaching. Notice how the ground ore is aligned on an angle to ensure the uranium leachate can be effectively collected. Taken from [7].

I’m sure you’ve heard the phrase “enriched uranium” before and I wonder what that makes you think. Perhaps you’re wondering why we even need to enrich our uranium? Why can’t we just use the uranium ore, or the yellowcake as fuel? Well it’s to do with which elements of uranium are fissile are which are not. That is, which can be induced to split (to fission) when they absorb a neutron and hence produce us energy. Well if you remember, the uranium we’ve been dealing with so far is predominantly uranium-238 (approximately 99% of it is 238). U-238 is not fissile, rather if it absorbed a neutron, it will transmutate to a different isotope but not decay by fission into two smaller atoms.

U-235 is a fissile isotope of uranium and it’s the fission process that produces the heat and therefore the energy that we need. To enrich the fuel basically means to try and separate out some of the U-235 from the U-238. Enrichment is a complicated and interesting topic but is closely related with weapons proliferation. Therefore it’s best that I don’t share the few details I know about the process with the wide world, so lets skip this part. All we need to know is that we are basically increasing the percentage of U-235 in our fuel so that we can get more fission!

A simple explanation of the difference between the two isotopes of uranium. They both have the same number of protons (so are both the element uranium) but have different numbers of neutrons (so are isotopes of one another). Taken from [8].

After our enrichment we’re left with uranium dioxide, UO2. The UO2 is the form of fuel that is used in most reactors and is formed into small pellets through a series of presses, sintered (basically baked in a furnace) to reduce its porosity and placed in the cladding. The cladding is the casing that the UO2 pellets sit within and for most reactors is made of a Zirconium alloy (e.g. Zircaloy-4) due to Zirconium’s high transparency to neutrons, which is, of course, what we want to have lots of neutron induced fission occurring. These fuel rods are normally grouped together into a fuel rod assembly and are then ready to be placed in a reactor!

The final fuel rod assembly, ready for use in a reactor. You can see the bundle is made up of many smaller fuel rods. This particular assembly is for the Canadian CANDU type reactor. Taken from [9].

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: