Thread by @nuclearkatie

Happy #fuelcyclefriday! To recap, previous weeks have talked about mining uranium-bearing ore, milling that ore to pull out the U, conversion to UF6 and enrichment to reactor grade. Now that we have a pile of reactor-grade uranium, let's talk about fuel fabrication! 📸: @NRCgov

I'm going to focus on the *processes* involved in making nuclear fuel. However, here's a moment I made about a year ago called Nuclear Fuel 101 for those who want to first review the basics on fuel
⚡️twitter.com/i/moments/9266… #fuelcyclefriday

I'm going to break down the fuel fab process into 4 major topics.
- producing UO2 powder,
- forming and sintering the UO2 into pellets,
- loading and placing the pellets into rods and assemblies,
- quality assurance #fuelcyclefriday
📸: @WorldNuclear

First, converting the enriched UF6 to the most common fuel form for power reactors, uranium dioxide (UO2). There are several ways to do this but I'm going to talk about the Ammonium DiUranate or ADU process. There are six steps as shown in the figure
#fuelcyclefriday 📸: @NRCgov

(1) First, the UF6 is vaporized at atmospheric pressure and ~57°C (135°F) (2) The UF6 reacts with deionized water to form uranyl fluoride (UO2F2) solution and hydrofluoric acid (HF). #fuelcyclefriday

(3) Adding ammonia (NH3 or NH4OH) causes precipitation of ammonium diuranate [(NH4)2U2O7], the namesake of this process #fuelcyclefriday

(4) At this point, the ADU is in a slurry form. Ammonium diuranate crystals are separated using a filter press or centrifuge and then fed into a (5) calcinating furnace at about 370°C (700°F)
#fuelcyclefriday 📸: Getty

Ammonium diuranate (ADU) and uranium oxide (U3O8) can both be present in what we know as "yellowcake", which also showed up as the final step in the milling process #fuelcyclefriday

(6) Finally, the U3O8 is reduced to uranium dioxide (UO2) powder, which is the desired ceramic used in most reactors.
#fuelcyclefriday 📸: @FAScientists

Another process of producing UO2 from UF6 is the integrated dry route process (IDR), which includes vaporization, hydrolysis, and defluorination/calcination steps. But this thread is already going to be quite long so we won't dwell on IDR
#fuelcyclefriday

Now that we have produced UO2, it's time to make pellets! The material is ground into a fine powder with homogenized particle sizes using techniques such as hammer milling, blending, and bulk rolling.
#fuelcyclefriday 📸: @Alchetron

Small amounts of additives may be added, but according to @NRCgov, these are usually proprietary. I do have a textbook that suggests the organic binder polyvinyl alcohol as one additive but I have no confirmation on that so ¯\_(ツ)_/¯ #fuelcyclefriday

Sometimes gadolinium (Gd2O3) is added as a burnable poison, which basically smooths out the reactivity in the core. It "burns" away as it absorbs neutrons and its effect is lessened near the end of fuel lifetime
#fuelcyclefriday 📸: @Alchetron

Next, pellets are formed using a pressure of about 12 to 15 tons per square inch. These pellets are called "green" because they haven't been sintered yet. I wish I had a photo of green pellets but I don't so here's an inaccurate "photo" of "green pellets" #fuelcyclefriday

Because I posted that Simpsons photo I wanted to note that fuel pellets, before being put in a reactor, aren't very radioactive at all. You could touch them with your bare hands (but no one would allow you to because they don't want you to contaminate them)
#fuelcyclefriday

Next, pellets are sintered at 1650+ °C (3000+ °F) in a hydrogen atmosphere to prevent oxidation
#fuelcyclefriday 📸: ScienceSourceimages

After sintering, pellets are ground down to their desired size and given a dish and a chamfer at the end, which is done to reduce pellet-cladding interactions (PCI). Pellets are inspected and move on to be loaded into rods
#fuelcyclefriday

Side note: I spent one summer working on modeling PCI in nuclear fuel and another on recording fuel failures at a utility, of which PCI is a common failure mode, so man have I thought a LOT about fuel pellet shape in my (short, so far) career

For those following along at home, we're now beginning the third step of the journey of fabricating nuclear fuel
📸: @WorldNuclear

Hope this doesn't mess up the thread, but I found some info from @iaeaorg on R&D of potential pellet additives. Those mentioned include TiO2, Nb2O5, CaO–TiO2 and V2O5. Research on V2O5 in the range of 0.1–0.3 wt% and 0.25% TiO2. Unsure if ultimately implemented #fuelcyclefriday

Fuel pellets need to be placed in a vacuum sealed and back-filled rod to protect them from interacting with the water and to ensure that (gaseous) fission products don't end up in the coolant.
#fuelcyclefriday 📸: @NRCgov

The materials used to hold nuclear fuel pellets, called cladding, must meet the following criteria
- Corrosion resistant
- Able to withstand high pressure, heat, and radiation environments
- Low cross section i.e. doesn't readily absorb neutrons
#fuelcyclefriday

Zirconium alloys are currently used as they are some of the best materials that meet all the above requirements. Nuclear grade alloys also contain Tin, Iron, Chromium, Nickel, Oxygen, and sometimes Niobium.
#fuelcyclefriday 📸: @cameconews

The zircaloy rods are loaded with fuel pellets and springs at the end to account for thermal expansion and fission products. Then the rods are backfilled with Helium to minimize compressive clad stresses and then welded shut
#fuelcyclefriday

Fuel rods are then inspected (hold that thought for one second, I’ll talk about quality assurance in a moment) and arranged in an assembly
#fuelcyclefriday

The exact design and number of rods per assembly depend on the type of reactor type. For example, PWR and BWR, the two reactor types we have in the US, have two notably different designs.
#fuelcyclefriday 📸: NFI

All right, we've now talked about the whole process of making nuclear fuel! The last area to talk about is quality assurance because it's REALLY important that all of these processes produce pellets within the desired tolerance
📸: @WorldNuclear

All pellets, fuel rods, and assemblies are inspected before being shipped off to commercial plants. Much of the inspection happens by hand, but some inspections are done by active interrogation with neutrons to identify small defects
#fuelcyclefriday 📸: @iaeaorg

These systems are pretty cool, as they're designed to scan every single rod individually. With tens of thousands (exact numbers vary) of rods in EVERY nuclear power plant at any given time, they have to maintain a high throughput and high accuracy
#fuelcyclefriday 📸: CASL

For decades, the industry has relied on Cf-252 (non-fissile) as a neutron source to scan every single fuel rod. However, Cf-252 is expensive and only produced in one facility in the U.S., @ORNL's High-Flux Isotope Reactor
#fuelcyclefriday 📸: SABIA

However, a nuclear startup company, @PhoenixLabs, has developed a fuel scanning system using their DD neutron generator that can replace Cf-252. This is super exciting since many areas of the nuclear fuel cycle have been quite (very) slow to innovate
#fuelcyclefriday 📸: Phoenix

I actually had the chance to tour @PhoenixLabs recently and see a unit they're testing and getting ready to ship off to a fuel manufacturer in the not-too-distant future. You can see the 2 small black & white conveyers that take rods in to be scanned continuously
#fuelcyclefriday

Anyway, thank you all for reading and feel free to ask questions if you've got them! Here's some good sources for further reading if you're interested
- Nuclear Fuel and its Fabrication by @WorldNuclear
world-nuclear.org/information-li…
#fuelcyclefriday

- The Nuclear Fuel Cycle (textbook) by Cochran and Tsoulfanidis
- Evaluation of UF6-to-UO2 Conversion Capability at Commercial Nuclear Fuel Fabrication Facilities by @ANL
web.ead.anl.gov/uranium/pdf/AN…
#fuelcyclefriday

- Module 5.0: FUEL FABRICATION of the @NRCgov Fuel Cycle Processes Directed Self-Study Course nrc.gov/docs/ML1204/ML…
and finally
- NUCLEAR FUEL FABRICATION & REFABRICATION COST ESTIMATION METHODOLOGY by @ORNL (old but still interesting)
osti.gov/servlets/purl/… #fuelcyclefriday

Finally, I do this all for free & for fun but if you learn something from one of my threads and want to give back, you can buy me a coffee on Ko-fi.com/nuclearkatie
or check out the cool uranium glass I sell on Etsy etsy.com/shop/NuclearKa… (10% off with code HAPPYHOLIDAYS)