Uranium enrichment
28-10-2024
Following a visit to an uranium enrichment facility, I figured I'd write a collection of my favourite facts about HALEU production. This is more of a quick "fact sheet" for future personal reference.
Profit margins
Whilst most steps in the nuclear fuel production chain hover around a 5% profit margin, enrichment tops all of them at a staggering 42.6%, with supply contracts often lasting decades, 99% of newly formed Urenco contracts spanning over 10 years. Urenco alone supplies 138 reactors worldwides with LEU and LEU+, with expansion plans aiming for 145 reactors in the near future.
Tails
Production remnants are processed and stored on site most of the time, being turned into depleted uranium.
Centrifuge operation
Spinning up centrifuges takes less than a week (accurate measures are not publicly available), with a similar figure for spindown procedures. When empty, due to the high angular velocity of the centrifuges, coupled with a lack of resistance from no uranium hexafluoride presence, centrifuges can spin for days on inertia alone.
Centrifuges often have a lifespan of around 15 years, with entire cascades being replaced simultaneously to ensure operational reliability. Some centrifuges have been operating continuously since the 1970s, and small GCs used by Urenco can achieve an average failure rate of 0.1%/year.
European/Japanese centrifuges are often smaller (25cm in diameter, 2 to 4 metres long) and require less maintenance, whilst American facilities employ larger centrifuges with up to 10x the production capacity.
Facilities often employ a large number of centrifuges, well into the tens of thousands.
Production
Centrifuges have a typical throughput of 17.6 kg of uranium per day, and enriched non-military grade uranium can reach up to 19.7% Uranium-235. Typical enrichment thresholds start at 5%, and advanced reactors often require a higher enrichment level. Higher levels of enrichment can be achieved by reprocessing already enriched uranium at slower speeds.
Gaseous centrifuges have a process operating region ranging around 25 degrees celsius, at 0.01MPa. Centrifuges have superseded most gaseous diffusion plants for at least 50 years in Europe, with all American fully functional gaseous diffusion facilities ceasing operation in 2013.
Centrifuges use roughly 5% of the power of a similarly sized gaseous diffusion facility.
Aluminium is often used in centrifuge operating conditions, for aluminium fluoride reacts with highly corrosive with UF6 to form a corrosion-resistant aluminium fluoride coating that protects the metal. Suitable rotor materials are aluminium/titanium alloys, low carbon steel with >25% nickel concentration, or composites (such as carbon fibre). Rotor peripheral speeds can exceed 500 m/s. Aluminium rotors typically achieve 350 m/s.
Storage
UF6 is stored in 12 ton drums, cooled down until solid (less than 50 degrees celsius at atmospheric pressure). To be handled as liquid, UF6 needs to be kept at above 64 degrees celsius and above 0.15MPa. Transfer is done by sublimating uranium and desublimating it on the target cooled surface. Stations are then denoted receiver/donor stations.
Unenriched, "base" uranium is denoted "feed", whilst depleted/discarded uranium is called "tails".
Stored enriched uranium can be "blended" to form product with the enrichment levels required by utilities. Highly enriched (often called "weapons grade") uranium can also be used in blending to form LEU, in a process called "downblending".
Weapons grade (97%) uranium is often blended with depleted uranium at a rate of 1:25.
Sampling
Some facilities operate mass spectrometry equipment, which allows for continous sampling of enriched uranium to ensure a consistent concentration of U-235. However, in some cases, manual liquid sampling is required, which is done by heating drums in a large autoclave until stored uranium turns liquid, and filling metal sample bottles with product. Sample bottles are cooled down over a period of two days, and then analysed off-site.
Sampling in some facilities occurs every 3 days, with samples returning to a cold tank for storage. Roughly 4 grams of uranium are used for sampling every year.
Failures
During an unplanned power loss or catasthropic event, product/tails are dumped into a NaF contigency system. The NaF absorbing beds bind to the UF6, and the remaining light gases are released through an effluent vent system, where they go through multiple steps of filtration.
Centrifuges are isolated in such a way that one failure will not hinder the operation of neighbouring centrifuges, even in violent mechanical failures. In such failures, the rotor disintegrates, heat is generated, and the stored uranium collects at the bottom of the centrifuge over the course of weeks.
Wastewater is neutralised with sodium hydroxide, with citric acid being used to decontaminate equipment.
Criticality is not a major risk, since assays of 5% have a critical mass of around 5 kg. However, transfers are done with UF6 in gaseous form to prevent accidents, and the cylindrical shape of the drums prevents a large surface contact area between vessels.