How is Uranium refined so it can be used in nuclear reactors?

[mathisrad]

TL;DR

How does Uranium get refined?

I just looked a little bit into how Nuclear power plants work out of curiosity, and I saw that the Uranium they use is U-235, but when I was looking at a website, it said that the Uranium required 3% or higher content of U-235. I wasn't sure if this was correct so I dug a bit deeper at the Uranium refinement process, and was confused about how it worked. I would appreciate it if someone could explain the Uranium refinement process to me (preferably in steps if it's not too much trouble)

 

[berkeman]

Doing a Google search on Uranium Refinement gets auto-corrected to Uranium Enrichment and here is the first hit on that search list:

https://geoinfo.nmt.edu/resources/uranium/enrichment.html

Does that help?

 

[mathisrad]

Yes that helps a lot, the previous website I was looking at wasn't the easiest to understand. Thank you

 

[Astronuc]

An additional resource - https://world-nuclear.org/informati...l-cycle/conversion-enrichment-and-fabrication

 

[mathisrad]

This was great, thanks!

 

[Astronuc]

TL;DR Summary: How does Uranium get refined?

I was looking at a website, it said that the Uranium required 3% or higher content of U-235.

Acutally, the statement applies to LWRs (PWRs or BWRs), which used 'light' water (H₂O) for cooling and moderation.

CANDU use heavy water (D₂O), which does not absorb thermal neutrons as much as (H₂O), therefore one can use natural U (~0.71% ²³⁵U). The enrichment is selected based on a number of performance requirement, e.g., residence time (burnup), reactivity requirements, temperature, voiding in the coolant, neutron spectrum, . . . . ). The enrichment must enable the core to achieve criticality during the designed cycle length for a given energy production.

Otherwise, uranium ore is mined and purified (separate the non-uranium portion from the uranium componds, which are processed to 'yellow cake', which is mostly oxides of U.
https://en.wikipedia.org/wiki/Yellowcake

The 'yellow cake' is then processed (converted) to a uranyl fluoride (UF₆), which is volatile above 56.2 °C (~133°F), and that gas is processed to increase the fraction of ²³⁵U in the gas.

The (UF₆) is sent to a fuel fabricator, where the (UF₆) is processed to UO₂ in conventional LWR and CANDU fuel. Remember, CANDU fuel is not nessarily enriched (but it can be for longer operational cycles), but the processing is much the same from oxides of U, to (UF₆) is processed to UO₂.

Uranium in nuclear fuel can be in the form of other compounds, e.g., UCO (kernel in TRISO pebble fuel)), UC, UN, UMo (U_1-xMo_x), UZr (U_1-xZr_x), and USi (U_1-xSi_x). The particular form depends on various performance requirements, such as U density, service and hypothetical accident temperatures, thermal conductivity (throughout design/service life), dimensional stability, chemical compatibility with encapsulating/cladding material, chemical compatibility with the coolant, and fission products retention.

In the case of LWR and CANDU fuel, cylindrical pellets of UO₂ are placed in cladding tubes of a Zr-alloy, which have evolved over the last 7 decades. Some earlier LWR fuel was clad in austenitic stainless steels, e.g., AISI types 304, 347 or 348. Stainless steel cladding (usually 316) is standard in liquid metal fast reactor fuel. Fast reactors use greater enrichments of about 20% equivalent, usually in the form of (U,Pu)O₂, where U is mostly depleted or natural U, and Pu is some mix of Pu-239, 240, 241, and some 242. Some fast reactor fuel has been UN and UC.

Fuel for gas-cooled, graphite-moderated reactors (e.g., in UK AGRs) have used stainless steel cladding, or graphite encapsulation (Pebble Bed, gas-cooled). In TRISO fuel, the fuel kernel (particle) is encapluated in three layers (PyC, SiC, PyC), where PyC is pyrolytic carbon, and SiC is sliicon carbide.

 

[snorkack]

Acutally, the statement applies to LWRs (PWRs or BWRs), which used 'light' water (H₂O) for cooling and moderation.

CANDU use heavy water (D₂O), which does not absorb thermal neutrons as much as (H₂O), therefore one can use natural U (~0.71% ²³⁵U). The enrichment is selected based on a number of performance requirement, e.g., residence time (burnup), reactivity requirements, temperature, voiding in the coolant, neutron spectrum, . . . . ). The enrichment must enable the core to achieve criticality during the designed cycle length for a given energy production.

Carbon also does not absorb thermal neutrons as much as protium, and graphite moderated natural uranium reactors are another common design.

 

[Astronuc]

and graphite moderated natural uranium reactors are another common design.

Magnox reactors are not that common; the UK had 26, which are now shutdown. They also use a special cladding, which achieves low burnup, and most units had relatively power ratings.