mathisrad said:
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 (H
2O) for cooling and moderation.
CANDU use heavy water (D
2O), which does not absorb thermal neutrons as much as (H
2O), 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 (UF
6), which is volatile above 56.2 °C (~133°F), and that gas is processed to increase the fraction of
235U in the gas.
The (UF
6) is sent to a fuel fabricator, where the (UF
6) is processed to UO
2 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 (UF
6) is processed to UO
2.
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 UO
2 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)O
2, 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.