Organic solvent extraction (OSE) is the selective transfer of components between two immiscible liquids. The organic solvent is 30% TBP diluted in odourless kerosene (OK). The aqueous feed from the dissolver, typically containing about 250 g U/L, is contacted with the solvent in a counter-current pulsed column.
How it works:
- The aqueous feed (containing dissolved U, Pu, fission products, and minor actinides in nitric acid) is pumped into the pulsed column roughly halfway up.
- The TBP/OK solvent is fed in at the bottom and rises through the column.
- As the two phases mix, uranium and plutonium are selectively extracted into the organic (solvent) phase via the following reactions:
- The fission products remain in the aqueous phase (containing >99% of the total activity).
- Fresh aqueous nitric acid is fed in at the top of the column to scrub any remaining fission products from the rising loaded solvent.
Why TBP works so well:
- TBP is highly selective for uranium and plutonium over fission products (distribution factors are orders of magnitude higher).
- TBP has reasonable resistance to radiation and nitric acid damage.
- It has low volatility and flammability.
- The kerosene diluent is cheap and readily obtainable.
- No salting agents are required (unlike earlier processes), which reduces waste volumes.
THORP uses three stages of pulsed column contactors in the highly active (HA) cycle to separate the uranium/plutonium from the waste stream, followed by further columns for purification.
Types of Contactor Equipment
The choice of contactor equipment in a PUREX plant significantly affects throughput, separation efficiency, and criticality safety:
| Contactor Type | Principle | Advantages | Disadvantages | Where Used |
|---|---|---|---|---|
| Pulsed column | Two liquid phases flow counter-currently through a tall column; a pulsing mechanism creates turbulent mixing at perforated plates | Continuous operation; no moving parts in contact with active liquor; good criticality geometry (tall, narrow) | Large physical size; slow to reach steady state; difficult to clean | THORP (UK), UP3 La Hague (France) — HA cycle |
| Mixer-settler | Liquids mixed in a stirred vessel, then separated by gravity in a settling chamber; stages connected in series | Flexible; easy to operate and maintain; high stage efficiency; well-understood technology | Large floor area; high liquid inventory (criticality concern); moving parts in active environment | Uranium purification cycles at THORP; La Hague plutonium purification |
| Centrifugal contactor | Liquids mixed and separated by centrifugal force in a rapidly spinning rotor | Very short contact time (seconds); very small liquid inventory (excellent for criticality safety); compact | Complex mechanical design; moving parts require maintenance | Savannah River (USA); some modern facilities |
In general, pulsed columns are preferred for the highly active first cycle (where the liquid inventory is most radioactive and criticality control by geometry is paramount), while mixer-settlers are used for the lower-activity purification cycles where their flexibility and high efficiency are advantageous.
Key Point: The aqueous phase containing fission products and other actinides is the highly active raffinate — this is high-level waste (HLW). It is stored in water-cooled, double-walled stainless steel tanks prior to vitrification.