Uranium: Under the sea

LCW Supercritical Technologies and Pacific Northwest National Laboratory (PNNL) have taken a milestone step in seawater extraction of uranium.

Using commercially-available acrylic fibres, chemically modified to allow adsorption, researchers extracted uranium from seawater sourced from Sequim Bay, located next to PNNL’s marine sciences lab, in Washington, US.

In three, month-long trials seawater was pumped through approximately a kilogramme of fibre, under conditions mimicking those of the open sea.

The fibres adsorbed enough uranium to produce 5 g of yellowcake – powder used in the preparation of uranium fuel for nuclear reactors.

This is a very promising step for the future of uranium extraction, opening up the possibility of renewable, commercial extraction from seawater.

Underwater uranium

Gary Gill, a researcher at PNNL, said: “The average concentration of uranium in the ocean is about 3.3 ppb,” which equates to an estimated  total of around 4.5bn t.

According to Uranium 2016: Resources, Production and Demand, a joint report by the Nuclear Energy Agency and the International Atomic Energy Agency, the total identified resources of uranium metal from land-based reserves, as of 1 January 2015, was 5,718,400 t (where recovery costs are less than US$130/kg of uranium).

“Therefore, the oceans contain approximately 788 times more uranium than total identified resources from terrestrial mines (where the recovery cost category is less than US$130/kg of uranium),” said Gill.

As land-based reserves become depleted, and land-based mining becomes more expensive, seawater extraction could become a major benefit to the industry.

Scaling up

For large-scale seawater extraction, the researchers envision adsorbents assembled to resemble a kelp field. The adsorbents would be long and kelp-like in shape, fixed to the bottom of the ocean, and rising up, “most likely in water depths of 50–100 m,” said Gill.

To put this into perspective for energy production, “to obtain the mass of uranium required to power a 1,000 MWe reactor for a year would require deployment of approximately 3,250 t of adsorbent, which would require a footprint of approximately 170 km²,” said Gill.

“This is roughly three times the size of Manhattan Island,” he added.

While the fibres are not-toxic, the physical deployment of the fibres could affect marine life through entanglement and other related issues, that would need to be identified and mitigated.

The next phase

The next phase in developing this technology will start in September and will involve deploying scaled-up adsorbent in the ocean.

Gill stated: “We plan to make several commercial-scale (length and mass to scale for a single adsorbent structure) adsorbents for deployment and test their performance in the warm coastal seawater of the Gulf of Mexico.”

To date a majority of the group’s work has been with bench scale reaction vessels, so a major effort for the next phase will be the development of larger, commercial-scale reaction vessels, and processes.