Recovering precious metals from nuclear waste

Materials World magazine
,
1 Jan 2018

A new method of spent nuclear fuel recycling may enable the recovery of precious metals. Khai Trung Le reports.

A means of recycling small amounts of precious metals from spent nuclear fuel (SNF) has been developed by physicists from Tomsk Polytechnic University (TPU), Russia. The method is said to be compatible with existing recycling measures, which the team hopes will facilitate the move away from SNF storage.

There are currently two means of SNF storage – dry storage, in ventilated rooms, or wet storage, under water. Both are far cheaper than recycling, but the Union of Concerned Scientists (UCS) has warned against current means of storage, noting, ‘Because no permanent repository for spent fuel exists in the USA, reactor owners have kept spent fuel at the reactor sites.’ As the amount of fuel increases, UCS states that the US Nuclear Regulatory Commission has authorised power plant owners to increase the amount of stored SNF by as much as five times what storage pools were designed to hold. TPU reports that there is around 160,000 tonnes of SNF in Russian storage facilities.

Precious breakdown

Currently, SNF recycling involves fragmentation and dissolution in nitric acid, followed by the extraction of uranium, plutonium and the production of Mixed-Oxide fuel. The leftover SNF waste consists of water-salt solutions containing a range of radioactive fission products of the 235Uranium group. According to TPU, much of this will be evaporated or removed with chemical reagents before vitrification, with the leftover briquette form stored underground.

Ivan Novoselov, a lecturer at the Department of Applied Physics Engineering at TPU, criticised this method, saying, ‘Unlike standard radioactive waste, SNF radioactive waste has value – their components can be used in the future. Briquettes obtained after vitrification takes up less volume and do not contain uranium and plutonium, but do contain isotopes of precious metals in the platinum group – palladium, rhodium and ruthenium, which are fission products.’ However, Novoselov added that extracting them from SNF in briquette form was previously impossible.

Alexander Karengin, Associate Professor at the Department of Applied Physics Engineering, TPU, used low-temperature plasma in existing recycling processes to immobilise SNF recyclable waste. He commented, ‘SNF recycling technology, when applied at radiochemical plants, is based on PUREX processes, in which uranium and plutonium are extracted from water-salt solutions with tributyl phosphate using various organic diluents including kerosene, and purified hydrocarbons among others. When exposed to radioactive irradiation, they gradually lose their effectiveness and convert into combustible materials. We designed a joint plasmonic immobilisation of combustible and non-combustible recycling products in air plasma that melts metal chlorides (sodium and potassium) resistant to radiation. As a result, we obtained a combustible composition, which is processed in air plasma at temperatures no less than 1,200˚C. This can be collected in special containers, solidified and sent for storage.’

After a waiting period, it is reportedly possible to extract platinum group metals from the SNF ‘via chemical means’, said Karengin. He added, ‘From one tonne of SNF, we can obtain up to 2kg of ruthenium, 1.5kg of palladium and up to 0.5kg of rhodium.’ TPU did not comment on whether the amount of precious metals recovered could be increased, the speed of the recovery or further details on how the process would be integrated into existing SNF recycling methods.