Advanced reprocessing of spent nuclear fuel

The advanced reprocessing of spent nuclear fuel is a potential key to achieve a sustainable nuclear fuel cycle and to tackle the heavy burden of nuclear waste management. In particular, the development of such advanced reprocessing systems may save natural resources, reduce waste inventory and enhance the public acceptance of nuclear energy. This strategy relies on the recycling of major actinides and the transmutation of minor actinides in appropriate reactors. In order to fulfill this objective, selective extracting agents need to be designed and developed by investigating their complexation mechanism.

Managing spent nuclear fuel

The estimated inventory of spent nuclear fuel discharged from nuclear power reactors worldwide up to the end of 2013 is about 370,000. To date, about 250,000 of this inventory is being stored. At the back-end step of the nuclear fuel cycle, two spent fuel management options could be potentially adopted:

Open fuel cycle: the cycle starts with the mining of uranium, goes through the fuel fabrication and ends with the direct disposal of the spent fuel.
Closed fuel cycle: the spent fuel stored for a long time can be safely handled and it undergoes reprocessing in order to recover and recycle a large amount of it.

Nuclide	Mass concentration
^U	95
^U	0.5
Fission Products	1 ; 2.5
Transuranic elements	0.9 ; 0.1

According to the first option, the spent nuclear fuel is considered complete waste. After an interim storage, all the used fuel is directly disposed of in a deep geological repository. Several geological media and repository designs are being studied according to different natures of fuel and their burnup, radioactive inventory and decay heat generation. The assessment of a geological disposal is based on the multi-barrier approach, which combines multiple effects of engineered and natural barriers in order to delay potential long-lived radionuclide migrations to the biosphere over time.
Reprocessing is the alternative option for the spent fuel after a long interim storage. The used fuel is not considered waste anymore but a future energy resource. A large amount of fertile uranium is discharged along with a small quantity of fissile and a non-negligible portion of high level waste products and transuranic elements, which strongly contribute to the long-term radiotoxicity of the spent nuclear fuel. The recovery and recycling of uranium and plutonium were the first steps in developing a closed fuel cycle. Furthermore, a strong reduction of the volume, radiotoxicity and heat load of the spent nuclear fuel can be efficiently achieved.
Despite the benefits of this first reprocessing approach, an amount of waste must be treated, stored and disposed of in a deep geological repository over a long period of time. Waste from reprocessing and spent nuclear fuel are classified as High Level Waste according to the IAEA guidance due to the high emission of radioactivity and decay heat.
The first reprocessing approach is based on the PUREX process, which is the standard and mature technology applied worldwide to recover uranium and plutonium from spent nuclear fuel at industrial scale. Following the dissolution of the spent fuel in nitric acid and the removal of uranium and plutonium, the generated secondary waste still contains fission and activation products along with transuranic elements that must be isolated from biosphere. Uranium and plutonium are recovered by the well-known tributylphosphate ligand in a liquid-liquid extraction process.
Reprocessing allows the recycling of the uranium and plutonium into fresh fuel and a strong reduction of volume, decay heat and radiotoxicity of the HLW. A measure of the HLW hazard is provided by radiotoxicity coming from the different nature of radionuclides. The SNF radiotoxicity is usually evaluated as a function of time and compared to the natural uranium ore. The spent nuclear fuel without reprocessing has a long-term toxicity that is mainly dominated by transuranic elements. Mainly due to plutonium, SNF without reprocessing reaches the reference radiotoxicity level after about 300,000 years. After uranium and plutonium removal, HLW is less radioactive and it decays to the reference level within 10,000 years. Since minor actinides also contribute to the long-term decay heat and radiotoxicity of the spent fuel, an advanced reprocessing could further reduce the radiotoxic inventory with a decay to the reference level of about 300 years.

Partitioning and transmutation strategy

Many efforts are being devoted to develop an advanced reprocessing approach with the aim to further reduce the radiotoxicity inventory of the spent nuclear fuel by removing all minor actinides and the long-lived fission products from the high active raffinate downstream of the PUREX process. Before the conditioning process, the long-lived radionuclides undergo a transmutation into short-lived or stable nuclides by nuclear reactions. This coupled approach is known as Partitioning and Transmutation strategy, which inclusion in an advanced closed fuel cycle could lead to strongly reduce long-term radiotoxicity, volume and decay heat of the final waste thus simplifying a performance assessment of a future nuclear waste repository and enhancing proliferation resistance criteria.
Two potential process options for the partitioning of spent nuclear fuel are being developed: hydrometallurgical and pyrometallurgical processes. The hydrometallurgical partitioning, also known as solvent extraction process, was born and developed in Europe thereby becoming the reference technology for future SNF reprocessing at industrial level, whereas the pyrometallurgical option started in the United States and Russia as an alternative to the aqueous processes. Unlike plutonium, americium and curium show a very low affinity towards TBP ligand, thus needing further advanced separation processes and different extracting agents such as nitrogen-bearing ligands, also known as soft donors, which have been showing a very good affinity towards actinides. An efficient and selective separation of actinides from lanthanides is crucial to meet the Closed Fuel Cycle goal. Lanthanide ions, present in a large mass ratio with respect to actinides in the PUREX raffinate, have a high neutron-capture cross section that would not lead to an efficient minor actinide transmutation. The presence of uranium isotopes and other impurities in the transmutation target could generate further radiotoxic transuranic isotopes by neutron capture, instead of more stable nuclides. Most of the radiotoxic nuclides could be transmuted by thermal neutrons in conventional reactors, but the process would take a lot of time due to the low transmutation efficiency. Recent research is focusing on innovative nuclear transmuters such as Gen IV fast reactors and hybrid reactors. The final product left by the P&T process will be a dense vitrified waste to be disposed of for a smaller period of time. All the benefits coming from the management of nuclear waste by this advanced approach will be a small step towards a sustainable energy source and an increased public acceptance of the nuclear energy.

Overview of European experience in nuclear partitioning

A lot of research funded by the European Commission is being devoted to hydrometallurgical processes for the partitioning and transmutation of trivalent actinides. These research programs have first led to multicycle processes, secondly to the development of simplified and innovative processes. The hydrometallurgical partitioning consists of two relevant steps: extraction and stripping. In the first step the organic phase, containing the extracting ligand dissolved in a suitable solvent, is contacted with the aqueous phase coming from the dissolution of the irradiated fuel. The solutes present in the aqueous phase are extracted by a complexation reaction with the extracting agent and transferred into the organic phase in which the formed complexes are soluble. The second step, known as stripping, is obtained by reversing the complexation reaction, where the solutes are back-extracted into another aqueous solution usually different in acidity compared to the previous one. The main goal is to develop reliable and affordable industrial separation processes by lipophilic and hydrophilic ligands to selectively extract minor actinides from the M acidic target waste downstream of the PUREX process, but with the more challenging goal to minimize the amount of solid secondary waste. The CHON principle was born to meet this further process requirement, according to which all extractants and molecular reagents used in the developed processes have only to contain atoms of carbon, hydrogen, oxygen and nitrogen, thus incinerable waste to easily release into the environment.
The industrial separation processes will be implemented stepwise by annular centrifugal contactors, developed for the first time at Argonne National Laboratory in the 1970s. The countercurrent process consists of the aqueous and organic phases moving continuously in opposite directions stage by stage. The two immiscible liquids enter each contactor unit, first contacted in the annular region between the housing and the spinning rotor and then centrifuged in the inner part of the unit. Two main ways are currently followed within the partitioning strategy: the heterogeneous and homogeneous recycling. All the first European research projects on hydrometallurgical partitioning started within the heterogeneous recycling, since none of the developed extracting agents were able to selectively extract actinides directly downstream of the PUREX process. This led research to develop first multi-stage and multi-cycle processes. A two-cycle process was developed for a selective actinide extraction downstream of a first co-extraction of actinides and lanthanides. The recent joint research projects point to develop innovative processes with a reduced number of cycles to directly extract minor actinides from the PUREX raffinate in one cycle, either by a lipophilic extractant or by a hydrophilic ligand. Recent research efforts are being devoted to the homogeneous recycling by Grouped Actinides Extraction, which consists in a previous uranium recovery and a successive group separation of plutonium, neptunium, americium and curium actinide ions.