As explained by Samira Bostanchi, Nicholas Barron, and David Pearmain, rapid sintering is a promising technology for producing nuclear fuel, but it can also support the fixation of nuclear materials to produce waste forms for disposal

Manufacturing ceramic-based nuclear fuel and waste forms is usually a time and energy intensive process. The latest developments in manufacturing provide the potential to disrupt these established processes, thereby providing economic and throughput advantages. Preliminary work shows that rapid sintering can achieve this through the application and optimization of alternative (nuclear) materials. Advanced manufacturing technology can produce a variety of microstructures for reuse or disposal applications as required, but further development is still needed to prove and overcome challenges.

Rapid sintering (FS) is a field-assisted sintering technique that uses an electric field to be directly applied to the particles to reduce production time. Since the heat in the ceramic body is directly dissipated, rapid heating can be achieved, thereby achieving a lower furnace temperature than traditional sintering. In this way, it is expected to greatly reduce energy and increase production, and promote research on a wide range of materials with economic and environmental motives. 

FS first reported zirconia stabilized by yttria in 2010, and has since been applied to other ceramics. Recent work has extended it to nuclear fuels, mainly focusing on uranium dioxide or cerium dioxide, the latter as an alternative. 

FS has been proven to be effective in producing high-density uranium dioxide pellets as a potential advanced manufacturing method for common nuclear fuel. The densification rate of uranium dioxide is very fast, and the combustion temperature is lower than the temperature that can be achieved by traditional sintering, making it a promising candidate for studying the densification of other nuclear ceramics. One such ceramic is a mixed uranium plutonium oxide (Mox) fuel. Recently published work has successfully applied FS to produce oxide-doped cerium oxide at a significantly lower temperature and time. Due to its high neutron cross-section, gadolinium is often added to nuclear fuel as a combustible (neutron) absorber, but it has also been studied as a neutron poison in the form of ceramic waste for disposal.  

The reprocessing of spent nuclear fuel will soon end at the Sellafield site, and its focus will be completely shifted to the decommissioning of existing nuclear facilities. 

A key component of the overall mission is the management of legacy nuclear material. The UK has the world's largest stockpile of separated civilian plutonium. The safe management of these stocks to a safe end is the primary task of the British government to avoid the burden of security risks and proliferation sensitivity to future generations. The government aims to determine a solution to make British civilian plutonium out of reach and provide for nuclear cleanup/decommissioning. 

A recent analysis emphasized the need for a dual-track approach to plutonium management because of the uncertainty in the capital and operating costs of Mox fuel manufacturing and plutonium fixed facilities and the associated technical risks. It is promised that any plutonium that has not been converted into Mox or reused in other ways will be fixed and treated as waste.

A typical nuclear fuel production route includes mechanical grinding and mixing of uranium or uranium-plutonium dioxide powder raw materials, then cold pressing into particles, and high-temperature pressureless sintering in a reducing atmosphere. Ceramic scrap can be produced in a similar way to immobilize any remaining plutonium. However, it is important to understand new developments in process technology that may improve process safety, environmental performance, or economic efficiency. 

FS is a promising technology for the production of Mox fuel, but it can also support the immobilization of nuclear materials to produce waste forms for disposal. If applied to the production of industrial fuel or waste, this can translate into a huge increase in productivity and a reduction in energy consumption. It can also support the retention of volatile but long-lived radioisotopes, especially trace actinides, such as americium oxide, which usually evaporate from ceramics during the high temperature and long-term retention required for conventional sintering. The addition of such volatile substances is not only necessary to reduce the amount of waste, but also to implement a sustainable fuel cycle through transmutation.

In addition, according to the microstructure requirements, FS can enable users to achieve the target microstructure by setting the flash parameters appropriately. If this is the case, in addition to the homogenization of the powder feed, additional mixing or grinding may no longer be required. Due to the reduced manual handling of powder, there will be less dust and tool contamination, and the operator's radiation dose will also be lower.

As part of the Advanced Fuel Cycle Program (AFCP) fast reactor fuel project, Lucideon has proven that FS is a reliable technology that can produce high-density and homogeneous cerium oxide at a lower time scale and furnace temperature than traditional sintering methods Pellets (as an alternative nuclear material). The £46 million AFCP is funded by the £505 million Energy Innovation Program of the Ministry of Business, Energy and Industrial Strategy.

In order to provide the best repeatable control of the sintering process, a high-speed non-linear controller is used. The controller was developed by Lucideon to meet the challenges of various FS projects. It controls the change rate and time of electric field parameters during the flashing process, thereby avoiding the sharp change of power consumption in the traditional constant current limit experiment, which leads to local sintering. The optimization of flashing parameters, together with the development of electrodes and interfaces, promotes crack-free, homogeneous pellet FS.

FS can produce a wide range of microstructures, but further development is still needed to overcome process challenges. The successful application of FS on cerium dioxide particles has formed a solid baseline. Lucideon will cooperate with the National Nuclear Laboratory of the United Kingdom and the University of Manchester to further demonstrate the process of uranium materials. This work is supported by further research and development studies to develop process operations and evaluate the applicability of FS as a new technology in the form of manufacturing fuel and waste. 

Samira Bostanchi is Lucideon's ceramic technology consultant

Nicholas Barron is the reactor core technology technical manager of the National Nuclear Laboratory Ltd.

David Pearmain is the head of enhanced sintering in the Lucideon field

Photo: Kwasi Kwarteng, Secretary of State of the Ministry of Commerce, Energy and Industrial Strategy, talking with Dr. David Pearmain in Lucideon in July (Source: Lucideon)