Bringing a new twist to nuclear fuel

THE NUCLEAR ENERGY INDUSTRY IS under more pressure than ever to operate plants more efficiently and increase public assurance of its safety.

Advanced fuel technologies are among the most promising developments to enable light water reactors (LWRs) to reduce operating costs and increase safety.

At Enfission we are developing such a fuel: Lightbridge Fuel. We believe it will provide significant, near-term benefits: increased power output; longer fuel cycles and plant lifetimes; reduced proliferation risk; and improved accident response.

The US-based Enfission joint venture was launched in January 2018, and it brings together Lightbridge Corporation’s years of advanced fuel R&D, and Framatome’s nuclear fuel engineering and production services to develop the innovative fuel.

Enfission is already making great strides to bring Lightbridge Fuel, with its distinctive helix shape and unique metallic composition, to market in a timeframe that can make a difference for utilities. Our team is hard at work developing the fuel for first demonstration in US nuclear plants.

As with any new fuel technology, development and demonstration will be supported by an extensive programme of research reactor irradiation, testing, and advanced modelling and simulation to ensure safe operation. These development activities will also generate qualification and validation data to ensure the fuel system meets the exacting demands of our customers and their regulators.

New shape and composition

The ingenuity of Enfission’s Lightbridge Fuel design lies in its novel shape and metallurgically-bonded metallic composition which, combined, drastically reduce fuel operating temperatures, improve its structural integrity, and enhance its response to abnormal events.

The shape of the fuel rod is unlike any other commercial nuclear fuel technology. The fuel rods are twisted, and this eliminates the need for complex fuel assembly spacer and mixing grids, because the fuel rods are naturally self-spacing and provide inherent coolant mixing.

Without spacer grids, the fuel assembly design complexity can be reduced, and a source of debris-trapping fuel failures is eliminated. Removal of the spacer grids also reduces the coolant pressure drop across the assembly, which makes natural circulation more effective as a passive safety mechanism.

The metallic Lightbridge Fuel rod has three components: a uranium-zirconium alloy fuel core, a corrosion-resistant zirconium alloy barrier, and a central displacer. These components become metallurgically-bonded during the fuel manufacturing process to form a composite solid metal fuel rod. 

Metallic nuclear fuels have been successfully used since the early days of nuclear, including in the EBR-II and many of the world’s research reactors. The Lightbridge Fuel uses a zirconium-rich alloy, designed for operation in commercial LWRs.

The distinct shape and high thermal conductivity of the metal rod enables the fuel to operate at significantly cooler temperatures. Conventional fuel in a modern plant can operate at temperatures over 1500°C, which is nearly 1000 degrees higher than the peak operating temperature of Lightbridge Fuel.

This temperature reduction is due to the fuel’s improved heat transfer capabilities. It has more surface area for coolant interaction, less distance between the fuel rod where the heat is generated and the water, improved coolant mixing, and higher fuel thermal conductivity. Initial calculations show that these thermal characteristics drastically improve the fuel’s ability to recover from off-normal conditions and we expect it will exceed the safety performance of current fuels during design-basis events.

Use in existing and new reactor designs

Lightbridge Fuel is designed to be compatible with the core internals and plant operating procedures of both existing and new commercial reactor designs, including some small modular reactors (SMRs). This gives existing and new nuclear utilities, across the market, flexibility to meet the needs of their plant.

The fuel enables plants to increase fuel cycle length and power output and provides the potential to economically load-follow without the constraints imposed on current fuels – a major advantage as the world continues to increase the capacity of intermittent renewable generation.

Lightbridge Fuel can operate with higher linear power density than is achievable with conventional fuels, while maintaining the cold fuel operating temperatures.

Combined with high burnup capacity, which enables longer cycle lengths, the fuel enables nuclear plants to generate more power for longer. The fuel can enable a conventional four-loop pressurised water reactor (PWR) to increase power output up to 4000MWt while operating with 24-month fuel cycles, a significant economic improvement.

New-build reactors, designed to take full advantage of Lightbridge Fuel, could achieve even higher power outputs. The increased power generation and cycle length represent significant cost savings for utilities. 

Another characteristic of the innovative technology is its ability to retain radioactivity within the fuel rod. The low operating temperature and composite structure of our metallic fuel introduces an additional, natural barrier to the release of fission products. As the fuel operates with an average temperature less than 400°C, fission product diffusion is significantly reduced, and fission gases are trapped within the fuel matrix. The fuel cladding effectively becomes a secondary barrier to radioactive release. Coupled with the metallurgical fuel-clad bond (i.e., the absence of a fuel-clad gap and plenum), there is no mechanism for rapid release of gaseous fission products to the reactor coolant.

Reactor load-following is enabled, in part, due to this metallurgical-bond. It creates a structural linkage between the fuel rod’s components, so the cladding is no longer the sole structural element of the fuel rod. This reduces the impact of fuel-clad mechanical interaction as the two components are designed to operate together. The fuel can withstand cyclic changes in reactor power (e.g., due to reactor startup and shutdown) more effectively than conventional fuel designs and enables faster and more frequent power ramps.

Combined strength

Enfission’s partners, Lightbridge and Framatome, have been developing new technologies to improve the safety and performance of nuclear power for decades.

Development of Enfission’s Lightbridge Fuel draws on nearly a decade of design engineering, testing and improvement, originating with Lightbridge’s research and development programme for thorium-based “seed-and- blanket” fuels. The need for a high-power density, high burnup fuel for the seed component of that technology led Lightbridge to the invention of the Lightbridge Fuel rod. Lightbridge announced the new fuel product in 2010 to meet the market’s need for a fuel system capable of increasing both reactor safety and economic performance.

The design of the fuel system has evolved over the years, based on continuing R&D and feedback from Lightbridge’s Nuclear Utility Fuel Advisory Board, with representatives from more than 50% of US nuclear generating capacity. The early fuel system design for LWRs shared similarities with the thorium-based seed-and-blanket technology, using the twisted metallic fuel rods in the centre of the fuel assembly, with a peripheral row of oxide fuel rods.  

Feedback from Lightbridge’s industry advisory group resulted in the removal of that outer row of conventional fuel rods to eliminate the operating constraints of oxide fuel on the new technology. The current fuel system design now includes only metallic fuel rods.

To move the design closer toward a product ready to be deployed in a US reactor, Enfission combines the Lightbridge design concepts with Framatome’s network of fuel designers and state-of-the-art tools. Framatome has designed nuclear power plants and nuclear fuel for 60 years and has a global network of expertise plus fuel fabrication facilities in France, Germany and the USA. Approximately 225,000 Framatome fuel assemblies have been loaded in more than 100 reactors in operation around the world.

Framatome’s latest offerings include ATRIUM 11 fuel for BWRs and GAIA fuel for PWRs, which offer better performance and flexibility compared to previous designs.

The road ahead

We announced the Enfission joint venture in March 2016. Since then, steady progress has been made on developing the fuel, while corporate governance and project management structures were put in place. Today, Enfission has more than 60 engineers developing the fuel and its advanced manufacturing processes and we are seeking additional staff who share our excitement for innovation, and dedication to quality and safety.

We plan to begin manufacturing process development and demonstration in mid-2019 at Framatome’s plant in Richland, Washington, USA.

The Lightbridge Fuel fabrication process is quite different from conventional fuels. It uses high-temperature metal coextrusion. However, the processes themselves are new only to the commercial fuel industry. Initial manufacturing efforts will focus on process optimisation to demonstrate safe, efficient commercial-scale production. The Richland facility will also produce fuel rods for our various test programmes.

When the Norwegian government announced plans to close the unique Halden research reactor in June 2018, Enfission modified its fuel development programme to move fuel testing to other facilities, including in the US, and we expect to formalise these arrangements by early 2019.

As part of the fuel demonstration programme, Enfission plans to load lead test rods of Lightbridge Fuel in a US PWR as soon as 2021 with lead test assemblies targeted to begin in 2024. The company expects to announce an agreement with a US utility to begin this testing soon.

Preparation for licensing by the US Nuclear Regulatory Commission is underway. While Enfission finalises the commercial design of the metallic fuel, it will also develop the licensing support packages that utilities can use with the NRC.

Recognising that the current NRC standard review plans are optimised for conventional nuclear fuel, Enfission will take regulator advice on the proper means to confirm safe operation of the Lightbridge Fuel. Our Regulatory Engagement Plan defines a process where we will interface early and often with the NRC and experts on nuclear regulations to determine the right standards to show compliance with the NRC’s safety standards.

A grand challenge

Advanced fuel technologies such as Lightbridge Fuel will change the game for the nuclear power industry.

Light water reactors will be the dominant commercial nuclear technology for many decades. At Enfission, we’re taking advanced fuel to the next level, developing an innovative technology and working with suppliers and various partners to do our part.

However, this vast undertaking will require industry- wide effort. Successful deployment of such innovative technologies in a highly-regulated industry requires both market demand and government support. The electric utilities and governments of the world must voice their demand, and show their support, for technologies that meet their needs.  

Author information: Aaron Totemeier, Project director at Enfission, LLC