Fuel rods pictured at AREVA's FBFC workshop, Romans, France  (Photo: Copyright, AREVA, Alex Bianchi)

Technology Readiness Levels (TRLs) were developed originally in the NASA space flight programme and are an ideal way to gain an understanding of the maturity of emerging technologies. As technology develops from a lower to a higher TRL, it moves from an idea to a fully-fledged application (see Table below).

TRL Description Fuel-/cladding-related example
1 Research into the principles underlying the technology Promising materials have been identified
2 Practical applications suggested Fuel and/or cladding designs have been established
3 Basic components successfully demonstrated Fuel and/or cladding components have been tested out-of-reactor and/or irradiated as a component only
4 Integration of components into a basic system Test rods subjected to out-of-reactor tests and/or irradiated in test reactor but with only limited success
5 Basic system successfully demonstrated Test rods have been irradiated and performed successfully in a test reactor
6 Prototype construction (much more representative than basic system) Lead use assemblies irradiated in a prototype or commercial reactor but with only limited success
7 Prototype successfully demonstrated Lead use assemblies successful in a prototype or commercial reactor
8 Actual system constructed and commissioned Assemblies fabricated in reload quantities (at least one third of a full reactor core) may include irradiation with only limited success
9 Successful operation of actual system Assemblies successful after irradiation in reload quantities
10* Long-term operation of many actual systems Long-term use within commercial reactor fleets (many thousands of hours of operating experience)
*Not part of original NASA scale, but added to distinguish between new and well-established


It should be noted that TRL values are more useful for comparisons between technologies than they are when considered individually as absolute values. The description of technology development in terms of TRLs has its own limitations. It is at best a crude and somewhat subjective measure. Nor does it indicate the time, cost or effort required to move to a greater TRL; likewise, TRLs of different technologies may increase at different rates. There is no guarantee that a technology will ever achieve the highest TRL as it may prove unfeasible.

"Fuel and claddings are materials whose properties often ultimately limit reactor capability"

Despite these limitations, the National Nuclear Laboratory (NNL) decided to perform a worldwide Technology Readiness Level assessment for advanced claddings and fuels relevant to current and near-term reactors (Gen III) and advanced future reactors (Gen IV). The assessment incorporated the results from a literature review, conference attendance, relevant facility visits and discussion with partners in the international nuclear community.

Fuel and claddings are materials whose properties often ultimately limit reactor capability. Key characteristics include mechanical strength and toughness, ability to maintain properties and structure at high temperature and dose, corrosion resistance and fission gas retention (FGR), as well as cost and ease of manufacture.

Nuclear fuel technology assessment

Standard fuels for Gen III reactors are uranium dioxide (UO2) and MOX (mixed uranium-plutonium oxide) in zirconium alloy cladding. This TRL assessment considered both evolutionary improvements to these as well as more revolutionary concepts, such as entirely new materials or radically different forms, which are more applicable to Gen IV reactors. These include High Temperature Reactors (HTRs) and Fast Breeder Reactors (FBRs), of which the latter can breed further fuel allowing for a much more sustainable fuel cycle.

"Thorium fuels were assessed as an alternative to the U-Pu fuel cycle"

Types of fuel assessed include evolutionary UO2 and MOX using new additives and manufacturing techniques in addition to new geometries such as dual-cooled fuel (DCF) which has ring-shaped pellets with a second cladding tube through the centre to allow internal cooling. Also assessed were revolutionary new materials including new U and Pu compounds such as carbides, nitrides and silicides as well as advanced metallic fuel alloys. Thorium fuels were also assessed as an alternative to the U-Pu fuel cycle, as were inert matrix fuels (IMFs) for the destruction of Pu or nuclear waste such as minor actinides. Finally radically different fuel forms were considered such as TRISO coated particles and molten salts where the fuel operates in a liquid form

Advanced claddings lag behind corresponding nuclear fuels

Five broad types of potential cladding materials were also assessed.

The first two are advanced zirconium alloys and steels, the latter including ferritic/martensitic (F/M), reduced activation (RA), and oxide dispersion-strengthened (ODS) versions.

Then there are semi-refractory (heat-resistant) alloys including those based on nickel (including those used in the second UK FBR at Dounreay and the Hastelloys® used in the US Molten Salt Reactor Experiment), vanadium (used as a liner in the first FBR in the Dounreay SFR and since developed extensively by international fusion programmes) and chromium.

Refractory alloys that could be used as liners include niobium, tantalum, molybdenum and tungsten, and have been tested as part of the US space reactor programme.

Finally there are ceramic-based clads. These include silicon carbide fibre composites (SiCf/SiC developed extensively for fusion), MAX phase ceramics (first irradiation trials now underway) and TRISO fuel particle coatings including SiC (developed for German/US HTR programmes) and zirconium carbide (currently in development for HTR prototypes).Ceramic clads were no higher than TRL 3, so metals may be generally expected to continue to dominate clad technology in the near future.

"Advanced claddings were found in general to have lower TRLs than their corresponding fuels. Therefore, claddings may be a limiting factor in deploying advanced reactors."

Advanced claddings were found in general to have lower TRLs than their corresponding fuels. Therefore, claddings may be a limiting factor in deploying advanced reactors. For most advanced reactor designs, a significant level of further clad development will be required before commercial operation is possible. Indeed, four of the six Gen IV systems (SCWR, LFR, MSR, GFR) were assessed to have a clad TRL of no higher than 5.

TRLs were also found to be generally lower for the cladding materials for reactor systems with the higher maximum operating temperatures. These are potentially more thermodynamically efficient and have greater potential to supply heat to chemical process applications such as hydrogen gas production for fuel cell-powered transport.

Therefore further development of clads needs to be pursued urgently as part of international programmes for advanced reactors and nuclear fuel cycles.

About the author

Daniel Shepherd, NNL Fuel Technology, NNL Preston Laboratory, Springfields, Salwick, Preston, Lancashire, PR4 0XJ. This work has been funded by the UK Department of Energy and Climate Change (DECC) under an Initial National Nuclear R&D Programme.