Enhancing economics with ATF

30 September 2020

Adoption of advanced technology fuels (ATF) is on the rise, but their economic impact is poorly understood. The UK’s Advanced Fuel Cycle Programme is shifting understanding. By Simon Middleburgh, Mustafa Bolukbasi and Dave Goddard

OVER RECENT YEARS, THERE HAS been a continuing desire across the nuclear power sector to improve the safety and operational performance of light water reactor (LWR) fuels. This has driven the development of ‘accident tolerant fuels’ — also referred to as advanced technology fuels (ATFs). Now ATFs are undergoing rapid innovation to harness their full potential within the Net Zero puzzle.

Safe, clean fuels must also be cost efficient. The UK’s Advanced Fuel Cycle Programme (AFCP), see box, is leading the transition of novel ATFs from being purely safety-driven to economic. Led by the UK National Nuclear Laboratory (NNL), AFCP is funded by the Department for Business, Energy and Industrial Strategy (BEIS). Leveraging partnerships across the nuclear sector, AFCP is investigating how novel ATFs would lower the overall cost of nuclear power. This timely study aims to assess feasibility of near-term ATF adoption by the global nuclear power industry.

Comparing costs

The overall cost of nuclear power is similar to that of other energy options. But when broken down its costs are structured differently from other baseload electricity- generating technologies, such as coal and gas. Its cost more closely resembles that of renewable technologies, such as wind and geothermal (Figure 1). Like renewables, the dominant cost for nuclear power is construction.

The cost of fuel for nuclear power is a small proportion (10-15%) of the overall levelised cost of electricity (LCOE). 

Within that, uranium is the dominant cost (50%) of nuclear fuel, with conversion (10%), enrichment (20%) and fuel fabrication (20%) making up the remainder. In contrast, for gas, biomass and coal, fuel is approximately 80% of the LCOE. Reducing the cost of fuel, therefore, does not reduce the overall LCOE for nuclear as it does for other CO2- generating technologies. However, the economic benefits of adopting new fuel cascade beyond immediate savings. ATFs can lower costs in other portions of the total LCOE, namely in operation and maintenance of the reactors. AFCP’s economic assessment has identified two ways that ATFs can contribute to reducing the LCOE of nuclear power: increasing fuel cycle lengths and reducing unplanned outages.

Extending fuel cycle lengths

In pressurised water reactors (PWRs) and boiling water reactors (BWRs), fuel residence times are typically split into cycles. At the end of each cycle – often 18 months – there is a planned outage. During these outages, old fuel is removed, fresh fuel is added and intermediate fuel is re- positioned. At the same time, fuels are inspected to avoid unexpected fuel failures. Planned outage times have been significantly reduced over the last few decades. At present, a typical outage time is one month, giving a maximum capacity factor of 94.4%. Extending the cycle length from 18 to 24 months can increase this figure to 95.8%. While seemingly moderate, this increase has a significant impact on the profitability of nuclear power.

AFCP is exploring a number of innovations and measures to increase overall residence times of fuel within the reactor and improve core flexibility, thereby reducing operational costs and making nuclear power more affordable and efficient.

Increase in enrichment: The commercial nuclear fuel industry generally limits 235U enrichments to under 5 wt.%. Modern fuel fabrication facilities are licensed to this value, with criticality safety measures in place to prevent accidents. A modest increase to 6-7 wt.% in enrichment limits would enable a significant increase in fuel residence times to extend fuel cycle length. Raising the enrichment limit for commercial fuels may also provide more flexibility for core design, further improving efficiency through longer residence times.

Advanced burnable absorber technology: The addition of burnable absorbers to fuel enables higher-enrichment assemblies to be loaded into a reactor, while maintaining a controllable neutron multiplication factor. As the absorbers burn away, fresh fuel is exposed, which enables longer residence times. Although burnable absorbers are regularly used in both BWRs and PWRs, advances in technology that provide longer suppression times, displace less fissile material and leave less unwanted residual neutron absorption are being developed as part of the AFCP and within the wider nuclear fuel industry.

Higher density fuels: UO2 is standard commercial nuclear fuel for LWRs. Its commonality is due to its acceptable material properties, fissile uranium density and benign reactions with coolant in the event of cladding failure. Other fuels with more uranium atoms per volume — therefore with higher uranium densities than UO2 — are now being considered in industry and being developed within AFCP. Such compounds include U3Si2, UB2 and UN.

Safety concerns related to how the materials react with water mean that water-proofing technologies are required for many of these concepts. Significant efforts are underway within AFCP and internationally to develop these revolutionary fuels.

Combinations of these three ATF technologies provide a powerful route to increasing fuel cycle durations. However, these innovations do not have to be packaged as a trio to have significant impact. For example, consider the combination of high density fuel with an advanced burnable absorber. Recently, AFCP and its partners at Los Alamos National Laboratory in the USA and KTH Royal Institute of Technology in Sweden explored UB2 as a combined fuel/burnable absorber system. Here, the efficient neutron absorber 10B is added back to the U11B2 fuel to act as an integrated burnable absorber. As UB2 has a 20% higher uranium density than UO2, fuel is not displaced by the burnable absorber in this compound, further improving the fuel cycle economics.

Composite fuels, including Triso particles, are also key. These fuels enable new, higher density uranium materials whilst ensuring the reactions between the fuel and coolant remain manageable. A leading composite fuel concept is the combination of UN and UO2.

Reducing unplanned outages

Alongside increased fuel cycle length, reducing unplanned outages is a route to reducing the costs of nuclear generation. Unplanned outages can result from equipment failure (including fuel failure), operational error or external circumstances (including severe weather or earthquakes). Most fuel failures are very minor, resulting in a small or negligible release of radioactive material into the primary circuit water at no risk to operators. If the failure is deemed too severe to continue operation — posing a safety risk in planned outages or during operation if allowed to progress — the reactor must be safely shut down for unplanned maintenance.

In LWRs, the majority of cladding failures are associated with fretting failures (debris or grid-to-rod fretting), pellet-cladding interaction or crud corrosion. Manufacturing errors cause ~5% of failures.

A number of new cladding technologies may increase the reliability of fuels. In accident scenarios, these materials may even provide additional benefits — combining the remit of accident tolerant fuel and advanced technology fuel.

The leading ATF cladding innovation is coated zirconium (Zr). In PWRs, chromium (Cr) is applied as a coating to the external surface of the cladding tube through techniques such as magnetron sputtering. Cr coatings are harder than the underlying Zr material, providing wear and fretting resistance while slowing oxidation in PWR environments – thereby improving the corrosion behaviour of the fuel cladding. Slower corrosion and lower hydrogen uptake also improve the mechanical property evolution of the cladding, enabling longer residence times in the reactor. This aids the increase in fuel cycle length with fuel pellet concepts.

In collaboration with UK and international industry, AFCP is developing advanced, next generation candidates that completely replace Zr-based cladding materials.

A leading candidate is SiC-SiC fibre-matrix composites. These materials oxidize at a much slower rate than Zr-based cladding and, as such, reduce any associated corrosion fuel failures. Challenges associated with these new materials include manufacturing, reactor operation behaviour deviations (compared to the current materials) and material availability – factors which all significantly impact the economic viability for deploying these systems today. However, advances in these low-technology readiness candidates may provide a useful path for future fuels — further enhancing their operational flexibility and reliability.

A proactive future for new fuel adoption

Novel fuels will require investment in new or updated fabrication facilities, out-of-core and in-core testing and updates to methods and regulation. However, with the long-term viability and payoff of ATFs so clear, nations and regulators are responding.

New fuel systems will require capital expenditure to improve existing facilities or build entirely new production lines. Some developments, such as increased enrichment pellets, may require fewer modifications, whereas some high-density fuels will require significant investment in manufacturing and throughout the supply chain.

Parallel developments in the nuclear industry — including the advanced and small modular reactor programmes (AMR and SMR) in the UK and internationally — will require new fuel materials. There is potential to enhance the economic impact of ATFs by ensuring that these fuels can also be suitable for next-generation reactors. This would reduce the costs associated with developing the next generation of nuclear power, increasing the operational and manufacturing experience of these fuel systems. In turn, this reduces the cost of licensing and operating with these new fuels, improving reliability in lieu of decades of operating experience that will further lower the LCOE for nuclear power.

Programmes like AFCP are taking a proactive approach to these challenges. As part of the nuclear answer to the UK’s 2050 Net Zero ambition, AFCP is demonstrating that fuel cycle economics can be better understood, improved and planned through careful consideration of new fuel systems. In illuminating the economic potential of future ATF deployment, these assessments establish the value of continued, collaborative nuclear innovation.

The Advanced Fuel Cycle Programme

The Advanced Fuel Cycle Programme (AFCP) is driving innovation to underpin a clean energy future in the United Kingdom. Part of the £180m Nuclear Innovation Programme — the biggest public investment in nuclear fission research in a generation — AFCP aims to understand the role of nuclear fuels and advanced fuel cycle in a Net Zero world.

Led in partnership by the UK National Nuclear Laboratory (NNL), AFCP pioneers a collaborative approach to innovation. The programme unites over 100 organisations across the globe, engaging with partners in over 10 countries. Domestically, AFCP is partnering with over 60 UK organisations spanning industry, national laboratories, government and universities — including Bangor University, where this novel economical assessment work is centred — to maximise the UK’s rich capability, infrastructure and multigenerational energy innovators.

Working together, AFCP’s research network covers the fuel cycle and considerations of spent fuel implications for ATF decisions. Under its four major focus areas – capability, capacity, cost reduction and collaboration – the programme is constructing the foundation for which tangible, future-focused advanced fuels and recycling concepts can fully evolve.

Author information: Simon Middleburgh, Ser Cymru Reader in nuclear materials at the Nuclear Futures Institute at Bangor University; Mustafa Bolukbasi, First-year Ph.D. student in Nuclear Futures Institute at Bangor University; Dave Goddard, Laboratory fellow, nuclear fuel manufacturing at National Nuclear Laboratory

Figure 1: Breakdown of the levelised costs of electricity for different electricity generation types in the UK Source: Projected Costs of Generating Electricity – 2015 edition – International Energy Agency https://www.oecd-nea.org/ ndd/pubs/2015/7057-proj-costs-electricity-2015.pdf
Two low-magnification scanning electron microscope (SEM) images show an early attempt at ZrB2 kernel manufacture. The particles are sub-millimetre in diameter and the AFCP research team at Bangor University is aiming to coat them to add directly into UO2 fuel before pelletizing and sintering. The additives will improve the thermal conductivity of the fuel whilst providing some burnable absorber behaviour. The process will be honed by Dr. Phylis Makurunje as part of the AFCP kernels project. Photo credit: Dr. Iuliia Ipatova at Bangor University
Figure 2: Schematic of burnable absorber behaviour. Without burnable absorbers, initial reactivity (kinf) is too high. Reducing enrichment to 3% reduces the activity at the end of life. Using a burnable absorber allows one to load higher enrichments
Magnetron sputtering is being used by AFCP for the Cr coating of Zr alloy cladding, with potential near term benefits of better wear resistance and reduced cladding corrosion. The images show 1) The plasma glow from the inside of the magnetron sputtering process, and 2) Some coated cladding tubes (the one at the bottom is uncoated which is why is has a slightly different colour) Photo credit: NNL
Figure 3: Increase in U density compared to UO2

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