Global efforts advance nuclear fuel performance

SIGNIFICANT ADVANCEMENTS IN INSTRUMENTATION AND control (I&C), non-destructive examinations, component replacements and repairs, and fuel help the current and planned reactor fleet operate safely, efficiently and reliably. No area has advanced as rapidly as developments in enhanced accident tolerant fuel (EATF). EATF is designed to withstand the loss of active cooling in light water reactors for longer and to improve fuel performance during normal operations.

During normal nuclear power production, fuel assemblies withstand high temperatures, managed by cooling systems that balance temperatures for optimal fuel performance. In the event of conditions that lead to a reduction or loss of cooling, emergency measures may have to be taken to shut down operations. Fuel engineers and manufacturers explore improved engineering of the materials involved to increase the robustness of fuel assemblies against these events.

The first line of defence in preventing the release of radioactive material into the reactor coolant is the fuel cladding. The primary reactor coolant is completely isolated from the secondary cooling and contained within the reactor vessel. The technical challenges in cladding development relate to minimising high temperature oxidation and preventing hydrogen generation, while maintaining or improving the structural integrity of the fuel assembly and all other safety parameters.

As changes are made to the cladding material, experts must ensure that it maintains its geometry and mechanical robustness, and prevents the release of radioactive material.

In response to nuclear energy industry actions taken after Fukushima and direction from the US Department of Energy (DOE), Framatome launched its PROtect programme and it currently has two EATF designs in development and testing.

The near-term design has chromia-enhanced pellets and chromium-coated cladding. These features will be added to Framatome’s latest pressurised water reactor (PWR) fuel designs (GAIA, HTP and AFA). Reload supplies of these fuel technologies with EATF features are targeted for completion in 2023–2025. Meanwhile, the long-term solution has chromia-enhanced pellets with a silicon carbide-based cladding.

The near-term solution: Chromia-enhanced fuel pellets

Chromia-enhanced fuel pellets have been used in boiling water reactors (BWRs) and PWRs in Europe since 1997. These pellets have a larger grain structure and improved viscoplasticity than earlier versions. Viscoplasticity is the mechanical response the pellet experiences under different loads. An improved viscoplastic response in the chromia- enhanced pellet design can reduce the stress on the cladding during pellet cladding interaction (PCI).

Improved PCI characteristics provide greater operational flexibility, such as load following capability. Load following allows for temporary power reductions to support fluctuations in other power supplies such as wind and solar. The larger grain structure could also reduce fuel fragmentation and thus improve fuel fragmentation, relocation and dispersal behaviour under severe conditions.

Framatome’s chromium-coated cladding samples have been undergoing irradiation in Europe since 2016, ahead of other EATF technologies. The chromium coating greatly reduces oxidation at higher temperatures, protects cladding from debris damage and significantly delays hydrogen buildup in the event oxidation does occur. In loss of coolant accident tests, the chromium cladding demonstrates reduced ballooning behaviour, maintaining the cooling geometry of the fuel rods and reactor core for longer. It also reduces fuel relocation, and in combination with chromia- enhanced fuel, it reduces the potential dispersal of fuel material.

These improvements also support increased burnup, meaning that each assembly could remain in the core longer and utilise its potential more efficiently. That would support extension of fuel cycle times for the US PWRs from 18 months to 24 months.

With the advancement of the Framatome physical vapour deposition (PVD) coating process, BWR applications for advanced coatings are under way in Europe and the USA. The first US commercial irradiation lead test rods are planned for 2021.

The long-term solution: SiC-based cladding and components

Silicon carbide can replace zirconium alloys with no adverse impact on fuel efficiency. At the same time, it provides substantial temperature and oxidation resistance, significantly reducing the of hydrogen that could be generated in the event of a severe accident. This makes silicon-carbide composite materials ideal candidates for an EATF cladding as well as for structural components such as BWR fuel channels.

The long-term PROtect EATF design features chromia- enhanced pellets with a silicon carbide-based cladding. A multilayer SiC concept was originally developed for Gen IV reactors in a programme initiated by France’s Alternative Energies and Atomic Energy Commission (CEA) in 2000. It became a LWR Gen III EATF cladding concept under a Framatome-CEA-EDF tripartite agreement. Framatome identified significant technical challenges with realising the projected benefits of SiC and, with the support of the CEA, it is making systematic changes to the sandwich design to address critical technical challenges such as hydrothermal corrosion, tightness and sealing the ends of the cladding tube.

The first irradiation tests under representative PWR conditions started in 2016 by irradiation of samples at the Go¨sgen plant in Switzerland.

Framatome plans irradiation testing of SiC fuel rodlets from 2020 in the Advanced Test Reactor (ATR) at Idaho National Laboratory (INL), with the primary objective of demonstrating the integral performance under PWR prototypic conditions.

The nuclear industry benefits from research and innovations from other industries. For example, Framatome’s PVD coating process is based on the industrial glass and electronic manufacturing industry’s use of PVD-applied coatings for its products. Framatome is learning from this industry on scalable versions of coating processes and equipment.

Recognising the potential to accelerate development of silicon-carbide for the nuclear fuel industry, Framatome teamed with General Atomics to study the application of silicon carbide for BWR fuel channel materials. The fuel channel is a square, 14-foot-long channel that surrounds each BWR fuel assembly. Advanced nuclear fuel designs using silicon carbide in the fuel channel will improve safety and fuel performance.

Framatome and General Atomics will test silicon-carbide materials for fuel channel applications to help remove 40% of zirconium (Zr) metal in BWR fuel designs. This directly reduces the risk of unwanted hydrogen production. This collaboration builds on General Atomics’ history of developing and deploying advanced materials for the nuclear energy industry and other sectors such as aerospace and defence.

Promising results from irradiated and non- irradiated samples

Chromium-coated and silicon carbide-based cladding test samples have been through three irradiation cycles at Go¨sgen, and samples show positive visual results. Oxidation characteristics are greatly reduced and there are no signs of coating delamination. Additional samples were removed after one and two cycles at Go¨sgen and are now undergoing further characterisation and testing at Switzerland’s Paul Scherrer Institute. Go¨sgen is the first PWR in the world irradiating samples of EATF cladding, which will support commercialisation of EATF concepts.

Extensive testing on samples continues at Framatome and CEA laboratories, where chromium-coating technology development started in the past decade. This extensive test programme has been built to create the comprehensive databases of material properties and performance data necessary to model, license and implement chromium- coated cladding in commercial reactors worldwide. There are out-of-pile test programmes planned and under way on Framatome’s EATF concepts that will complement irradiation test programmes.

In June 2018, chromium-coated rodlets with chromia- enhanced pellets were inserted for testing at the INL Advanced Test Reactor. These rodlets are the first complete combined (cladding and pellets together) EATF concept rod to be irradiated under PWR conditions. In total 26 rodlets are being tested in a special loop that mimics the coolant conditions of a commercial light water reactor. The test results will be used to help qualify the fuel design with the US Nuclear Regulatory Commission (NRC).

Following irradiation, the rodlets are scheduled to undergo transient testing at INL’s Transient Reactor Test Facility. The results from these tests will also help qualify the fuel with the NRC. The Framatome team is working with Oak Ridge National Laboratory to test chromium-coated cladding test specimens planned for irradiation in the High Flux Isotope Reactor test facility in 2020.

Making advances thanks to continued financing

Advancing nuclear technologies requires a significant commitment from industrial organisations in research and development funding and resource availability.

In 2010, Framatome started researching options for EATF. In 2016 the company won a $10 million, two-year grant to initiate in-house design and development activities. With the additional funding and lab support, Framatome accelerated its schedule and was the first to introduce test samples to the market.

Progress stepped up in 2018 with funding from the US DOE’s Accident Tolerant Fuel programme, which gave fuel designers funding and additional support to prioritise EATF studies. Framatome received $49 million in grant funding from US DOE in November 2018, along with the opportunity to continue to use, and receive support from, DOE’s national laboratory facilities.

While testing on PROtect EATF technologies is under way, Framatome has also developed and qualified the manufacturing process. For the fuel rod cladding, development has advanced to the stage of small batch, full-length rod coating capability, necessary for lead test assembly and test rod production. In March 2018, the team produced its first full-length fuel cladding coated with chromium. The coating was performed on prototype equipment in France.

With the funding awarded by DOE, Framatome’s fuel manufacturing facility in Richland, Washington, is being upgraded and qualified to begin production of chromia- enhanced pellets for BWR reload quantities and to support lead test rod fabrication for PWRs. Framatome also invested in upgrades to its fuel manufacturing facilities in Lingen, Germany, and Romans, France, which are ready to begin production.

With the lessons learned from the small-batch prototype, the company is accelerating a pilot programme to develop the large-batch capability required to support a high volume of fuel rods and the initial reload batch. A PWR fuel assembly can contain more than 200 fuel rods, depending on the design, and one reload could require 14,000-17,000 fuel rods, depending on fuel design and batch size. While much progress has been made with promising results, more work is being done to manufacture this quantity of rods.

Based on the promising results obtained using its advanced PVD coating process, Framatome will expand its research into the applicability of this process and beneficial coatings for the BWR market.

Advancing accident tolerant control components

Framatome is also developing advanced technology for the components that allow operators to control the nuclear reaction and power levels in the core during operations. While EATF solutions tolerate higher temperatures and provide operators with more response time, accident- tolerant control component materials allow operators to control the reactors at even higher temperatures.

Framatome is independently developing an accident-tolerant control rod (ATCR) comprising advanced ceramic pellets. These pellets exhibit extremely high temperature tolerance and do not show any eutectic (ie where an alloy has a melting point lower than its component metals) up to at least 1600°C. This is at least a 400°C better for these new absorbers than for the current ones (AIC and B4C). The reactor control system would be better able to maintain a fully shutdown condition during the unlikely case of a severe transient, a temporary condition or situation outside normal operating conditions that can present challenging conditions for the fuel. It also allows more time for the emergency core cooling systems to inject borated water.

Ceramic pellets exhibit another significant benefit to normal operations: substantially less swelling than AIC control rods as a function of radiation exposure over time (fluence). Framatome’s ATCR pellets’ improved performance is being demonstrated by irradiation in the high flux isotope reactor. This reduced swelling allows the control rod to be operated over a longer lifetime and inserted into the core during power operation without the issue of swelling. As a result, these control rods can be replaced less frequently and can be used in plants that require power manoeuvring capabilities to support load following (ie flexible operation).

The road to full reload supply

Research, development and testing work on EATF designs is the result of decades of experience and expertise. In early 2019, a team of experts from Framatome, Southern Company and Georgia Power placed full-length (not segmented) lead fuel rods with chromia-enhanced fuel pellets and chromium-coated cladding into Georgia Power’s Alvin W. Vogtle 2 plant. These rods were the first complete, full-length and fuelled EATF concepts loaded into a commercial reactor anywhere in the world. The schedule was as follows:

• October 2018: LFA production began.
• December 2018: LFA production completed.
• January 2019: LFAs delivered to Vogtle.
• March 2019: LFAs inserted during the spring outage. These rods will remain in core for three, 18-month cycles then be removed for further examination and testing at a hot cell facility. Southern Company and Georgia Power have been key supporters of the EATF programme and instrumental in the success achieved with this delivery. Additional reload activities include:
• Summer 2019: two LFAs with 10 full-length rods with chromia-enhanced fuel pellets and chromium-coated cladding were loaded in the Go¨sgen plant. The data collected will support licensing up to very high burnups.
• Autumn 2019: Framatome loaded fuel assemblies with chromium-coated rods into Entergy’s Arkansas Nuclear One unit 1. These LFAs will provide important data and insight on the fuel’s performance in B&W design commercial reactors. Framatome also completed an agreement with Exelon for full LFAs to be loaded in early 2021 at Calvert Cliffs. All fuel rods in these assemblies will be chromium coated and will contain chromium- enhanced pellets.
• 2021: Framatome is targeting completion of the first lead test rods in a BWR for its advanced PVD coatings and silicon carbide-based channel samples in a US commercial reactor, in addition to a PWR lead rod demonstration in France.

Author information: Jeff Reed, Programme director, Advanced Fuel Development at Framatome Inc.; Nicolas Vioujard, Framatome Fuel Business Unit – Products, Technologies and Quality; Nico Vollmer, Global marketing manager, Framatome GmbH