In the aftermath of the 2011 accident at Fukushima Daiichi, the nuclear industry undertook a worldwide research and development effort to further increase fuel margins in the case of a severe accident, leading to the development of different enhanced accident tolerant fuel (EATF) programmes.

These fuel designs must provide reactor operators with additional time to cope with accident scenarios in light water reactor cores, such as loss of active cooling or beyond-design-basis events. At the same time they must offer improved fuel performance during normal operation.

Three programmes support and advance Areva NP’s work on EATF:

  • The US Department of Energy’s (DOE’s) EATF programme, currently in Phase 2.
  • France’s Joint Research programme agreement with the Alternative Energies and Atomic Energy Commission (CEA) and EDF.
  • A first-of-a-kind irradiation programme of EATF concepts in the pressurised water reactor (PWR) at Gösgen in Switzerland; part of a long-term research and development partnership.

The Areva NP team is developing and testing two EATF concepts: a near-term solution that involves chromia-enhanced fuel pellets and fuel rods with a high-quality chromium coating; and a solution to be used at a later date with a silicon carbide-based cladding.

Collective work

The Areva NP team is supported by its facilities around the world and the global nuclear sector, including utilities, research organisations, US national laboratories, universities and industry organisations. It has progressed over decades of research and testing, and nearly five years of work with DOE.

During the initial phase of the DOE programme, Areva NP worked with the University of Florida, University of Wisconsin and the Massachusetts Institute of Technology and with DOE’s national laboratories at Savannah River, Oak Ridge and Idaho. They explored a variety of advanced materials, alloys and manufacturing application methods. Simultaneously, Areva NP partnered with CEA and EDF to draw on their expertise in researching advanced materials and manufacturing methods.

In Phase 2 of the DOE programme, the team narrowed its focus to those materials that can be licensed using existing regulations, show near-term potential and for which advanced manufacturing methods can be developed to realise the fuel concepts.

Future technologies

Areva NP’s development work on chromia enhancements to fuel pellets started at the turn of the century. Chromia-enhanced fuel pellets have larger grains and improved viscoplasticity. This results in lower fission gas release under normal and accident conditions, and better resistance to pellet-to-cladding interaction. The higher burnup capability and density improves fuel cycle economics during normal operation. Increased resistance to pellet washout in the event of fuel rod failure means reliability is improved.

Adding a chromium coating to existing M5® zirconium alloy cladding offers an advanced and mature EATF cladding solution for near-term implementation. It consists of a dense, 15μm-thick chromium coating deposited on the surface of an M5® cladding tube. The coating is deposited using a proprietary physical vapour deposition (PVD) technique that does not modify the underlying zirconium substrate’s microstructure and forms a dense layer with no porosity at the chromium-zirconium interface. The coating is physically bonded and very adherent. This solution shows improved high-temperature steam oxidation resistance with reduced cladding creep and ballooning, providing the utility with improved performance under beyond-design-basis conditions. It also demonstrates improved performance during normal operation, with enhanced resistance to wear from grids, debris or baffle jetting.

Areva NP is also researching and developing a silicon carbide-based cladding solution with the following characteristics:

  • Very low oxidation kinetics in high-temperature steam in light water reactor conditions;
  • High strength at high temperatures;
  • High melting temperature;
  • Potential to allow for limited operation in post-critical heat-flux heat transfer.

These developments have some way to go and Areva NP is focusing on resolving the main technical challenges. Furthermore, to take full advantage of silicon carbide-based cladding, other plant and system changes must be evaluated. For example, to benefit from cladding that can perform at higher temperatures, new materials or manufacturing techniques may be necessary to increase the ability of control rods, and fuel assembly and reactor components to resist the higher temperatures. In response, Areva NP has initiated research and development in advanced control rods and fuel assembly components to offer a holistic EATF solution.

Continued research and development

Irradiation of Areva NP’s chromium-coated and silicon carbide-based cladding is taking place at the Gösgen plant, the first PWR in the world irradiating samples of EATF cladding. When inspected after the first cycle of irradiation, chromium-coated segments showed that the chromium coating remains tightly bonded, and oxidation performance improvements seen in out-of-pile testing are confirmed with irradiation. Post-irradiation examinations are under way and hot cell examinations are scheduled to start later in 2017.

Extensive out-of-pile testing is under way at the Areva NP and CEA laboratories, where technology for chromium coating has been under development over the past decade and shows promising results. This testing includes characterising corrosion performance in normal and accident conditions up to and including severe accidents, and mechanical and thermal-mechanical properties at room, operating and accident temperatures. Out-of-pile testing shows that the optimised coating provides significant improvement in oxidation performance and resistance to ballooning of the cladding, up to the extreme conditions experienced in severe accidents, with no detrimental impact on the mechanical properties of the underlying cladding structure. The chromium coating also exhibits excellent adhesion even under extreme strain conditions.

Several irradiation campaigns of chromia-enhanced fuel pellets in lead test assemblies (LTAs) have been conducted in European reactors. Hot cell examinations, integral tests and in-pile ramp tests were performed. The results demonstrate the potential for safety gains in normal operating conditions, accident conditions and transients. Additional irradiation testing of chromia-enhanced pellets was conducted in a US BWR over three operating cycles. Areva NP will remove some of these fuel rods and send them to a US national laboratory for further hot cell validation, expanding the range of data to high duty US cores in early 2018.

As part of Phase 2 of the DOE EATF programme, the Areva NP facility in Richland, Washington state, manufactured chromia- enhanced pellets and other components for irradiation test rodlets. Areva NP produced chromium-coated cladding tubes for these rodlets in France, before sending the tubes to Richland for final assembly.

The rodlets are due to be inserted into the Advanced Test Reactor at the Idaho National Laboratory in early 2018. They will be the first complete combined (cladding and pellets together) EATF concept rod in the world to be irradiated under PWR conditions. Initial post-irradiation and hot cell examinations are scheduled to start in 2019. Following irradiation, rodlets are scheduled to undergo transient testing at the laboratory’s Transient Reactor Test Facility. The Areva NP team is also working with Oak Ridge National Laboratory to design and fabricate chromium-coated cladding test specimens to be irradiated in the High Flux Isotope Reactor test facility, beginning in late 2017 or 2018.

Looking ahead

Areva NP anticipates beginning limited production of LTAs with chromia-enhanced pellets and chromium-coated cladding in 2018.

Building on its extensive research and testing, the Areva NP team has decided to implement EATF features – chromia-enhanced fuel pellets and chromium-coated cladding – in its GAIA and other fuel designs for PWRs, to help operators immediately benefit from these fuel technologies.

It is currently completing facility upgrades to implement and qualify chromia-enhanced pellet manufacturing processes at its fuel manufacturing facility in Richland.

A physical vapour deposition prototype machine to manufacture full- length rods has been fabricated and is currently being commissioned and calibrated. In spring 2019, Areva NP will load four of these lead test assemblies at Vogtle 2 in Georgia. It will be ready for production of full reload fuel batches featuring the chromia-enhanced pellets and chromium-coated cladding at the conclusion of the lead fuel assembly demonstration programme. 

Jeff Reed, Enhanced Accident Tolerant Fuel EATF US Programme Manager at Areva NP.