THE TWO MAIN DRIVERS OF materials development in the nuclear fuel domain are safety and fuel cycle cost, says Alain Frichet, vice president of Fuel Products and Technologies at Framatome. Frichet, who joined Framatome in 1995, is leading the company’s research and development of all pressurised water reactor (PWR) and boiling water reactor (BWR) fuel technologies. This includes projects at Framatome’s sites in France, Germany and the USA, as well as worldwide technical partnerships.

In the aftermath of the 2011 accident at Fukushima Daiichi, the nuclear industry undertook a global effort to further increase fuel margins in the case of a severe accident. It led to programmes to develop enhanced accident tolerant fuel (EATF).

Framatome, as part of its PROTect programme, is developing and testing two EATF concepts: a near-term solution that involves chromia-enhanced fuel pellets and fuel rods with a chromium coating; and a longer-term solution with a silicon carbide-based cladding.

Frichet says adding a chromium coating to Framatome’s existing M5® zirconium alloy cladding, which has been demonstrated at burnup of 75MWd/kgU, improves resistance to oxidation at high temperatures and reduces hydrogen generation during a loss of cooling. The chromium coating also reduces creep and improves resistance to debris fretting during normal operations. Chromia-doped pellets can be used with UO2 and MOX (PuO2/UO2), and they reduce fission gas release, improve behaviour during loss of cooling and minimise pellet clad interaction (PCI).

“We introduced EATF in commercial reactors as early as 2015, at the Goesgen nuclear power plant in Switzerland,” says Frichet. Test rods with advanced cladding materials have now been in the reactor for two cycles, and are confirming the positive results from out-of-pile tests. “For coatings, the main challenge is ensuring the quality of the product, so that at the end of life the coating remains on the cladding and retains its functions,” he says.

Frichet stressed the importance of collecting data from different countries to satisfy operators and regulators.

Framatome recently delivered the first lead test assemblies containing chromium-coated cladding and chromia-enhanced fuel pellets to Vogtle 2 in Georgia. The assemblies were loaded into the PWR in March, and the unit is now operating at full power. “The rods will stay in the reactor for three 18-month cycles,” Frichet says.

A comprehensive qualification plan is needed to deliver commercial fuel. Framatome has worked for several years with the support of the US Department of Energy’s Accident Tolerant Fuel (ATF) programme, which it says has allowed the company to significantly improve on its initial target of 2022 to deploy the technology. Framatome is now aiming to have reload quantities of ATF ready by 2025.

The next goal is to replace the metallic cladding with a ceramic silicon carbide composite material that can withstand higher temperatures and more extreme conditions. Demonstrating and licensing this material is more challenging. “SiC is a ceramic, which means that it doesn’t have the same behaviour as metals,” says Frichet. As a result, there will be many tests looking at SiC during and after irradiation to test whether there is any atypical behaviour because it is a ceramic.” Framatome is developing a ‘triplex’ concept, comprising a SiC matrix plus metal fibres. The metal will act as a barrier to fission product release, which could help accelerate the licensing process.

Future technologies

Frichet also sees opportunities to leverage new technologies for fuel manufacture and qualification. Big data is a “very interesting” way of predicting fuel behaviour, he says. As a fuel designer, Framatome has lots of operating data, which could be used to optimise the design of future fuel rods.

Additive manufacturing (AM) also offers “many potential benefits” for nuclear fuel components, according to Frichet. It can be used to manufacture 3D shapes that are not feasible with standard manufacturing processes or would require many parts. It also offers opportunities to investigate composition gradients in components. Key applications include prototyping, manufacturing customised tooling and producing spare parts.

“Additive manufacturing is opening the field of creativity for the designer,” Frichet says. “Framatome is equipped with selective laser melting technology and has designed an anti-debris filter and other components.”

The next stage would be to validate the behaviour of additively-manufactured components under irradiation — a process which could take 5-6 years. However, a question remains as to whether AM will reduce costs. “This is not obvious so far because the manufacturing speed is low and the maturity is not enough to know if these techniques will provide cost reduction,” Frichet says.

Technologies have to be “fully mastered”, and the microstructure of the materials qualified, to show that the materials meet the design requirements. This means that widespread application is unlikely before the mid 2020s.

Framatome is working with Lightbridge as part of its EnFission joint venture to develop metallic nuclear fuel with helical twists (see NEI September 2018, pp24-26), which could bridge the gap between current LWRs and advanced reactors, Frichet notes. Due to the unique geometry, this technology could benefit from AM. Framatome is also working on small modular reactor (SMR) fuel in partnership with NuScale, which is expected to install the first SMR in the USA.