The Fukushima Daiichi event has demonstrated the need for improved nuclear energy safety, which can be ensured by the development of accident‐tolerant fuels (ATFs). ATFs are expected to overcome the inherent technical shortcomings of the standard zircaloy/UO2 fuels, thus relieving the industry from the huge financial penalty associated with beyond‐design‐basis accidents leading to fuel cladding material failure and release of radioactive fission products to the power plant containment and the environment.
In February 2017, the European Commission has approved the large-scale H2020 project "Innovative cladding materials for advanced accident-tolerant energy systems" - IL TROVATORE (October 2017-March 2022), where SCK•CEN is the coordinator. IL TROVATORE combines scientific excellence with industrial support and has been brought forth as an international collaboration between Europe, the USA and Japan.
The main objective of IL TROVATORE is to identify the best candidate ATF cladding materials for use in Gen‐II/III LWRs and to validate them in an industrially‐relevant environment, i.e., under neutron irradiation in PWR‐like water. Ideally, fuel cladding materials must demonstrate leak tightness and containment of fuel pellets and fission products during the fuel residence in the reactor, even under transient/accidental operation conditions. The key material irradiation foreseen in IL TROVATORE will be carried out at the BR2 test reactor in Mol.
A critical pre‐selection of material candidates with the potential to address the stringent requirements of the ATF cladding application has already been performed, taking into account the latest global academic and industrial achievements. The pre‐selected ATF cladding material concepts to be further studied in IL TROVATORE are:
- SiC/SiC composites: select composite concepts produced by different processing routes and with different – but equally promising in terms of performance –critical fibre/matrix interfaces. Apart from their refractoriness, the great advantage of SiC/SiC composites is their damage tolerance, i.e., the fact that they exhibit pseudo‐ductile fracture with high strain at failure (εf > 0.5%) in contrast to the brittle fracture of monolithic SiC (εf < 0.1%). This type of ATF cladding materials is considered as mid‐term technology, due to the challenges associated with the qualification of such advanced ceramic clads.
- Coated cladding material concepts: the application of protective coatings on commercial clads (e.g., zircaloys) is considered a near‐term technology, due to the fact that the performance of these new clads will essentially rely on the performance of the well‐known substrates that remain largely unaltered by the applied coatings. The following coating types are considered:
- Nanolaminated ternary carbide (MAX phases) coatings: select MAX phases (Mn+1AXn, n=1, 2, 3) and solid solutions thereof with high‐temperature stability and oxidation resistance, as well as self‐healing potential for both cracks and radiation‐induced damage will be applied on commercial clads;
- Nanocrystalline oxide coatings: substrate‐specific, nanocrystalline oxide coatings will be deposited on commercial cladding materials. Some of the considered oxide coatings (i.e., alumina) have already demonstrated high‐temperature stability, radiation tolerance and capacity for damage self‐repair, but have not been validated in an industrially‐relevant environment (water, neutrons) or under loss of coolant (LOCA) conditions (high‐temperature steam). Other candidate oxides have already shown excellent corrosion resistance in both water and high‐temperature steam, but have not yet been systematically studied with respect to their irradiation tolerance. Whenever possible, the resistance of the oxide coatings to hydrothermal ageing will be improved by the addition of judiciously selected dopants.