Accident Tolerant Fuel: safer fuel for the future?
The safety of nuclear power plants depends, among other things, on the behaviour of the fuel and its cladding – particularly under accident conditions. Since the Fukushima reactor accident in 2011, there has been increasing international research into so-called Accident Tolerant Fuels (ATFs). The aim is to further develop the fuel–cladding system in such a way that it remains intact for a longer period under accident conditions.
Wide range of sometimes very different concepts
ATF does not refer to a single product, but rather to a collective term for a wide variety of concepts that differ in terms of the materials used, the design and the level of development. Some of these concepts are already at an advanced stage, while others are still in an early phase of development.
„ATF is not a silver bullet, but it is one building block in enhancing the robustness of the reactor core under accident conditions.“
Dr. Isabel Steudel,Project manager
ATF concepts include, among others, the following:
- Chromium-coated zirconium cladding: The aim is to improve oxidation resistance at high temperatures; however, the formation of a eutectic can lower the melting temperature.
- FeCrAl cladding: These iron–chromium–aluminium alloys exhibit high resistance to corrosion by steam; however, they can lead to changes in the neutron spectrum, for example, which may affect the effectiveness of control rods.
- SiC/SiC composite cladding: This ceramic composite material shows very high temperature resistance, but also low thermal conductivity, which further decreases under irradiation.
- Doped UO₂ fuel: Targeted doping (e.g. with Cr₂O₃) is intended to improve fission gas retention and reduce mechanical stresses; however, these fuels also exhibit higher diffusion rates.
Uranium nitride and uranium silicide: These alternative fuel compounds offer higher thermal conductivity and uranium density, but are more chemically reactive.
Safety-related investigations of ATF concepts
On behalf of the Federal Office for the Safety of Nuclear Waste Management (BASE), scientists at GRS carried out a research project focusing on the safety assessment of ATF concepts. To this end, they first analysed the international state of development of various ATF concepts – both on the part of fuel manufacturers and within publicly funded research programmes.
Subsequently, they assessed selected concepts with regard to their safety-relevant characteristics under normal operation as well as under accident and severe accident conditions.
In addition, they carried out their own simulation calculations for FeCrAl cladding to investigate the impact that the hypothetical use of ATF would have had on the progression of real reactor accidents – specifically Three Mile Island (1979) and Fukushima Daiichi (2011).
More time for countermeasures
The simulations carried out by GRS show that, under the assumptions made, FeCrAl cladding can lead to significantly reduced oxidation, lower hydrogen generation and improved maintenance of the core geometry. These properties can be particularly beneficial in accident scenarios by providing additional time for countermeasures.
„The simulations show that, under certain conditions, ATFs can extend the so-called grace period, that is, the time during which intervention is still possible before core damage occurs and thus a severe accident may develop.“
Dr. Isabel Steudel,Project manager
However, it should be noted that the actual effectiveness strongly depends on the specific scenario, the material properties and the reactor configuration. In addition, many ATF concepts have not yet been sufficiently tested to allow for a robust safety assessment. Furthermore, simulation codes still need to be further developed to adequately account for ATF-specific characteristics.
Technological maturity: between research and application
A key outcome of the project is the classification of the various ATF concepts according to the so-called technology readiness level (TRL). While doped UO₂ fuel and chromium-coated cladding are already being tested in some commercial reactors, SiC/SiC cladding and uranium nitride fuels are still in the experimental stage.
„Some concepts are technically promising, but still far from industrial deployment.“
Dr. Isabel Steudel,Project manager
Opportunities, challenges and open questions
The development of ATF is a dynamic field with international participation. Manufacturers such as Framatome, Westinghouse, GE Hitachi and TVEL are driving development forward, supported by government programmes in the United States, Japan, Russia and the EU.
Research on ATF is also being conducted in Germany, for example at the Karlsruhe Institute of Technology (KIT) and at GRS.
This raises a number of additional questions, for example:
- What is the impact of ATF concepts on normal operation and neutron economy?
- What long-term behaviour do new materials exhibit under irradiation?
- How can ATF concepts be integrated into existing safety analyses and codes
- What regulatory requirements need to be met?
It should be noted that ATF is not intended to replace existing safety concepts, but can contribute to enhancing nuclear safety. However, further detailed safety-related investigations are required, including experimental research on the individual concepts, taking into account their respective stages of development.