Accident tolerant fuels (ATF) are a series of new nuclear fuel concepts, researched in order to improve fuel performance during normal operation, transient conditions, and accident scenarios, such as loss-of-coolant accident (LOCA) or reactivity-initiated accidents (RIA). Following the Fukushima Daiichi accident, a review of fuel behaviour has been initiated. Zirconium alloy clad fuel operates successfully to high burnup and is the result of 40 years of continuous development and improvement. However, under severe accident conditions, the high temperature zirconium–steam interaction can be a major source of damage to the power plant.
These upgrades include:
- specially designed additives to standard fuel pellets intended to improve various properties and performance
- robust coatings applied to the outside of standard claddings intended to reduce corrosion, increase wear resistance, and reduce the production of hydrogen under high-temperature (accident) conditions
- development of completely new fuel designs with ceramic cladding and different fuel materials
Current fuel cladding is the outer layer of the fuel rods, standing between the reactor coolant and the nuclear fuel (i.e. fuel pellets). It is made of a corrosion-resistant material with low absorption cross section for thermal neutrons (~ 0.18 × 10–24 cm2), usually zirconium alloy. It prevents radioactive fission products from escaping the fuel matrix into the reactor coolant and contaminating it. Cladding constitute one of barriers in ‘defence-in-depth‘ approach, therefore its coolability is one of key safety aspects.
Special Reference: Nuclear Energy Agency, State-of-the-Art Report on Light Water Reactor Accident-Tolerant Fuel. NEA No.7317, OECD, 2018.
FeCrAl alloys consist of mainly iron, chromium (20–30%) and aluminium (4–7.5 %). These alloys are known under the trademark Kanthal, which is a family of iron-chromium-aluminium (FeCrAl) alloys used in a wide range of resistance and high-temperature applications. FeCrAl is highly corrosion resistant due to the formation of a thin aluminum rich oxide, Al2O3.
FeCrAl-based ATF utilises an FeCrAl alloy material as fuel rod cladding in combination with uranium dioxide (UO2) fuel pellets currently in use. FeCrAl alloy clad fuel rods (with UO2 fuel) appear to exhibit properties that meet or exceed current fuel design technical requirements (with the exceptions noted below) while providing increased safety benefit during design-basis events and severe accident conditions. The concept’s key advantage over Zircaloy is its substantially slower oxidation kinetics up to 1773 K (1500°C). FeCrAl alloys have mechanical strength similar or superior to that of zircaloy, with plastic yielding (ballooning) and perforation characteristics similar or better than zirconium alloys.
There are two main disadvantages of FeCrAl-based fuel clads:
- Increased parasitic neutron absorption. Due to increased neutron absorption cross-section of iron.
- Tritium releases. There is a potential increase in tritium release into the reactor coolant. Tritium is produced as a fission product (FP). FeCrAl does not react with hydrogen to form stable hydrides similarly to a zirconium-based alloy, resulting in higher permeability of tritium through the cladding to the reactor coolant.