Accident tolerant fuels (ATF) are a series of new nuclear fuel concepts researched to improve fuel performance during normal operation, transient conditions, and accident scenarios, such as loss-of-coolant accidents (LOCA) or reactivity-initiated accidents (RIA). Following the Fukushima Daiichi accident, a fuel behavior review was 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 corrosion-resistant material with a 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 constitutes one of the barriers to the ‘defence-in-depth‘ approach; therefore, its coolability is one of the 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.
Refractory Metals for Fuel Cladding
Refractory metals and alloys are well known for their extraordinary resistance to heat and wear. The key requirement to withstand high temperatures is a high melting point and stable mechanical properties (e.g., high hardness) even at high temperatures. The most common refractory metals include five elements: niobium and molybdenum of the fifth period and tantalum, tungsten, and rhenium of the sixth period. They share some properties, including a melting point above 2000 °C and high hardness at room temperature. Poor low-temperature fabricability and extreme oxidability at high temperatures are the main disadvantages of most refractory metals. The application of these metals requires a protective atmosphere or coating.
Zr-alloy lined Molybdenum Cladding.
In 2012, EPRI initiated an independent research project with conceptual designs of coated molybdenum alloy as an ATF cladding to achieve accident resistance to a temperature range of 1,200–1,500°C. Molybdenum (Mo) is a candidate because of its high melting point (2623°C) and high strength at elevated temperatures. At the same time, Mo and its alloys are known to be susceptible to forming volatile MoO3 in oxidizing environments at temperatures > 600°C. Therefore, this research program uses a composite design in which the Mo alloy cladding is covered with an outer protective coating of either a Zr-alloy or an Al-containing alloy.
The Zr-alloy lined Mo-cladding is anticipated to possess sufficient corrosion and hydriding resistance for the current fuel burnup limit and beyond. The fully metallic Mo–Zr and Mo–FeCrAl duplex claddings are anticipated to achieve accident tolerance by forming a protective oxide during an accident. The thin Zr-alloy coating will completely oxidize to ZrO2 as the temperature reaches 1,000°C or higher. The ZrO2 will maintain its integrity and stability with proper alloying and protect the underlining Mo alloy. A thin FeCrAl coating is highly corrosion resistant in LWR coolants due to the formation of a chromium-rich protective oxide, mainly Cr2O3. FeCrAl is highly corrosion resistant in high-temperature steam due to the formation of a thin aluminum-rich oxide, Al2O3. FeCrAl alloys consist mainly of iron, chromium (20–30%), and aluminium (4–7.5 %). These alloys are known under the trademark Kanthal, a family of iron-chromium-aluminium (FeCrAl) alloys used in a wide range of resistance and high-temperature applications.
Molybdenum is highly resistant to oxidation in high-purity or reducing steam. Therefore, the lined molybdenum cladding is anticipated to maintain good integrity in the event of steam ingress into a failed fuel rod and under a design-basis LOCA. If the outer coating is locally removed, such as grid-to-rod fretting, localized corrosion of molybdenum cladding may occur.