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.
Refractory Metals for Fuel Cladding
Refractory metals and alloys are well known for their extraordinary resistance to heat and wear. 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 all 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 main disadvantages of most refractory metals. 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 very high melting point (2623°C) and its high strength at elevated temperatures. At the same time, Mo and its alloys are known to be susceptible to the formation of volatile MoO3 in oxidizing environments at temperatures > 600°C. Therefore, this research programme 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 be completely oxidized to ZrO2 as the temperature reaches 1,000°C or higher. With proper alloying, the ZrO2 will maintain its integrity and stability and provide protection to the underlining Mo-alloy. A thin FeCrAl coating is highly corrosion resistant in LWR coolants due to formation of a chromium-rich protective oxide, mainly Cr2O3. In high temperature steam, FeCrAl is highly corrosion resistant due to the formation of a thin aluminum rich oxide, Al2O3. 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.
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, as well as under a design-basis LOCA. In the event the outer coating is locally removed, such as due to grid-to-rod fretting, localised corrosion of molybdenum cladding may occur.