Fuel & fuel cycle | Corrosion
Alloy M5 cladding performance update9 May 2011
Alloy M5 is the reference alloy for fuel rod cladding and structural components for all Areva PWR designs. Recently, concerns about the axial growth of fuel assemblies with M5 guide tubes have been resolved. A mature alloy, M5 continues to offer substantial performance improvements over other zirconium alloys. By G. L. Garner and J. P. Mardon
Alloy M5 is a zirconium alloy containing 1% niobium with oxygen, iron and sulphur content controlled. This composition, combined with a fully recrystallized microstructure that remains stable under irradiation, permits M5 to achieve optimum performance in PWR operating environments whose severity has limited the continued use of other zirconium alloys.
M5’s stable microstructure is the result of a low temperature process adhered to throughout the fuel assembly component manufacturing processes. These low-temperature processes produce beta-niobium second phase particles with optimum size and distribution in the zirconium matrix. The absence of tin as an alloying element results in very low corrosion and hydrogen pick-up rates at high burn-ups and reactor duty conditions.
At the end of April 2010, more than 3.2 million M5 clad fuel rods in 13,000 fuel assemblies in arrays from 14x14 to 18x18 have been irradiated in 80 commercial PWRs in 13 countries to a maximum fuel rod burn-up of 80,000 MWd/tU. M5 cladding batches were first launched in 1997; cladding and guide tubes were first used in a US reload in 2001. Cladding, guide tubes and grid batches were launched the same year.
Alloy M5 in the fully recrystallized condition induces stability in the material during manufacturing and results in the ability to accurately predict its in-core behaviour, principally its creep resistance, pellet-cladding interaction (PCI) behaviour and irradiation growth.
With its sulfur content controlled at a value between 10 and 35 ppm, M5 exhibits high creep resistance and low growth. For burn-ups as high as 80,000 MWd/tU, no acceleration in growth is observed with M5 fuel rods (Figure 1). This phenomenon is explained by the fully recrystallized state of M5 cladding, and the high stability of its microstructure under irradiation.
In recent times much attention has been given to the growth of Areva fuel assemblies with M5 guide tubes, principally those in the US. Unlike fuel rod growth, whose predictable growth with increasing burnup is largely insensitive to fuel assembly design, fuel assemblies with M5 guide tubes displayed a variation according to specific design features. This trend is consistent with historic performance of other alloys such as Zr-4.
An extensive root cause assessment programme was implemented to evaluate the growth performance of fuel assemblies and judge the impact of growth on current product and design programmes. This activity, conducted in the combined Areva worldwide research centers, resulted in some fundamental conclusions and multiple preventative measures to ensure well-defined fuel assembly growth behaviour for current and future products.
Paramount among the conclusions of the programme was that alloy M5 was not implicated in the variation in assembly growth observed beginning in 2007. In addition, a correlation with MOX fuel and growth variation was not established either. These conclusions were the result of a rigorous programme of hot cell and poolside examinations of high and low burnup fuel assemblies containing M5 guide tubes and fuel rods. Subsequently, all Areva fuel assembly growth laws were revised to reflect a greatly increased worldwide assembly growth database. Figure 2 shows the results of these actions in terms of the number of unexpectedly high fuel assembly growth measurements reported in post-irradiation examinations (PIE) through the end of 2009. Additionally, fuel assembly growth measurements made in the first half of 2010 are within the design limits.
The absence of tin in M5 (<100 ppm) contributes greatly to its low corrosion rate. Cladding oxide thicknesses of less than 40µm under high duty/high burnup conditions eliminates the risk of breakaway corrosion. The stability of the microstructure discussed earlier also plays an important role in the oxidation performance of alloy M5. Unlike the beneficial second-phase particles in other zirconium alloys, the beta-niobium particles of alloy M5 do not amorphize and agglomerate with increasing fluence. They remain crystalline and do not migrate and leave areas of the microstructure unprotected.
Because of its high resistance to corrosion, M5 is an excellent cladding alloy for demanding reactor conditions, high burnup fuel management strategies and modified primary water chemistry conditions requested by utilities. This has been widely demonstrated by experience feedback and international programmes in which Areva participates.
The irradiation experience of M5 covers a wide range of PWR operating conditions from nominal to severe. In-pile testing and more importantly, experience feedback from PWR exposure, demonstrates that M5 possesses the performance properties required for reactor upgrade conditions including new in-core fuel management approaches, demand for flexibility and high-duty reactor conditions.
Alloy M5 has demonstrated significant improvements over Zircaloy-4 and other zirconium alloys for fuel rod cladding. M5 fuel rods have been operated in PWRs to fuel pin burnups of 71,000 MWd/tU in the United States and 80,000 MWd/tU in Europe.
The corrosion resistance of alloy M5 cladding is shown in Figures 3 and 4. Of specific interest is its performance in high duty conditions (including high local heat generation regions and heat flux in 14x14 to 16x16 reactors). Also significant is its performance when exposed to increasing varying reactor coolant system Li levels where the maximum oxide thickness has not exceeded 40µm in all conditions.
Zinc injection in PWR primary coolant is gaining in popularity among nuclear utilities to reduce personnel dose rates. Areva’s range of experience is increasing with zinc injection in the range of 5 to 10 ppb in several reactors in Germany, the United States and Brazil. Figure 5 illustrates the corrosion of M5 in zinc injected plants against the backdrop of M5’s world wide corrosion database. No impact of zinc injection on the oxidation of M5 cladding is observed.
As with fuel rod cladding, the benefits of M5’s low oxidation kinetic also apply to guide tubes. Guide tubes operate at lower temperature than fuel rods but the benefits of low corrosion are important because unlike fuel rod cladding they experience double-sided corrosion. Numerous recent hot cell examinations of M5 guide tubes confirm poolside eddy-current measurements as well as the uniformity of the oxide layer, as shown in Figure 6. Typical guide tube oxide thickness for M5 guide tubes are 6µm to 10µm on each surface.
The hydrogen uptake in alloy M5 is also very low. The availability of hydrogen is necessarily low in a low oxidizing material and the lower pickup fraction of M5 further limits the amount of hydrogen ingress into the alloy. The hydrogen content of M5 and Zircaloy-4 with increasing burnup is plotted in Figure 7. At operating temperatures, the hydrogen content in M5 fuel rod cladding is below the solubility limit for hydrogen in the alloy matrix. This means that, unlike Zr-4, there is no brittle hydride in the material in operation. It has been well-demonstrated that a high in-service hydrogen pickup has a strong negative impact on the behaviour of zirconium alloys in loss of coolant and reactivity-initiated accidents. Simply put, the less cladding hydrogen content going into a loss-of-coolant accident (LOCA) or reactivity-initiated accident (RIA) event, the better.
The development of a no-tin completely recrystallized material for fuel rod cladding and structural components was a breakthrough in the 1990s. Today, the benefits of Alloy M5 have been verified by extensive PWR experience. The challenge for fuel suppliers to keep pace with ever changing reactor operating environments continues, however. Areva is engaged in a wide range of R&D activities and enhanced performance modeling directed at increased understanding of the functioning of fuel assembly components and materials under evolving operating conditions and environments.
M5 is a trademark of Areva NP registered in the USA and in other countries.The figures an be found in the attached PDF.FilesFigures 1-7