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Nuclear fuel

Tailor-made nuclear fuel

Though the small-sized pellets of uranium oxide have hardly changed shape since the 70s, their composition and behaviour are still the subject of in-depth research  at the CEA. Nuclear fuel needs to be optimised according to the different applications: power generation, propulsion, test reactors, or fourth generation reactors. Yet the overriding aim is to constantly improve its safety and performance requirements.

Published on 25 July 2016

Optimising nuclear fuel behaviour regardless of the challenges

For decades now, the French nuclear fleet has been using the same fuel material: uranium oxide (UOx) in the shape of small cylindrical pellets, stacked in metal cladding called rods and grouped in devices called assemblies. All of these objects have more or less retained their original geometry from the start of pressurised water reactors (PWR) in the 1970s. The fuel used in nuclear power plants is therefore a well-defined and highly proven industrial product, as is that used in nuclear propulsion submarines and aircraft carriers. Yet at the CEA Cadarache centre, researchers working for the Nuclear Energy Division are still relentlessly studying the subject, whether this be the fuel itself at the Fuel Studies Department (DEC), or the behaviour of fuel assemblies, which is one of the specialities of the Nuclear Technology Department (DTN)1.

Indeed, even for well-established technologies, the fuel is still modified to improve its performance. "Our industrial partners are always turning to us for help since the exact composition of the material depends on the economic context. We can adjust the quantity of plutonium or change the uranium enrichment level according to the fluctuating costs of raw materials, or even add additives to further improve its performance. We have to study the behaviour of the new fuel before implementing the change, to make sure the safety requirements are met, which is one of our primary tasks. Compared with the 1980s, this approach has allowed us to double the residence time of PWR fuel, and especially to increase the energy produced by each fuel assembly in the reactor by a factor of two", said Philippe Prené, head of the DEC department at the CEA.

1 The DMN (nuclear materials) and DPC (physical chemistry) departments are also involved in this research.
2 From 2000 onwards.

Leca-Star facility
Leca-Star facility at Cadarache where fuels that are being manufactured are analysed © G.Lesénéchal/CEA

Fuel sample
Fuel sample prepared for metallographic examination in a shielded cell at the Leca-Star facility © CEA

Experimental furnace
Experimental furnace used in a shielded cell to heat the fuel to temperatures of about 2,000°C to study its behaviour under extreme conditions © CEA

Another example: instead of operating reactors continuously at their rated power, EDF decided to implement load following in some of its reactors, i.e. adjusting their power to meet demand thanks to the reactor's control rods. These rods are usually brought down during the night and raised again to meet the morning peak demand, though even faster manoeuvres can be used to quickly balance the grid. "This practice saves fuel but it puts extra stress on the material which, instead of working under stable conditions, is subjected to varying temperature and irradiation cycles. We must therefore make sure that the fuel retains all its characteristics" stressed Bruno Collard, head of laboratory at CEA.

The CEA's fuel research activities can also be oriented with respect to strategic choices. For instance, when France decided to recycle the plutonium in spent fuel using about twenty EDF reactors fuelled2 with a mixture of uranium and plutonium oxide (MOX), this new fuel material had to be tested, characterised and validated by the Nuclear Safety Authority (ASN). The CEA is also responsible for designing and developing fuels for future reactors like the Jules Horowitz reactor (JHR), currently being built at Cadarache site, or the fourth generation of fast reactors including the Astrid integrated technology demonstrator. And if we decide to deploy the transmutation of minor actinides in Astrid, we will need to know how to incorporate it into the fuel for future fast reactors.

First fuel pellets for the Astrid integrated technology demonstrator
First fuel pellets for the Astrid integrated technology demonstrator © CEA

Controlling the resistance of nuclear fuels in extreme and complex environments

In all cases, the issue is all the more complex since nuclear fuel has to operate in difficult conditions.
"Its standard properties - shape, microscopic structure and basic composition - vary dramatically under normal operating conditions" pointed out Philippe Prené. Designed to release energy, the fuel does indeed reach extremely high temperatures: in the case of PWRs, the centre of the pellet reaches 1,100°C whereas the water around the cladding is at about 300°C. Under the effect of stress due to greater thermal expansion in the centre of the pellet compared with its periphery, the pellets end up fragmenting and taking on an hour-glass shape. Furthermore, irradiation not only changes the composition of the fuel, but also its microstructure: the fission products generated by the fission reactions in turn provoke swelling in the pellet and a pressure increase inside the cladding. The cladding material - just like that of the fuel assemblies - must also resist this very aggressive irradiating environment and mechanical stresses generated by the fuel itself, the pressure and the movement of water.

To understand all these phenomena, CEA teams have established a comprehensive approach to the entire value chain when requested by the industry or to meet the needs of long-term research products. The designers are the first to step up to the block; they have to define the fuel material, the fuel assembly geometry and the operating rules by using simulation codes. Next are the manufacturers who must make what the designers have developed, followed by those who must test what has been made. Lastly come those who characterise it on all scales. All this data is amassed in the knowledge database needed to better understand and simulate the behaviour of fuels.

Resisting accidents

Beyond those irradiation levels considered to be normal, extreme experiments are also performed on small quantities of radioactive materials. Issue: to determine the limits of the material so as to apply suitable safety margins. Before it can be validated by the French Nuclear Safety Authority (ASN), a fuel must have proven its capacity to operate under normal conditions and have shown that it will not release radionuclides into the environment in the case of an accident.

Among the main risks tested in specifically designed facilities, Cabri and Phébus reactors respectively: runaway fission reactions (reactivity insertion accident, RIA) which can be caused by the inadvertent ejection of the control rods, or core meltdown following the loss of primary coolant (case of the Three Mile Island accident in 1979 in the US).