Clad tidings

Despite all the technology used to enrich uranium and package the fuel efficiently, nuclear reactors are not particularly fuel-efficient.

A large proportion of the fuel in each pellet goes unused. This has nothing to do with the fuel itself, the engineering of the reactor or its ancillary systems. it's the fault of the cladding material surrounding the pellets.

A team from three UK universities is collaborating with several of the largest nuclear industry players in a five-year project to develop better cladding materials. The project could result in reactors which have to shut down less frequently for refuelling, making them more efficient and reducing the amount of waste they produce, explained project leader Michael Preuss of

. And just as importantly, he said, it will provide valuable training for a new generation of nuclear engineers.

The current generation of nuclear reactors, using light water cooling technology, consume fuel packaged in a cladding made from alloys of zirconium, explained Preuss. These are transparent to neutrons, so they don't interfere in the nuclear reactions which take place in the reactor.

They're also relatively corrosion resistant, which is important in the hostile environment of a fusion reactor; the cladding is exposed to water and steam at 300-350°C. 'The zirconium also has sufficiently good mechanical properties for these high temperatures,' he added.

However, corrosion is still the downfall of the cladding. Like many other metals, zirconium develops an oxide coating which acts as a protective covering, preventing oxygen from reaching the metal surface and slowing down the rate of corrosion. But oxidation still occurs, and the layer slowly thickens. 'You can't have an oxide layer thicker than about 100µm, because the oxide acts as a thermal barrier, preventing the heat generated by the fuel from escaping. Also, you lose the integrity of the cladding, so it can't sustain the load of the fuel.'

Because of this, the time the fuel assemblies can stay in the reactor — known as the burn-up time — is limited to about 18 months. When the fuel assemblies are removed, the reactor has to be shut down. 'If you had cladding that allowed a longer burn-up, you could enrich the fuel more so you can run it longer,' said Preuss. 'You could conceivably have fuel assemblies that stay in the reactor for many years.'

Preuss's team is the lead partner in the £1.5m project, which also involves

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. The project aims to develop methods for modelling how the zirconium alloys used for cladding corrode within the nuclear reactor, as a first step in developing more robust alloys.

'What we'd like to happen is to have an alloy that develops a perfect, dense oxide layer to passivate the corrosion process, and which wouldn't develop cracks through which oxygen can percolate,' said Preuss. At the moment, there is no understanding of the corrosion mechanisms.

The researchers will start by using samples of the commercial alloys, which include small amounts of niobium, iron and tin, and subjecting them to high-pressure and temperature steam in autoclaves.

The Manchester team will then use high-energy synchrotron X-ray diffraction and electron microscopy to study the oxide layer, the metal and the interface between the two at varying stages of oxidation, looking particularly at the stress states induced where the phases of the alloy change as the metals oxidise. The Oxford team, meanwhile, will use a 3D atom probe to gain insights into the chemical processes involved in the oxidation.

The collaboration will then develop its own alloys, said Preuss. 'These will be simplified versions of commercial alloys, so we can look at issues we think are related to one of the alloying elements.' The experiments won't include extensive use of irradiated samples, he added; although irradiation will have some effect on the corrosion, it won't affect the main mechanisms, and extensive use of irradiation would increase the cost of the project ten-fold. 'You would have to have the irradiation facility in the same place as the characterisation equipment to do that effectively,' he said, 'and they aren't.'

At the end of the five years Preuss said the group hopes to generate the right mechanistic understanding to develop accurate mechanistic modelling. That will allow metallurgists to develop alloys with the right sort of corrosion performance for long burn-up.

'But one of the most important aspects is that we're going to train nuclear materials researchers who'll have expertise in zirconium alloys for light water reactors,' he said. 'That expertise doesn't exist in the UK at the moment; we only have one pressurised water reactor. All the rest are Magnox or AGR, and they don't use zirconium.'

If a new generation of reactors is to be built, they will use a more advanced version of light water technology, so zirconium expertise will be vital.