Testing in extreme environments - challenges facing testers in the nuclear sector

Materials World magazine
30 Apr 2013

Innovations in testing not only make it easier to spot problems before they occur, but can increase safety and efficiency. Michael Bennett went along to Zwick Roell’s 4th Academia Day in Manchester, UK, to learn about challenges facing testing for the nuclear sector.

‘My mother was always surprised that you can make a living by breaking things,’ said Dr Jan Stefan Roell, CEO of Zwick Roell Group, a destructive and non-destructive testing manufacturer and host of the conference. The delegates chuckled at the anecdote, kicking off an event at which testing specialists from a number of European universities gathered to share ideas.

The nuclear industry featured heavily in the day’s talks, unsurprisingly perhaps, given the demanding environments faced by materials used in fusion facilities. Testing for damage by irradiation is of critical importance, and Dr Hans-Christian Schneider from Karlsruhe Institute of Technology (KIT), in Germany, described how instrumented high-temperature indentation can be used to determine irradiation damage in steel.

Characterising materials by indentation has a number of advantages, said Schneider, in that small volumes are sufficient, it is non-destructive, can be applied several times to a single specimen and layers and graded materials can be examined. However, he added that one major shortcoming is that no adequate high-temperature devices are commercially available. In terms of indenter materials, Schneider and his team have been looking into diamond and sapphire, as well as exploring the use of cubic boron nitride. However, diamond has a risk of carbonising testing materials at high temperatures, sapphire is too soft for tungsten and cubic boron nitride can oxidise at high temperatures. Schneider said that a new set of materials for indenters was required.

An indentation device for the materials used in fusion must be capable of performing its function at the operating temperature of the reactor and have a maximum test temperature of 650oC, which is the upper limit for steel during fusion. The device would also need a maximum indentation depth of 30µm and an indentation force of 200N in order to test both steel and tungsten. Remote operation in a hot cell is essential, as is the ability to perform multiple indents in one vacuum cycle.

Schneider’s team has developed an indentation depth sensing unit that provides a contactless, camera-based method of monitoring the relative displacement between the indenter and a sample, which helps minimise errors by thermal expansion and the influence of machine stiffness. He added that current research is looking into the geometrical stability of indenters in terms of oxidation and fracture as well as attempting to identify thermal drift effects caused by thermal expansion of the identer after contact. Schneider noted that high temperature indenting is feasible, but requires customised solutions, for example sophisticated penetration depth measurement, and will be one of the main focuses of Schneider’s team at the KIT Fusion Materials Laboratory for the foreseeable future.

Testing conditions
Professor Grace Burke, Director at the Materials Performance Centre at the University of Manchester’s Dalton Nuclear Institute, UK, described her research into resolving a number of issues in reactor pressure vessels, including steel irradiation embrittlement, the problem of irradiation-induced growth and creep in zirconiumbased materials. She also explained how electron backscatter diffraction techniques can be used to monitor corrosion fatigue and differences in corrosion fatigue crack growth rate.

The internal components of a reactor vessel are often made from stainless steel, which can buckle after sustaining irradiation damage. In addition, the closure head and steam generator of the reactor vessels are often made from nickel-based alloys, such as Alloy 600, which can suffer from stress corrosion cracking. Extensive testing has informed the development of an improved alloy for this purpose.

To help speed the development of the high temperature-resistant core materials that will be needed for Generation IV reactors, Burke said that researchers should share information more readily, which could save time and effort for testers. ‘When you are processing lead zirconate titanate and magnesium alloys, you find similar issues due to them having essentially the same crystal structure,’ she said. ‘Understanding the relationship between processing and performance requires a high degree of precise testing, and with that in mind it’s important to identify common ground with other materials engineering research communities.’

John Yates, Professor of Computational Mechanics at the University of Manchester, UK, agreed, saying that transfer of knowledge from one discipline to another was crucial, and that testers should keep communication channels open to avoid reinventing the wheel. ‘For example,’ he said, ‘we are using ideas that come from fluid mechanics to solve problems in solid mechanics.’