Materials testing: radioactive materials for nuclear fusion

Research into fusion as an energy source has as its aim the development of hydrogen isotopes as a new, clean fuel for our future energy supply. The physics behind this energy source are complicated. However, the real challenge presently lies in developing materials which can withstand the extreme loads to which they are subjected within the reactors, with plasmas at a temperature of 100 million degrees and extremely strong particle and neutron streams.

Even ignoring the particular requirements of nuclear fusion, the basic demands placed on materials are formidable: up to 10 MW/m², operating temperatures up to 1000°C and several tens of thousands of thermal cycles, with gradients in some cases of over 100 K/min. With fusion come hydrogen and helium embrittlement, radiation damage up to 100 DPA (every atom in the material is struck and transformed one hundred times) and transient events with frequencies > 10 Hz. The effect of these loadings on material properties is being tested on a wide range of materials at the Institute of Energy and Climate Research at Forschungszentrum Jülich GmbH, Germany. ZwickRoell's instrumented indentation test is a particularly powerful tool for this work. Because of the heavy expenditure involved in experiments in radiation damage to materials, only small specimens in the millimeter size range are available, in very limited quantities. In the Institute, new or optimized materials produced in-house and based on tungsten and iron are in many cases also only available without radiation damage in small amounts. Classical mechanical tests are therefore often out of the question. The instrumented indentation test nevertheless allows a wide range of material parameters to be determined, while yielding good statistics, and has thus become a de facto standard in this field of research. A new zwickiLine 2.5 ZHU is therefore being installed in a hot cell in the Institute of Energy and Climate Research's high-temperature materials laboratory. The intention is for materials from international research reactors, fusion reactors and the local cyclotron to be gathered here and analysed, in the process bringing together mechanical, thermal and physical analysis under one roof. Short channels and a highly integrated laboratory are among the great strengths of the Jülich site, particularly for highly radioactive specimens. The specially adapted zwickiLine can be integrated into the compact hot cell, allowing the entire experiment to be controlled from outside. The instrument's sensitive electronics will remain outside the cell to shield them from the extreme specimen radiation of up to 100 GBq. New Inspector X software options allow additional material parameters to be determined from the indentation test, enabling even more comprehensive qualification of materials before their use in fusion reactors.