Research on Steel Alloys Looks at Reducing Impurities
Ryan Hunt from Lawrence Livermore National Laboratory contributed this post:
Reduced Activation Steel Development
There is significant activity underway around the world to develop the process of fusion to the point where it produces net energy – with applications in power production, national security and basic science. The ITER project in France and the National Ignition Facility in the USA are two of the largest such projects, each representing investments of many billions of dollars.
In a future fusion energy plant, structural materials will be irradiated with high-energy neutrons during the operation of the device. If commonly-available steels were to be used, then certain alloying elements and impurities would become highly radioactive, while others would rapidly decay to non-hazardous levels. To enable safe disposal of these steels upon the end of their service life, work is underway at Lawrence Livermore National Laboratory (LLNL) and elsewhere to understand the requirements and possible solutions to this problem. One such avenue is to develop types of steel that have extremely low levels of any of the high-activation alloying elements or impurities, while maintaining the required high strength characteristics at the high temperatures required for a fusion environment (typically in the range 400-600 deg C).
Sophisticated Alloys were contracted by LLNL to develop a steel with extremely low impurities levels across the periodic table, with specific limits set by LLNL’s analysis of the anticipated fusion output. Sophisticated Alloys developed a suite of candidate steels with levels of the non-desirable elements in the parts per million range. This required careful control not only of the ultra-low impurity alloying powder, but also a clean production line and stage-by-stage diagnostics. As an example of this process development, initial attempts yielded higher than expected Niobium content (a non-desirable element), requiring analysis samples from each step of the production line to track down the source of the impurity.
The ultimate objective is to ensure that eventual disposal of the steels after a 60 year service life can be categorized as Class-C waste, permitting shallow land burial. In fact, work to date has turned out so well that we are considering an even higher cleanliness level of Class-A waste (similar to medical equipment waste), making the nuclear fusion byproducts even more benign than originally envisioned.
This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.
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