NSUF Article

First innovative NuScale experiment cycle completed, en route to post-irradiation examination

Thursday, February 15, 2024 - News Release, Nuclear Research

By Alexis Starks, NSUF at Idaho National Laboratory  

In a major step towards commercial, small modular reactor deployment, progress is being made in testing new materials for NuScale's small modular reactor lower containment vessel.

In 2019, NuScale Power LLC received access to Idaho National Laboratory’s Advanced Test Reactor and Westinghouse Churchill Laboratory Services through the Nuclear Science User Facilities (NSUF) as part of the Consolidated Innovative Nuclear Research program. The arrangement allowed NuScale to obtain irradiation data on F6NM, a martensitic stainless steel, with the intent of using F6NM for its lower containment vessel as part of the NuScale power module design for small modular reactors.

(Left to right) Idaho National Laboratory Operations Supervisor Geremy Schuldt, and Nuclear Operators Colt Farmer and Dalton Dummer, configure the experiment in preparation for installation at ATR. 

The NuScale lower containment vessel, which is capable of withstanding pressures during reactor operations, is a safety feature that prevents fission products from being transported to the outside atmosphere. In its current design, NuScale uses FXM-19, an austenitic stainless steel that is resistant to irradiation embrittlement and corrosion and has higher American Society of Mechanical Engineers design specifications than other common austenitic stainless steels.

The downfall? FXM-19 is expensive to cast and fabricate in large sizes needed for the containment vessel.

F6NM, to compare, is also corrosion resistant, more cost effective and more readily available than FXM-19 – and it has higher design specifications, so long as it performs as expected after irradiation.

September 2023 marked the completion of the first of four cycles, which began in July 2023 at the Advanced Test Reactor. After irradiating the high-fluence target specimens, NSUF shipped them to Westinghouse, an industry partner facility, for post-irradiation examination to include Charpy and tensile testing.

This experiment is uniquely complex, requiring novel approaches, such as direct specimen contact with water, the reactor’s primary coolant, to achieve the experiment’s low temperature requirement, and the use of three high-power, short-duration Power Axial Locator Mechanism cycles to meet the experiment’s low-fluence requirements.

According to
NuScale’s 2019 CINR research proposal, the qualification of F6NM would “reduce manufacturing cost and schedule for NuScale module production.” The data and material specimens generated by this experiment will be publicly available through the NSUF Nuclear Fuels and Materials Library
and would “[benefit] the industry by promoting the adoption of higher strength steels with improved thermal efficiency for other nuclear vessel applications.”