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In 2017 the Transient Reactor Test (TREAT) Facility was restarted after having been placed in a standby state in 1994. The TREAT reactor's restart has since enabled the progressive development of new nuclear technologies within the United States that previously were outsourced to other countries. While the reactor's restart was a large feat worthy of recognition, the experimental use of its characteristics has required further development of an in-pile experimental infrastructure sufficient to support programmatic needs. This hardware has taken the form of capsule designs (compact and elongated) as well as loop concepts representing the phenomena of interest for a subset of the separate effects tests desired for each respective testing campaign. The transient testing program has been a large integrated effort that aligns with the U.S. Department of Energy's current needs. This study complements those programmatic elements by developing, fabricating, and demonstrating a full-scale flowing water loop in an out-of-pile environment. The goal of this effort is to develop a pragmatic understanding of the engineering capabilities and limitations associated with geometric form factors, metering technology, and controls logic under the representative thermal-hydraulic conditions that would be experienced within the TREAT reactor during an in-pile reactivity-initiated accident test. The outcomes of this study result in an evaluation of the conceptual design of a comprehensive flowing water loop, including objective figures of merit for comparing unique instrumentation and the basis for their selection during operations. These efforts directly contribute to and are required for the further advancement of transient testing capabilities within the United States.

The effort to perform transient (TR) testing of nuclear fuel and materials in the United States took an important step in 2017 with the resumption of operations at the Transient Reactor Test (TREAT) Facility at the Idaho National Laboratory. As part of this restart effort, a U.S. Department of Energy-funded grant was tasked with developing a set of computational and experimental benchmarks for prior TR activities in order to assess the capabilities of historically developed codes used to provide the safety case for experimental design and evaluation. One subset of tasks in this project was the development of an experimental facility-the Transient Reactor Test Loop (TRTL) Facility-that would emulate a pump-driven flowing water loop that would be implemented at the TREAT Facility for TR testing of light water reactor fuel. The TRTL Facility was designed and developed at Oregon State University and began operations testing in 2017. Empirical data produced by this new experimental flow loop was benchmarked against a set of codes that represent the standard for use in industrial [Reactor Excursion and Leak Analysis Program-Three Dimensional (RELAP5-3D)] and regulatory [TRAC/Reactor Excursion and Leak Analysis Program (RELAP) Advanced Computational Engine (TRACE)] settings. The findings from those benchmarking activities from the TRTL Facility and the comparisons with the established safety codes are presented here.

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