**2. Facility history**

The Arco Desert, where TREAT and ANL-W were built, has also housed many other test reactors as part of the National Reactor Testing station and Naval Reactor Facility missions. A series of water-based transient test reactors were constructed under the Special Power Excursion Reactor Test (SPERT) program that was contemporary to TREAT in its early years [9]. Together, SPERT and TREAT used water capsules to conduct most of the foundational research on overpower fuel performance thresholds for LWRs. During this time, TREAT also continued to perform research on sodium fast reactor (SFR) fuels and nuclear thermal propulsion (NTP) fuels using specialized test capsules. Later, two additional landmark facilities were built out in the Arco Desert to advance research on the accident behavior of LWR fuels. The Power Burst Facility (PBF) offered unrivaled capabilities for reactivity-initiated-accident testing of fuel rods in an integral pressurized flowing loop [10], while the Loss-of-Fluid Test Facility (LOFT) addressed system-scale safety testing via its seminal work in loss-of-coolant-accident testing [11]. These features, along with the postmortem exams performed by facilities in the Arco Desert on fuel from the Three Mile Island accident made Idaho the nexus of fuel safety research throughout the 1980s.

With PBF and LOFT focusing on LWR safety research TREAT's latter historic era naturally shifted toward a focus on SFR fuels using clever sodium loop test vehicles. The Mk-series loops could test bundles of up to seven pins using compact electromagnetic pumps to recirculate sodium through a small pipe weldment [12]. The entirety of these loops was small and self-contained to foster transportation between TREAT and the adjacent Hot Fuel Examination Facility (HFEF) on the main ANL-W campus. Casks established for this purpose could house sodium loops and other experiments measuring up to 25 cm in diameter by 3.6 m tall. HFEF was used to assemble fuel pins irradiated in other test reactors (e.g., EBR-II) into these TREAT test loops, and to extract/examine these pins after transient irradiation. Today, HFEF remains in operation as a global hub for post-irradiation examinations.

Unlike reactors such as PBF, TREAT was not designed from the ground up with integral piping for test loops. Thus, the most common type of TREAT experiment design is well represented by the successful Mk-series sodium loops. Referred to as package- or cartridge-type experiments, this design approach used a compact, robust, experiment containment vessel to provide the desired specimen boundary conditions and contain all chemical, radiological, and mechanical hazards associated with the test (see **Figure 8**). These devices, which fit entirely within casks, were installed by being lowered into the reactor and then connected to power/signal lead on the top flange. These leads were routed through the slot in the rotating shield plug and to the necessary control and data acquisition equipment. The absence of liquid coolant or pressure vessel surrounding the reactor simplified lead routing for facilitating transient tests in which real-time experiment data was crucial for understanding the data objectives. This package-type approach was key for enabling TREAT to address specimen coolant conditions and research needs for a variety of reactor designs [13].

TREAT performed numerous tests on oxide-type SFR fuel designs in Mk-series loops to produce much of foundational transient behavior data for these systems. The TREAT facility was upgraded in numerous ways to enable testing of larger oxide fuel bundles in an upsized sodium loop in order to address further data gaps, but shifts in national research priorities prevented this upgrade project from being fully completed. The major upgrades that were realized included a larger building with increased crane capabilities and experiment storage holes, modernization of the automatic reactor control system, and reconfiguration/upgrading of the control rod configuration for the reactor trip system (described earlier). While the upsized sodium loop was never deployed, a special set of new TREAT driver fuel assemblies was also fabricated—using higher uranium loading and Inconel canisters to support higher temperature operation—in an inner converter ring meant to increase the fast neutron flux delivered to the test. These new upgrade driver fuel assemblies remain unused in storage at TREAT to this day.

TREAT was upgraded and maintained in state-of-the-art condition up through the early 1990s. By this time, SPERT, PBF, and LOFT had all ceased operation. TREAT continued to perform work related to SFR metallic fuel until funding was canceled for the Integral Fast Reactor Program in the mid-1990s, causing both TREAT and EBR-II to cease operation. EBR-II was eventually decommissioned, and unique specimens irradiated therein were placed in storage to await future use. However, TREAT's unique, simple design required virtually no maintenance to remain in a safe condition. As a result, electrical power to TREAT's control rod drive systems was simply disconnected to ensure it could not operate, fuel was left in the reactor, and it remained unchanged in this state for approximately 20 years.
