*The Transient Reactor Test Facility (TREAT) DOI: http://dx.doi.org/10.5772/intechopen.101275*

Owing to its floorspace, vertical headroom, and authorization as a nuclear facility, TREAT was still used throughout these years for various other nuclear research applications, but the reactor itself was not operated. These efforts required TREAT to remain in active status and maintain its safety basis authorization. Throughout this period, occasional efforts surfaced to champion the resumption of reactor operations at TREAT [14], but none garnered enough momentum to realize this goal. The events of Fukushima Daiichi in 2011, however, gave rise to renewed interest in developing and researching enhanced safety characteristics for nuclear fuels. The U.S. Accident Tolerant Fuels (ATF) program was launched shortly thereafter and, along with the other mission needs that had accumulated over the years, finally justified the resources needed to resume reactor operations at TREAT [15].

The TREAT restart project then followed. The entirety of the TREAT restart project is summarized in a journal special issue in Ref. [16]. Articles from this special issue are referenced throughout this paper as appropriate. The facility was thoroughly characterized and refurbished as needed, with a focus on age-related degradation of systems and components. In some cases, basic industrial equipment in the plant was replaced or repaired, but most of the plant's systems were found in good working order. Key staff previously involved in TREAT operation, many of whom had since retired, rallied to this project to train new staff and transfer knowledge. The facility's safety basis authorization was updated and modernized to reflect new standards and needs [17]. As a testament to the facility's simplicity, the orderly way it was shut down, and the dedication of the restart project staff, TREAT achieved its "second first-criticality" in 2017 [18]—both ahead of schedule and under budget [19].

A few years prior to TREAT's restart, contractor reorganization caused the ANL-W campus, along with its key facilities (i.e., TREAT and HFEF), to come under the same management structure responsible for operating many other key nuclear research assets, including the Advanced Test Reactor (ATR). The resulting national laboratory was termed the Idaho National Laboratory (INL). Upon TREAT's successful restart, INL attained a powerful partnership in research reactor facilities (e.g., a high-flux thermal spectrum material test reactor [ATR], a multipurpose transient test reactor [TREAT], and a sizeable hot cell with abilities to examine and transfer specimens between these reactors [HFEF]) (**Figure 9**).

#### **Figure 9.**

*Modern-day aerial view of INL's materials and fuels complex. (ATR is just out of view on the left side, ~30 km west of TREAT.)*
