**3. Current efforts and future outlook**

Efforts to prepare for transient experiments began shortly after the TREAT restart project commenced. The FMMS detectors were refurbished, and its data acquisition system was replaced with a modern digital system at this time. The FMMS works as fast neutrons born in the experimental fuel specimens travel through the experiment's containment structure, the core's void "slotted" assemblies, and one of several slits in a collimator installed in the reactor's concrete shielding. A fast neutron detector resides at the end of each slit. The slits are arrayed to focus on different axial and transverse locations in the experiment cavity. The FMMS detectors interact with fast neutrons to cause scintillation and luminescence. This phenomenon is proportional to the number of fast neutron interactions, becomes amplified by photomultiplier tubes, and is converted into an electrical signal for high-speed digital data acquisition. This FMMS is able to observe the location of test fuel throughout the duration of the transient. Phenomena such as the expansion, disruption, and meltdown of test fuel can be observed in real time by the FMMS. A cross-section image of the FMMS can be seen in **Figure 10**.

Similarly, the new digital experiment data acquisition and control system (EDACS) was installed. EDACS relies on commercially available equipment and is designed with modularity and expandability to support new instrumentation and control system functions. Dedicated controllers work redundantly with this system to ensure that functions significant to safety are highly reliable (e.g., overtemperature control of electric heaters for heating experiments prior to transient operation). Similarly, wire routing options and facility locations were established for specialpurpose signal processing and data acquisition equipment to support special test sensors that do not require integration with EDACS.

In the years preceding its restart, numerous experimenters had expressed interest in using TREAT. The interests of these users encompassed LWR-, SFR-, and NTP-type reactors. A new test system, referred to as the Minimal Activation Retrievable Capsule Holder (MARCH), was designed to fulfill these various research needs shortly after resuming reactor operations. The MARCH system took inspiration from historic package-type experiments by using a stainless-steel containment pipe weldment, inside a sheet metal enclosure, referred to as the Broad

**Figure 10.** *FMMS cross section of collimator/detector locations and the reactor shielding interface [20].*

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

Use Specimen Transient Experiment Rig (BUSTER). BUSTER can be handled, installed, and connected to support leads in the same way as the Mk-series loops. However, the MARCH system departed from the historic approach in that the sealed capsules are placed inside its pipe. Since many of the first fuel technologies tested in TREAT were emerging (e.g., ATF specimens), only fresh fuel specimens were available. Hence, by combining this capsule-in-pipe mechanical layout with capsule materials that do not transmute into significant radioisotopes (principally titanium alloys), the MARCH system enabled fresh fuel capsules to be irradiated, removed from BUSTER on the TREAT working floor by using the storage holes, and shipped for post-transient exams using glovebox facilities, all in a matter of weeks. A detailed characterization of the BUSTER nuclear environment was performed via Monte Carlo neutronics modeling and can be found in [21]. This approach enables BUSTER to function as a reusable device manufactured in accordance with exacting pressure vessel code and quality assurance requirements, whereas capsules are typically treated as consumable hardware with function-specific engineering requirements. This strategy helps reduce costs as well as the design innovation cycles between test series and capsule adaptations.

The inaugural irradiations performed in BUSTER were sponsored by the ATF program and featured LWR rodlets composed of UO2 pellets in zirconium-alloy cladding. These tests used a helium environment capsule design known as the Separate Effects Test Holder (SETH). These tests focused on quantifying core-to-specimen energy coupling factors, commissioning new experiment support systems such as EDACS, demonstrating use of the FMMS, and assessing the performance of instrumentation in concurrent tests placed in TREAT coolant channel positions [22]. The SETH tests hosted new technologies for world first applications in transient testing, including additively manufactured capsules and multispectral pyrometry. Post-transient exams were performed as intended using a glovebox facility [23], and a second round of capsules were irradiated shortly thereafter on ATF technologies including as U3Si2 fuel pellets and silicon carbide composite cladding [24]. The design was adapted to perform power ramp testing on unclad ceramic fuel specimens inside solid metal holders acting as heat sinks to create thermomechanical gradients in order to investigate transient fuel fracture behaviors.

Building on the successes of the SETH series of experiments, three new major capsule categories were created to provide more prototypic specimen boundary conditions. One capsule was created to support new NTP fuel specimen testing in the SIRIUS series of experiments. The SIRIUS capsule design can house hydrogen in its gas environment, as well as support repeated high-temperature irradiations. The SIRIUS capsule has been used to perform repeated power ramps and to measure specimen temperatures ranging from room temperature to well beyond 2000°C in order to simulate NTP engine startup cycles.

Another capsule, termed the Static Environment Rodlet Transient Test Apparatus (SERTTA), was created to house pressurized water environments for reactivity-initiated-accident testing on LWR rodlets. To date, several studies have been performed using SERTTA, including a series of tests focused on the elucidation of in-reactor transient critical heat flux boiling behavior, and aided by a novel electro-impedance sensor able to detect water voiding in real time [25]. The SERTTA capsule was also recently used to test an LWR rodlet previously irradiated in the ATR. This test marked the first modern use of HFEF to assemble TREAT experiments. Tests assembled in HFEF are expected to become prevalent as more previously irradiated specimens become available for end-of-life fuel safety testing.

A new sodium capsule, referred to as the Temperature Heat-Sink Overpower Response (THOR) capsule, was very recently designed and underwent commissioning tests in TREAT. THOR's key feature is a thick-walled metal heat sink

surrounding the specimen. Embedded electrical heaters liquify sodium between the heat sink and test pin cladding prior to transient operation. The liquid sodium enables tight thermal coupling between the pin and heatsink. Working in concert with TREAT's flexible transient power-shaping capability, THOR can simulate transient overpower temperature responses in test pins. THOR can house up to a single full-length EBR-II rod and is currently being prepared for a test series using legacy rods irradiated in EBR-II that were retained for many decades for this very purpose. See **Figure 11** for an overview of the test capsules currently used in the MARCH system at TREAT.

As of 2021, TREAT offers a variety of experiment capabilities and capsules for testing fuel specimens in water, liquid metal, inert gas, and NTP reactor environments. As the only remaining U.S. transient test reactor with significant fuel testing capabilities, TREAT's mission in the modern era remains as diverse as ever. Still, TREAT and its supporting infrastructure are not yet as capable as they were in the past, especially considering that TREAT must now absorb missions that would historically have been addressed by other reactors. This need is particularly important for test devices able to house larger specimens/bundles and actively manipulate thermal hydraulic conditions. For this reason, a new enlarged version of BUSTER (i.e., Big-BUSTER) has been engineered and slated for deployment in TREAT in 2022. Big-BUSTER allows for test devices up to 20 cm in diameter (as opposed to the 6 cm

**Figure 11.**

*Overview of MARCH system and experiment capsules used to date.*
