**1. Introduction**

In the late 1950s, the Transient Reactor Test facility (TREAT) was designed, constructed, and commissioned within the span of only a few years [1]. The facility was built just over 1 km away from the Experimental Breeder Reactor-II (EBR-II) sodium-cooled fast breeder reactor as part of the Argonne National Laboratory West campus (ANL-W) located in the Arco Desert, west of Idaho Falls, Idaho. As with most facilities at ANL-W, TREAT was originally envisioned to help support research and development pertaining to EBR-II, but its mission diversified in later years to support other nuclear technology areas. TREAT was a specialized graphitebased test reactor able to safely perform extreme transient power maneuvers to research the effects of postulated accident conditions on nuclear fuel specimens placed in its core [2, 3]. A modern aerial image of TREAT is shown in **Figure 1**.

TREAT's unique abilities stem from its fuel assemblies, in which uranium oxide, graphite, and carbon powders are mixed with binders, pressed into blocks, and fired at high temperatures [4]. The resulting fuel blocks were stacked inside zircaloy-3 sheet metal canisters (a uniquely oxidation-resistant zirconium alloy that was being researched at the time, but which is no longer in production, having been superseded by other zirconium alloys for light-water reactor [LWR] use). These canisters were evacuated and sealed. Aluminum sheaths and end fitting hardware were fastened to the tops and bottoms of these fuel assemblies to house graphite reflectors and provide mechanical interfaces for gridplate placement and handling. These fuel assemblies had a ~10 cm2 cross section with 0.6 m of unfueled axial

**Figure 1.** *Modern day aerial image of TREAT.*

reflector top and bottom with 1.2 m of active fueled length in the center. Various special fuel and graphite dummy assemblies were also produced, including some with central cylindrical cavities for control rods, some with integral thermocouples, and some with a void region (i.e., containing no fuel or moderator) in the core's axial center (see **Figure 2**) [5].

The resulting fuel assemblies were produced in sufficient quantity to fill the reactor's 19 × 19 square-pitch gridplate array. Despite thousands of reactor startup and transient cycles over the decades that followed, the fluence experienced during short transients was small, and these same fuel assemblies accumulated very little burnup. Hence, TREAT operates to this day using the original fuel assemblies produced in the 1950s. Occasionally, these fuel assemblies are shuffled into different reactor positions or stored below grade in adjacent storage holes. Core reconfigurations are performed to optimize the core parameters for experimental needs rather than to equilibrate burnup as is typical of most nuclear reactor shuffling schemes. The radionuclide inventory of these fuel assemblies is minimal, and they can be handled without shielding, especially after an extended decay period

**Figure 2.** *Historic image of TREAT fuel assembly types.*

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

between transient operations. Still, these fuel assemblies are typically handled in a lead-shielded cask outside the reactor to reduce personnel radiation exposure.

TREAT's active core region resided just above ground level. The reactor is surrounded by a thick wall of graphite reflector blocks. TREAT's graphite reflector is surrounded by thick walls of concrete that comprise both the reactor's main structural shell and its radiation shielding. Blocks can be removed from some parts of the graphite reflector and concrete shielding to create a void slot for viewing the core center from each of the four cardinal directions. Presently, the west slot is occupied by a collimatedbeam neutron radiography facility adjacent to the reactor. The north slot is occupied by the Fuel Motion Monitoring System (FMMS), also known as the hodoscope. The east slot area is filled with normal fuel assemblies with a large graphite region in the concrete wall and rolling shield door give access to a highly thermalized neutron environment referred to as the thermal column. The south slot is currently unused, but could be outfitted with other scientific instruments or facilities in the future.

The concrete walls support a ~30 cm-thick circular upper shield plug approximately 3 m in diameter. This shield plug can rotate 360 degrees on bearings via a gear drive. A rectangular slot through the shield plug extends from its center to its periphery. All fuel assemblies, experiments, and other hardware are installed in TREAT through this slot, using bottom loading shielded casks and/or overhead cranes. A ~1 m gap between the top of the fuel assemblies and the bottom of the rotating shield plug provides space for TREAT's control rods to protrude above the core. See **Figures 3** and **4** for an overview of some of the reactor's key features.

**Figure 3.** *Section view of TREAT's key reactor features [6].*

#### **Figure 4.**

*Top view of TREAT's core, reflector, and shielding layout [6].*

Apart from experimental devices that may contain various liquids to support the desired specimen boundary conditions, TREAT does not house liquid coolant for the reactor itself. Instead, a blower system pulls air from the reactor building through debris filters located atop the reactor, down into the core (primarily through ~1 cm2 coolant channel gaps where the corner chamfers of four fuel assembly canisters meet), and out through a filtration system and stack. This air-cooling system is adequate to enable the reactor to operate in low-level steady-state (LLSS) mode for several hours at a time. Presently, TREAT is authorized to operate in LLSS mode at up to 120 kW thermal power, but this power level does not challenge facility physical limitations and could likely be uprated if needed. LLSS mode is useful for calibrations, system check outs, dosimeter irradiations, and neutron radiography. This cooling system is inadequate for removing significant heat within the time duration of a fast transient; hence, it is not credited for transient safety calculations. Therefore, the core's heat capacity and high-temperature oxidation of fuel assembly canisters typically set the core transient energy capacity at around 2500 MJ, depending on the core configuration. The cooling system also helps cool down the core after large transients, thus boosting operational efficiency. In this manner, TREAT can typically perform one large transient per day—and occasionally two moderate-energy transients in a one-day shift.

TREAT's unique core design is complimented by its specialized control rod systems, thus enabling its unparalleled transient capabilities. All TREAT's control rod types use boron carbide in the absorber section, along with graphite-filled zirconium alloy followers. Reactor operation is initiated by withdrawing compensation and transient rod sets (the compensation rods' purpose is to ensure that hold-down reactivity margins are maintained during the removal of large experiment devices, many of which are net neutron sinks). The reactor is then brought critical by moving the control/shutdown

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

rod sets out of the active core. LLSS operations are typically performed with the rods in this configuration. Transient control rods can then be inserted incrementally to prepare for transient operations, while the control/shutdown rods are withdrawn to maintain criticality until the desired excess reactivity is available in the transient rods. The reactor is then switched into transient mode, and a preprogrammed transient power shape is executed by an automatically controlled computer system with active feedback from ion chamber neutron detectors located in TREAT's concrete shielding. See **Figure 5** for an example core map showing these control rod locations.

Transient rods are driven by fast-acting hydraulics in the TREAT basement sub-pile room (see **Figure 6**). These rod drives can move the rods at a velocity of ~3.5 m/s in both directions (i.e., up and down), permitting split-second manipulation of the reactor's power shape. A tremendous number of transient shapes can be executed, including prompt pulses, ramps, flattop regions, and combinations thereof [7]. (See **Figure 7** for examples of possible power shapes in TREAT.) Transient operation can be "clipped," based on the desired test conditions, by rapidly inserting the transient control rods to narrow the TREAT natural pulse width to <90 ms (full width at half max) and terminate the reactor power. Further upgrades are planned for expanding TREAT's clipping capability to include even narrower pulses when needed [8].

**Figure 5.** *Example core map showing current control rod types and locations.*

**Figure 6.** *View of TREAT rod drive mechanisms from the sub-pile room.*

**Figure 7.** *Example transient shapes possible at TREAT.*

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

Under certain conditions, a state-of-the-art reactor trip system will initiate the rapid insertion of all rods; however, as with the air-cooling system, the trip system is not credited in the reactor safety basis. Instead, TREAT's strong negative temperature feedback behavior is credited as the primary means of limiting transient energy. Since TREAT's uranium oxide particles are dispersed in the fuel blocks, power excursions cause the moderator temperature to rise, resulting in higher neutron energy, increased neutron leakage, and self-limiting power excursions with reliable negative temperature reactivity coefficients. This key feature of TREAT enables it to safely perform research on nuclear fuel specimens under extreme conditions.
