**2.1 The advanced test reactor**

The ATR is a water-moderated, beryllium-reflected, pressurized water reactor with a serpentine arrangement of plate fuel taking on a four-leaf-clover likeness [1]. Each of the cloverleaves plus the center region are each referred to as lobes. The design power rating is 250 MW. However, it is currently operated at about 110 MW, and occasionally at powers approaching 250 MW to support higher power experiments. The metallic fuel plates consist of a highly enriched uranium (HEU 93wt% 235U/U) uranium-aluminide (U-Alx) dispersed in aluminum. This dispersion is sandwich clad in aluminum alloy, Al-6061. The fuel serpentine contains 40 fuel elements, each containing 19 curved plates. The fuel plates are swaged into sideplates forming the fuel element. The angular separation between the two side-plates is 45 degrees. The inner and outer four fuel plates contain natural boron carbide (B4C) to suppress radial (i.e., plate-to-plate) power peaking. The inner 11 fuel plates do not contain B4C. Also, the UAlx concentration is varied by plate to minimize radial power peaking.

Initial criticality as well as the power share among the lobes is maintained using hafnium plates on rotating control drums, called Outer Shim Control Cylinders (OSCCs). The burnup reactivity decrement is made up partly with OSCCs but also with annular hafnium neck-shims (i.e., 24 hafnium control rods) which are removed from the four aluminum neck arms in the center region. Numerous penetrations in the reflector and neck arms allow for non-instrumented "drop-in" as well as instrumented capsules, in addition to the nine flux traps. A picture of a typical reactor configuration is provided in **Figure 1**.

**Figure 1.** *The ATR basic configuration.*
