The reactor core is cylindrical, approximately 2 ft (0.61 m) high and in diameter. A 5-in. (12.70-cm)-diameter hole, referred to as the "flux trap," forms the center of the core. The target is typically loaded with curium-244 and other transplutonium isotopes and is positioned on the reactor vertical axis within the flux trap. The fuel region is composed of two concentric fuel elements. The inner element contains 171 fuel plates, and the outer element contains 369 fuel plates. The fuel plates are curved in the shape of an involute, thus providing a constant coolant channel width. The fuel (93% U235 enriched U3O8-Al cermet pg.22) is non-uniformly distributed along the arc of the involute to minimize the radial peak-to-average power density ratio. A burnable poison (boron-10) is included in the inner fuel element primarily to flatten the radial flux peak providing a longer cycle for each fuel element. The average core lifetime with typical experiment loading is approximately 23 days at 85 MW.