50 YEARS OF OPERATION reactivity without disturbing the vertical power profile. The design also included small neutron-absorbing control rods. Unlike control rods in earlier reactors, these were not moved slowly up or down during reactor operations to effect their control, but either fully inserted or fully removed. Another groundbreak- ing—and aesthetically satisfying—innovation in the ATR was how it wrapped the reactor’s fuel around the samples in serpentine fashion, more than doubling the neutron flux (available in the Engineering Test Reactor) to the sample. As de Boisblanc relates, it was during the long drive home from the Site that the“Aha!” moment occurred. As was the custom, I was driving Byron Leonard, our consultant from Internuclear Company, to his hotel in Idaho Falls. It was one of those lingering twilight evenings, still quite light. On that straight stretch of Highway 20 across the desert, with its sagebrush and the frequent lava flow patches, there wasn’t much to distract us. I started to describe a novel way to look at the problem before us. I thought of breeder reactors, where the effort is to minimize the leakage of neutrons. I tried to think how we might make the neutrons leak in the direction of the sample, where we wanted to maxi- mize the number of neutrons absorbed into the Navy’s samples. If we placed water between the ATR fuel and the sample, the fast neutrons would “leak” into the water and collide with hydrogen. This would slow them down and they would pile up to create a high slow-neutron flux. This is the so-called “flux trap,” which I didn’t invent. I reached over across the front seat of the car and with my finger drew four circles for test loops, and then a snakelike fuel line partially around each loop. Immediately, I saw that we could place another loop at the very center because the four arcs that surrounded the center loop were almost as effective as a circle. It soon became obvious that by placing a beryllium reflector properly we could gain four more attractive loop locations. The more we looked at that strange arrangement, the better it looked. Possible new locations for control elements became apparent. Byron was so excited he volunteered to lay out the configuration. He didn’t get much sleep that night, but what he produced was remarkable. His plan view showed that the entire serpentine fuel arrangement could be produced with only one type of fuel element. The number of test loops grew from the original four to nine. The next several days brought the usual questions from devil’s advocates. As always with a“rich”design, each negative, when resolved, revealed new capability. They sensed they had a winner. Arranging the core into multiple different flux-trap regions—in which the power level could be different in each simultane- ously—was something that de Boisblanc did invent. Satisfied, the AEC and the Navy selected the ATR cloverleaf design. The more we looked at that strange arrangement, the better it looked. “ProvingthePrinciple, aHistoryoftheIdaho NationalEngineeringand EnvironmentalLaboratory 1949-1999,” bySusanM.Stacy Chapter17