Realistic Evaluation of Shadow Shields for Space Nuclear Systems

The Outside Test Tank is the only nuclear shielding test facility in the United States suitable for investigating the nuiclear characteristics of shadow shield geometries such as will be required to protect components and man near nuclear systems operating in outer space. The OTT is designed so that only the radiation of interest, that is, neutrons and gamma rays penetrating or originating in the shadow shield materials, is measured. No radiation is measured that is scattered by air or ground. The only radiation that reaches the detectors penetrates shield components on the direct line of sight between the test reactor, the shield, and the test radiation detectors. The design and operation of the Outside Test Tank are described. Problems encountered during testing are discussed, as are the instruments and devices used to measure neutron and gamma ray dose rates and fluxes. A number of unusual phenomena that were discovered are examined. Tested configurations included such high temperature shielding materials as W and LiH. In summary, the information presented in this paper proves that realistic evaluation of actual nuclear shadow shield configurations can be made conveniently and relatively inexpensively without having to test in the vacuum of outer space. The data measured in the Outside Test Tank clearly demonstrate that the design of efficient shadow shields for space nuclear systems is not nearly as straightforward as has been suggested.