Neutron transmission properties of concrete for a HEU storage vault from time of flight transmission measurements with a {sup 252}Cf source

Abstract

Prompt fission neutron and gamma rays have been used to measure the transmission of concrete samples that were cast from the concrete mixes for the modular highly enriched uranium (HEU) storage vault at the Oak Ridge Y-12 plant. These data and the samples will be retained for future measurements to track the drying process that may reduce the unbound water content of the concrete. The hydrogen atomic density from bound water is {approximately}5.9 x 10{sup -3} and that for unbound water is {approximately}7.3 x 10{sup -3} atom per barn centimetre. If the unbound water diffuses out of the concrete, the reduced hydrogen content will result in increased interaction of fissile material and in higher neutron multiplication factors for the fissile arrays stored in such a vault because the main effect of the hydrogen is to reduce the transmission of neutrons through the concrete by slowing down so that the neutrons can be absorbed in the concrete. Time-of-flight (TOF) transmission measurement with a {sup 252}Cf source was used to measure the time distribution of neutrons and gamma rays after {sup 252}Cf fission. These measurements can be used to verify the calculated transmission from neutron and gamma-ray Monte Carlo transport methods.