Nuclear car wash status report, August 2005

Citations Over Time

Abstract

A large majority of US imports arrive at seaports in maritime cargo containers. The number of containers arriving is nearly 10 million per year, each with a cargo of up to 30 tons of various materials. This provides a vulnerable entry point for the importation of a nuclear weapon or its components by a terrorist group. Passive radiation sensors are being deployed at portals to detect radioactive material and portable instruments are carried by port personnel to augment detection. Those instruments can detect the neutrons and g-rays produced by {sup 240}Pu that is normally present in weapons grade plutonium in cases where cargo overburden is not too great. However, {sup 235}U produces almost no neutron output in its normal radioactive decay and its principal {gamma}-radiation is at 186 keV and is readily attenuated by small amounts of wood or packing materials. Impurities such as {sup 232}U, often present in reactor irradiated material at the 100-200 ppt level, can provide a detectable signal through significant cargo overburden but the wide variations among samples of HEU make this an unreliable means of detecting SNM. High quality radiography may be useful in determining that the majority of containers are clearly free of SNM. However, some containers will lead to ambiguous results from radiography and passive radiation sensing. For these reasons active neutron interrogation is proposed as a means to produce fission and thus greatly amplify the radiation output of fissionable material to facilitate its reliable detection even when well shielded by large cargo overburden. Historically, the fission signature utilized as the unique identifying feature of fissionable materials is the detection of delayed neutrons. However, these neutrons have very low yield {approx} 0.017 per fission in {sup 235}U, and their low energy results in very poor penetration of hydrogenous materials such as fuels, water, wood, or agricultural products. That signature alone does not provide reliable detection in thick cargos. A new signature has been identified and has been developed within the current project for the detection of well shielded SNM. This SNM signature is based on high-energy {beta}-delayed {gamma}-radiation produced by fission products following neutron or photon induced fission. These {gamma}-rays are high enough in energy (E{sub {gamma}} > 3 MeV) to be readily distinguished from any natural background radioactivity since the latter does not extend above 2.6 MeV. Their abundance is nearly a decade greater than delayed neutrons and their short half-lives deliver nearly all of the signature radiation on time scales of one minute or less and thus facilitate rapid scanning. Finally, for this {gamma}-radiation in the 3-6 MeV range attenuation occurs only by Compton scattering and is in the range where minimum attenuation occurs in all materials. Even the thickest cargos of any material attenuate these {gamma}-rays by only a factor of 10-30X so that the signature is readily detected even with the most challenging shield materials. The goals of the current program are to detect significant quantities (much less than IAEA ''significant'' amounts) of well-shielded SNM, and to do so with detection probability P{sub d} {ge} 95% and with false alarm rates P{sub fp} {le} 0.001. It is the goal to meet these requirements in a scan that requires less than one minute to complete and does so without damage to the cargo or to people who may be hidden inside. We intend to meet these requirements even when the cargo overburden is up to {rho}L {le} 150 g/cm{sup 2} of any material ranging from fuels and agricultural products to steel and lead.

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