Abstract
Oak Ridge National Laboratory is developing a new capability to perform passive fast neutron emission tomography of spent nuclear fuel. The goal of this capability is to detect vacancies or substitutions of individual fuel pins in spent nuclear fuel assemblies for international safeguards applications, such as verifying the integrity of an assembly prior to transfer to “difficult to access” storage. Emission tomography uses collimation to isolate activity along “lines of response” through an object. By combining many collimated views through an object, the neutron emission from each fuel pin can be mathematically extracted and an image of the fuel assembly can be constructed. However, performing fast-neutron imaging is challenging for the very reason that it is desirable, namely, that fast neutrons penetrate a good deal of shielding and are consequently difficult to collimate and measure with high resolution. For spent fuel, additional challenges include the modest neutron source strength and the overwhelming gamma-ray emissions. While the International Atomic Energy Agency (IAEA) is presently investigating the use of passive gamma emission tomography for the same application, it is useful to investigate neutron emission tomography because fast neutrons better penetrate larger fuel assemblies and because fast neutrons (originating primarily from curium-244 which is mainly produced at the end of the exposure cycle) may be sensitive to replacement fuel pins that are subsequently irradiated. In the present work, we present a novel collimator concept that will enable rapid transaxial tomographic imaging of spent nuclear fuel using the spontaneous fast neutron emissions from the fuel. Initial design simulations of an imager based on this collimator indicate sufficient resolution to identify individual fuel pins. Employing such a collimator, the resulting imager can be sufficiently compact, efficient, and radiation resistant to make fast neutron emission imaging practical.
