Because of the growth of the nuclear power industry in the United States and the policy to ban reprocessing of commercial spent nuclear fuel, the spent fuel inventory at commercial reactor sites has been increasing. With the Yucca Mountain project on hold, more spent fuel is expected to be stored in dry storage casks (DSC) at independent spent fuel storage installation (ISFSI) for extended periods of time. These fuel assemblies are practically inaccessible for inspection purposes, as reopening a DSC would require special facilities and be tremendously expensive. There is currently no practical method to verify the content of a DSC once continuity of knowledge is lost, but cosmic ray muon imaging is under development as a method that could meet this need. Imaging with these muons has been demonstrated to be a viable non-destructive assay method for high-Z materials, such as those inside used nuclear fuel assemblies. Most often a gas-based detector system has been used. In this work, we report on a proof-of-concept muon tomography system made out of plastic scintillator and wavelength shifting (WLS) fibers.
The prototype muon tomography system were designed, built, assembled and tested for the purpose of monitoring used nuclear fuel content inside dry storage casks. First, the simulation study suggested muon was a promising tool to image dense objects and benchmarked the idea of utilizing muon image for cask inspection. Two versions of detector designs (i.e. single scintillator panel and independent scintillator bars) were explored. The first version of detector design was demonstrated to be position-sensitive, yet the position resolution for muon is not good enough for the system to do tomography. The second version was demonstrated to have about 1 cm resolution and was able to detect muon trajectories. And this version was shown to be able to image some simple objects. The study of additional radiation indicated that the radiation emitted from a typical cask does not significantly impact the imaging capability and image quality. The image spatial resolution of lead bricks was quantified to be around 2 cm