Verification of Spent Nuclear Fuel in Sealed Dry Storage Casks via Measurements of Cosmic-Ray Muon Scattering

Most of the plutonium in the world resides inside spent nuclear reactor fuel rods. This high-level radioactive waste is commonly held in long-term storage within large, heavily shielded casks. Currently, international nuclear safeguards inspectors have no stand-alone method of verifying the amount of reactor fuel stored within a sealed cask. Here we demonstrate experimentally that measurements of the scattering angles of cosmic-ray muons, which pass through a storage cask, can be used to determine if spent fuel assemblies are missing without opening the cask. This application of technology and methods commonly used in high-energy particle physics provides a potential solution to this long-standing problem in international nuclear safeguards.

Figure 1

The two muon trackers around the MC-10 cask. One tracker is elevated by 1.2 m relative to the other to sample the muon flux at smaller zenith angles. Both are housed inside weatherproof containers. One of the lifting trunnions that is used as a reference point to align the detectors with the fuel basket is visible on the bottom left of the cask body.

Figure 2

A diagram showing the cask loading configuration and the detector positions during the measurements as viewed from above. Muons moving between the two detectors pass through columns in the fuel basket containing (from left to right) zero, one, six, five, four, and two fuel assemblies. Data are recorded with the detector on the lower right, but winds at the testing site shook the detector during this portion of the measurement, rendering the data unusable for analysis.

Figure 3

(a) One of the horizontal double layers of drift tubes, which gives a position where the muon crosses the detector along the vertical direction but provides no information on the horizontal position where there is no segmentation. In the complete muon tracker (b), an identical double layer that is oriented with tube segmentation along the horizontal direction is placed behind this one. This pattern repeats three times to give a total of six double layers (three horizontal and three vertical) in each detector. The blue and red boxes contain low- and high-voltage supplies.

Figure 4

An illustration of the analysis procedure (not to scale). Tracks measured in the elevated upper detector are projected to an array of voxels at the center of the cask. Each voxel has a corresponding histogram where the scattering angles of muons that pass through it are collected.

Figure 5

The average muon scattering angle as a function of horizontal position across the cask. The gray shaded areas indicate the inner and outer boundaries of the 25-cm steel shielding around the fuel, and the boundaries of the columns in the fuel basket are denoted by dashed lines. A geant4 simulation of muon scattering in a full (empty) cask is shown by the solid blue (dashed red) line.

Figure 6

The muon scattering signal averaged across the positions of each column in the fuel basket for data and simulations of a full and empty cask. The number of assemblies in each column for the full cask and the configuration as measured are indicated in text.