Most nuclear reactors are powered by pellets of uranium or uranium mixed with plutonium. The pellets are inserted into long thin fuel rods which are then bundled together into fuel assemblies. When fuel assemblies are removed from a reactor, they are placed in a cooling pool for five years to allow heat and radioactivity to dissipate. When the assemblies are removed from the pool, they are stored in massive stainless steel and concrete containers known as dry casks which can hold between twenty and thirty fuel rod assemblies.
Once a dry cask is sealed, there is currently no way to know how many fuel rod assemblies are actually stored in the cask because it cannot be opened once sealed. This is of grave concern to individuals and organizations who are working on insuring the non-proliferation of nuclear weapons. It would be possible for a country to claim that there were more spent fuel rod assembles in their dry casks than were actually in the casks. The missing fuel rod assemblies could be diverted to a nuclear weapons facility for recycling and recovery of U-235 and/or plutonium.
Scientists at the Los Alamos National Laboratory (LANL) in New Mexico under the leadership of Matthew Durham have developed a method for checking on the contents of dry casks without opening them. Their method relies on monitoring the path through the dry casks of muons generated by cosmic rays. They say that International Atomic Energy Agency inspectors could use the process that LANL researchers have developed to make sure that spent nuclear fuel is not being diverted to a nuclear weapons program.
The LANL process requires muon monitors to be placed on either side of a dry casks. The muons will easily penetrate the concrete and steel of the casks but their paths through the casks will be influenced by the presence of heavy elements such as uranium and plutonium.
The team set up a test cask at the Idaho National Laboratory. The test cask has twenty-four slots that can receive spent fuel rod assemblies. For the test, six of the twenty-four slots were left empty. Previous tests had suggested that the angle of muon scattering was influenced by missing fuel rod assemblies, but the area of the cask tested was too small for the tests to be conclusive.
In the new experiments conducted by the LANL team, sensors were placed all over the test casks in nine different patterns. They found that they could accurately determine when two or more fuel rod assemblies were absent. They say that if they increase measurement time or use larger detectors that can capture more of the passing muons, it should be possible to tell if only one fuel rod assemble is missing.
Considering that the cooling pools at nuclear power plants are quickly filling up, more and more spent fuel rod assemblies will be placed in temporary dry casks storage to await the construction of permanent geological repositories. The new muon monitoring system will be a valuable addition to the tools of nuclear plant inspectors.
