Part 1 of 5 Parts
I have blogged about muons before. They are often generated in particle accelerators. Physicists can use them to identify more exotic particles in the debris of particle collisions. Muons are also produced by natural forces in the atmosphere and researchers have turned to them for probing dense materials. This use is referred to as muon radiography or muography. Muon detectors have been used by archeologists to probe for cavities in pyramids and by geologists to check for the rise of magma in volcanoes. Muography is especially well suited for use in probing containers of nuclear waste which are inaccessible to other forms of radiation.
Dealing with spent nuclear fuel is complex, expensive and dangerous. The goal is to keep the public and environment safe while ensuring that the fuel can be traced. The International Atomic Energy Agency (IAEA) was establish partly to ensure that no spent nuclear fuel will fall into the hands of groups that could extract plutonium to create dirty bombs or fulling operations nuclear bombs.
Nuclear fuel for commercial power reactors is produced in the form of pellets of uranium which are inserted in to long metal tubes that are bundled into fuel assemblies for insertion into the core of the reactors. As the uranium in the pellets undergoes fission and generates power, it also creates plutonium and other highly radioactive isotopes which generate a lot of heat.
After a fuel assembly is removed from the core, it is placed in a cooling pool near the reactor for several years to cool off. Cooling pools in the U.S. are filling up rapidly. Some fuel assemblies are removed after cooling and placed in steel and concreate cylinders referred to as dry casks. These casks are several meters high and lined with radiation absorbing material. They are stored near the reactor or in a temporary storage facility somewhere else.
In order to account for spent nuclear fuel, the IAEA inspectors utilize cameras to observe the process of loading and unloading the fuel assemblies. They can also use Cerenkov radiation (Cherenkov radiation is electromagnetic radiation emitted when a charged particle such as an electron passes through a dielectric medium at a speed greater than the phase velocity of light in that medium.) to check the assemblies in the cooling pool. However, once the assemblies are placed in the radiation proof casks, no direct monitoring is possible.
All the inspectors can do is place seals on the casks to indicate if they have been tampered with. However, the seals could corrode with time or be damaged when the casks are moved. The only way find out if a damaged seal indicates tampering would be to open the casks to inspect their contents. This would be complicated, expensive and dangerous.
Muons offer a solution to the problem of monitoring the contents of a sealed cask without opening it. Matt Durham works at the Los Alamos National Laboratory. On the matter of monitoring spent nuclear fuel, he says, “This issue is only getting worse as more plutonium piles up around the world. Muons offer a way to establish how much waste there is in a container without having to open or move the container in question.”
Please read Part 2