There are a few options for testing for voids in your cask or vessel.
This is a process of using a nuclear source to inspect the walls of a cask to look for voids. A grid pattern is established for the testing, such as a 4 inch by 4 inch layout. A detector can be used to look for radiation spikes from an established baseline level and then documented. Film can also be used to actually capture an image of the dark and light spots that reflect higher and lower density spots in the wall. This is a popular inspection method in the nuclear power and nuclear waste industries.
This is the same process as the gamma scan except that an X-ray source or Betatron is used to scan. Due to the size of the X-ray source, this testing can be challenging at times but Betatron can punch though 12 inches of steel. This allows for a good inspection method for a product that can be moved to the inspection lab since a Betatron cannot be brought to the product.
A water height test is a process in which the volume of a vessel is verified with water. A cask or vessel is weighed when empty and the weight is recorded. The cask is then filled with water while on a scale until full. The filled weight is recorded. The water is then removed and the cask is dried. The cask is then filled with lead and weighed again.
We use a formula to compare the density of the water to that of the lead in order to establish the theoretical weight for the filled container. The difference between the theoretical weight (the water calculation) and the actual weight is the loss created by voids. Voids are a normal part of the pouring due to temperature variation, freeze off, turbulence, flow, etc. How well these items are controlled determine how solid of a pour you will have upon completion.