What Is One Method of Long-Term Spent Fuel Rod Storage?

Spent Fuel

Every 12 to 18 months, roughly one-fourth to one-third of the nuclear fuel in a reactor is spent and must be replaced. The spent fuel is highly radioactive. Even 10 years after it has been removed, the dose of radiation you would receive standing a meter from spent fuel is four times higher than the dose needed to cause death within one week. Some of the elements in this spent fuel decay very quickly, but others, such as plutonium-239, decay very slowly and have half-lives measured in millennia. Consequently, spent fuel must be safely stored for a long period of time.

In dry storage, spent fuel is entombed in specially designed concrete casks made from materials that absorb radiation. The casks must meet a variety of stringent conditions. For example, casks must be able to survive undamaged a fall from 30 feet onto a concrete surface, a 40-inch drop onto a 6-inch steel spike, 30-minute exposure to a 1,475-degree Fahrenheit fire, and immersion in 3 feet of water. The casks incorporate materials, such as boron, that absorb neutrons to prevent any uncontrolled nuclear reactions from taking place. These casks are welded shut once the fuel is inserted, then stored either standing upright, or kept horizontal in concrete bunkers.

In wet storage, spent fuel rods are stored in a specially designed pool beneath at least 20 feet of water The water shields anyone near the pool from the radiation. The fuel rods are inserted into the pool via canals, so workers need never come in contact with the radioactive material. At the bottom of the storage pool, the fuel rods are arranged in a square array.

Wet and dry storage both are safe, but pool storage requires more ongoing maintenance. The pool must be kept full of water, and the system must be monitored closely. In wet systems, it's important that the rods be kept well-spaced to avoid the possibility of an uncontrolled reaction. Dry storage is simpler and requires less maintenance. It will not work for spent fuel fresh from the reactor, however, because radioactive decay generates considerable heat. Once the spent fuel has been out of the reactor for a few years, it can be transferred to casks for dry storage, which is a simpler alternative for long-term storage.