Spent nuclear fuel pools
Spent nuclear fuel (SNF) refers to fuel after it has fueled a reactor. This fuel looks like new fuel in the sense that it is made of solid pellets contained in fuel rods. The only difference is that SNF contains fission products and actinides, such as plutonium, which are radioactive, meaning it needs to be shielded. Just as with the fuel rods in a shutdown reactor, the SNF produces decay heat because most of the decay radioactivity from the fission products and actinides is deposited in the fuel and converted into thermal energy (aka heat). As a result, the SNF also needs to be cooled, but at a much lower level than fuel in a recently (<12 hours) shutdown reactor as it produces only a fraction of the heat. In summary, the SNF is stored for a certain time to: 1) allow the fuel to cool as its decay heat decreases; and 2) shield the emitted radiation.
To accomplish these goals, SNF is stored in water pools and large casks that use air to cool the fuel rods. The pools are often located near the reactor (in the upper floors of the containment structure for a BWR Mark-1 containment). These pools are very large, often 40 feet deep (or larger depending on the design). The pools are made of thick concrete, lined with stainless steel. SNF assemblies are placed in racks at the bottom of these pools, so almost 30 feet of water covers the top of the SNF assemblies. The assemblies are often separated by plates containing boron which ensure a neutron chain reaction cannot start. The likelihood of such an event is further reduced because the useful uranium in the fuel has been depleted when it was in the reactor, so it is no longer capable of sustaining a chain reaction. The water in the pool is sufficient to cool the SNF, and the heat is rejected through a heat exchanger in the pool so the pool should stay at fairly constant average temperature. The water depth also ensures the radiation emitted from the SNF is shielded to a level where people can safely work around the pools.
Under normal operating circumstances, spent fuel can be stored in the pools indefinitely. An active cooling system is in place to remove the residual decay heat and the water also provides effective radiation shielding. The amount of fuel that can be stored into the pool can vary according to the capacity of the pool itself, but most spent fuel pools are design to be able to store many reactor cores at once.
During the refueling operation the reactor is shut down, all the areas between the reactor and the spent fuel are flooded with water (to provide radiation shielding) and fuel elements are moved one by one from the reactor to the spent fuel pool where they are re-racked. Refueling can occur every 12-18 months and during a single refueling shut down, up to one third of the fuel elements of the core are replaced. All the operations are conducted remotely under water through cranes and special equipment to avoid radiation exposure to the workers.
The spent fuel is usually stored in the spent fuel pool for a number of years, depending on the spent fuel capacity and on regulations, and after that period they are usually dry stored in concrete casks located on the site outside the reactor buildings.
If there is a leak in the pool or the heat exchanger fails, the pool temperature will increase. If this happens for long enough, the water may start to boil. If the boiling persists, the water level in the pool may fall below the top of the SNF, exposing the rods. This can be a problem as the air is not capable of removing enough heat from the SNF so the rods will begin to heat up. If the rods get hot enough, the zirconium-based cladding will oxidize with the steam and air, releasing hydrogen which can then ignite. These events would likely cause the clad to fail, releasing radioactive fission products like iodine, cesium, and strontium. It is important to note that each of these occurrences (cooling system failure, pool water boiling, fuel rod overheating in air, zirconium oxidation reaction) would each have to last sufficiently long in order to cause an accident, making the total likelihood of a serious situation very low.
The most significant danger if such an event were to occur is that there is no robust containment structure (like the one housing the reactor,) surrounding the SNF pool. While SNF pools themselves are very robust structures, the roof above each pool is not as strong and may have been damaged, meaning the surface of the pool may be open to the environment. As long as the water covers the fuel, this does not pose a direct threat to the environment, however it does allow for a possible dispersion of these fission products if a fire were to occur. But if the water level stays above the fuel, the threat of a large dispersion event is low.