In the last article I explained what happen to uranium inside the nuclear reactors (I advise that you read that first). Now i want to close this topic with an article that explain what is usually happening to the nuclear wastes when they are removed from reactor. As we already saw, the majority of nuclear power plants use uranium 235 as fissile fuel, and requires uranium to be enriched to 3%-5% before it can be used in the slow neutron reactors. The enriched uranium are “sold” in a form of small cylinder of 1 cm in diameter x 1 cm long called uranium pellets and are covered by a Zirconium cladding for protection. This pellet is then assembled in Rods (can be several meters long) and rods are assembled into fuel assemblies, that are going to be inserted into the reactor itself. Inside a core there could be 100-200 fuel assembly each consisting of several hundreds rods, depending of reactor type. Fuel rod are alternated with control bars (silver, cadmium, etc), and in some cases (depending from the reactor) with moderators bars such as graphite (however usually is used water as moderator). By inserting or removing the bars it is possible to control the reaction speed.
So when the core is shut down (EG SCRAM), there is still heat produced by the beta decay of the fission particles. As soon as the core is shut down, this residual heat decay is around 7% of the one with reactor was on, and after one week is only around 0.2%.
So, despite they are not so hot as when in the reactor, spent nuclear fuel needs to be cooled down for a period of time. Because they need cooling and because they are high radioactive, the best place to keep these rods is under water. So, each Nuclear power plant, has spent fuel pools, usually 10-15m deep and on the bottom of these pools are placed the spent fuel rods. This will make sure that the rods are properly cooled and that the radiation are shielded. Boron walls are placed in between the rods to reduce the probability that the fission reaction restart. The rods can be stoked in the pool for decades, and these pools usually have space for many times the reactor full fuel load.
So every 12-18 months, around 1/3 of the fuel rods are considered spent, and thus removed from the reactor and placed into the spent fuel pool. So in average a fuel rod stay in a core for 36-54 months. Moving the rod from/to the pool is done remotely to reduce the contamination risk. After enough time the spent fuel is moved from the pool to concrete rooms where are left to cool down in air.
So as far as the rods are under water, they do not create any problem. Indeed is also possible to swim in the spent pool; is enough that you do not get too close to the rods where you will die within minutes. But the surface is safe and because the temperature is kept to a comfortable 30 to 50 degree Celsius it will also be a pleasant swim!. Remember to keep your mouth closed and avoid drinking that water, because there could be dissolved radioactive particles in it.
The problem arise when, for some reason, the cooling system does not work anymore. Every single major incident in nuclear power plants has happened because failure in cooling systems.
So in case that the spent fuel pool does not get cooled anymore, there is a chance that the water will boil down and expose the rods to the air. This will increase the rod temperature even more, and it will most likely burn off the zirconium cladding covering the uranium pellet. This oxidation will release hydrogen that can lead to explosions and dispersion of radioactive material in the air. So during nuclear incident is vital to keep the spent fuel covered with water.
As last i want to close with a curiosity.. not sure if you are aware but Chernobyl power plant continued to run for 14 years after the incident, and was completely shut down in year 2000.
So, is very important to stay cool!