From the NEI website
Fact Sheet
Used Nuclear Fuel Storage at the Fukushima Daiichi Nuclear Power Plant
(Last updated 3/15/11)
Key Facts
Used nuclear fuel at the Fukushima Daiichi plant is stored in seven pools (one at each of the six reactors, plus a shared pool) and in a dry container storage facility (containing nine casks).
Sixty percent of the used fuel on site is stored in the shared pool, in a building separated from the reactor buildings; 34 percent of the used fuel is distributed between the six reactor fuel storage pools, and the remaining six percent is stored in the nine dry storage containers. There are no safety concerns regarding the used fuel in dry storage at Fukushima Daiichi.
Used fuel pools are robust concrete and steel structures designed to protect the fuel from even the most severe events. Pools are designed with systems to maintain the temperature and water levels sufficient to provide cooling and radiation shielding.
The water level in a used fuel pool typically is 16 feet or more above the top of the fuel assemblies.
The used fuel pools at the Fukushima Daiichi reactors are located at the top of the reactor buildings for ease of handling during refueling operations.
The used fuel pools are designed so that the water in the pool cannot drain down as a result of damage to the piping or cooling systems. The pools do not have drains in the sides or the floor of the pool structure. The only way to rapidly drain down the pool is if there is structural damage to the walls or the floor.
What Could Happen During an Accident?
The systems that cool and maintain water levels in the pools are designed to withstand severe events. If these systems are unable to function, the heat generated by the used fuel would result in a slow increase in the temperature of the spent fuel pool water. The operating temperature of the pools is typically around 40 degrees C or 100 degrees F (the boiling point for water is 100 C or 212 F). This slow increase in temperature would result in an increased evaporation rate. Rapid evaporation of the water will not occur.
Exact evaporation rates would depend on the amount of used fuel in the pool and how long it has cooled. The rate at which the pool water level would decrease (due to evaporation or mild boiling) in the absence of cooling system function would not be expected to lower water levels by more than a few percent per day. Given that there is approximately 16 feet or more of water above the used fuel assemblies, operators would have ample time (days to weeks) to find another way to add water to the pools before the fuel would become exposed. For example, water could easily be added using a fire hose.
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If the water level decreases below the top of the fuel assembly, oxidation of the zirconium cladding could occur. This oxidation could result in some hydrogen generation. However, only the fuel assemblies with the least cooling time (on the order of weeks after discharge from the reactor) would be susceptible to this oxidation. The temperature of the fuel assemblies decreases exponentially with cooling time. The rate of hydrogen generation depends on the temperature of the fuel assembly, with hotter temperatures leading to higher gas generation rates. However, the temperature of the cladding must rise to approximately 1,000 C before significant hydrogen generation rate occurs. This is extremely unlikely to occur after as little as 120 days (16 weeks) of cooling. As a reference, the melting point of zirconium is approximately 1,800 degrees C.
Even if the water level in the pools was to decrease sufficiently so that the fuel were exposed to air, the same level of overheating that can occur in a reactor accident would not occur in the used fuel pool because the used fuel assemblies in the pool are cooler than in the reactor. It is highly unlikely that used fuel temperatures could reach the point where melting could occur, although some damage to the cladding cannot be ruled out. The likelihood of cladding damage, as with hydrogen generation, decreases substantially with temperature and cooling time.
There has been some speculation that, if the used fuel pool were completely drained, the zirconium cladding might ignite and a “zirconium fire” might occur. Studies performed by the Department of Energy indicate that is virtually impossible to ignite zirconium tubing.
At the surface of the used fuel pool, the gamma dose rate from radiation emanating off the used fuel assemblies is typically less than 2 millirem per hour. If the water level decreases, gamma radiation levels would increase substantially. This increase would be noticed at the radiation monitors near the reactor buildings.
I would suspect that there is some structual damage to the spent fuel pools. I don't believe they have caught on fire at this point. However, the only explanation of the rumored excessive levels is that the spent fuel is somehow, partially exposed.
