Because of their high radioactivity, fuel rods continue to produce very significant heat even after they are no longer useful for generating electricity and are removed from the reactor core. Such “spent fuel” rods need to be continually cooled for many years to prevent them from heating to a level where they would suffer damage.
To cool the rods after they are removed from the reactor core, they are placed on racks in a spent fuel pool that circulates cooled water around them. This water is circulated by pumps that are run using electricity from the power grid. Typically these pumps do not have backup power from deisel generators or batteries, so if power from the grid is interrupted, as it is in the case of the Japanese earthquake, they will stop operating.
Once the cooling pumps stop, the water in the spent fuel pools will begin to heat up and will eventually start to boil off. The pools are typically 45 feet deep with the fuel rods stored in the lower 15 feet of the pool, so 30 feet of water would have to boil off before exposing the rods. That could take several days, so this issue may only be appearing now.
The pool at Fukushima Dai-Ichi Unit 4 is a particular problem since the fuel rods in it were only removed from the reactor core during a refueling of the reactor in December 2010. Therefore, they still have a very high level of radiation and are generating more heat than the spent fuel at the other reactors at the Dai-Ichi site.
The spent fuel pools for the Dai-Ichi reactors are located on an upper floor in the reactor building (see red circle in figure). The pools are outside the primary containment of the reactor, but inside the reactor building, which is the secondary containment. Typically any radioactivity released from the pool will be contained by the reactor building, which is maintained at less than atmospheric pressure so air flows into the building rather than out. The air in the building is filtered to remove the radioactivity before it is released outside.
As the water in the pool heats up and evaporates, the vapor will carry some radiation with it. This includes tritium and radioactive particles in the water.
If the water level in the pool drops low enough to expose the spent fuel, the fuel rods can suffer the same kinds of damage as fuel in the reactor core that is expose. If only a small length of the rods is exposed, they will get hot enough to create steam, but the steam flowing along the exposed surface of the rod will cool it enough that the rod’s cladding will not reach the high temperature required to react with the steam and create hydrogen.
Once the water has dropped low enough to expose several feet of the length of the fuel rods, they can become hot enough that the zirconium cladding of the rods will react with the steam and release hydrogen.
Tokyo Electric Power Company (TEPCO) has said there was a hydrogen explosion that damaged the Unit 4 reactor building on Tuesday morning in Japan (Monday afternoon U.S. time), reportedly blowing a 26-foot wide hole in the side of the building. If this explosion was due to hydrogen, that hydrogen very likely came from the spent fuel since there is no other clear source (this reactor was not operating when the earthquake hit). And if the spent fuel produced the hydrogen, that indicates that the water level in the spent fuel pool must have been low enough to have exposed a significant fraction of the fuel rods.
Shortly after that—at 9:38 am Tuesday (8:38 pm EDT Monday)—TEPCO discovered a fire on the fourth floor of the building in the spent fuel pool, which reportedly burned for three hours. That fire may have been the oxidation of the zirconium cladding, as it was continuing to produce hydrogen.
The other effect of heat damage to the fuel rods is that radioactive gases such as iodine-131 and cesium-137, which are produced in the fuel during the operation of the reactor, can be released. The hole in the secondary containment at Unit 4 means that any emissions from the spent fuel will be vented directed to the outside.
If water cannot be added to the pool, or if the pool has been damaged and is leaking, the fuel may remain uncovered. The exposed fuel can get hot enough to melt, depending on how long it has been out of the reactor. If the fuel melts, it would release significant additional radioactivity into the air.
This same scenario could occur at Units 5 and 6 if the water in the spent fuel pools is not replenished, although the fuel there has apparently been in the pools longer and is not as radioactive as at Unit 4. For rods that have been in the pool for long enough, their decay heat will have dropped sufficiently that they will not undergo the same rapid oxidation as newer fuel rods will, and would not produce as much hydrogen.
Thus, depending on the age of the spent fuel in Units 5 and 6, there may be less hydrogen produced if the water level in the spent fuel pool drops. There may still be enough heat to damage the fuel and release radioactive gases, but if the secondary containment is not damaged by a hydrogen explosion, that gas may not be released to the atmosphere.
If mechanisms to fill the pool at Unit 4 are broken, or if there is a need to repair the pool, it will be difficult to get workers close enough to do this. If spent fuel has been in the pool for a relatively short time, even if the water level is at the top of the fuel rods, the radiation dose to someone at the railing of the pool would give them a lethal dose in well under a minute. This would explain why there have been reports of requests to use helicopters to deliver water to the pools. However, it appears that this is not a practical way of delivering water.
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