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Sunday, March 20, 2011
Lowdown on nuclear crisis and potential scenarios
By ALEX MARTIN
Frantic efforts to cool down the Fukushima No. 1 nuclear plant's overheating reactors and spent fuel rods are continuing, as workers rush to prevent highly toxic radiation from being released into the atmosphere.
Below are questions and answers regarding the stricken reactors and spent fuel pools:
What is the current situation?
There are six reactors at the Fukushima No. 1 plant.
Reactors 1, 2 and 3 were in operation when the massive earthquake and tsunami devastated the Tohoku region on March 11.
The three reactors immediately shut down and control rods were inserted to neutralize them. Reactors 4, 5 and 6 were already shut down for maintenance at the time of the disaster.
Hydrogen explosions have since blown the roof off reactors 1 and 3, and the ceiling of the No. 4 reactor unit burned down during a fire.
Damage to fuel rods is suspected in reactors 1, 2 and 3, while the suppression pool connected to the containment vessel of the No. 2 reactor has apparently cracked following an explosion Tuesday, which also blew a hole in the reactor building.
All six reactor buildings also have spent fuel pools that contain hundreds of spent fuel rods. A single fuel rod is 4 meters long, and is composed of hundreds of small, 1 cm by 1 cm fuel pellets.
While the rods have been withdrawn from the reactors, they remain highly radioactive. The spent fuel pools are located adjacent to the containment vessel, and are not nearly as protected as the reactors themselves, which are carefully designed to contain radiation and prevent leaks.
How are the reactors holding up so far?
Reactors 1, 2 and 3 are presumed to have all been damaged to varying degrees as their water levels receded and temperatures spiked following the quake and tsunami.
At the time of the catastrophe, all reactors except No. 4 were loaded with fuel assemblies, which refer to several dozen uranium-filled fuel rods packed into a bundle, according to the Federation of Electric Power Companies of Japan.
Reactors 1, 2 and 3 contain 400, 548 and 548 fuel assemblies, while No. 5 and No. 6 have 548 and 764 fuel assemblies. The reactor buildings housing the reactors and assemblies are approximately 46 meters tall.
Seawater has been pumped into reactors 1, 2 and 3 to maintain sufficient water levels around the fuel assemblies and prevent the reactors from overheating.
Will electricity be restored?
Work is currently under way to restore electricity to the reactors for restarting the pumps necessary to prevent the fuel rods from overheating. Without electricity to pump in and circulate coolants in the spent fuel pools, experts say there could be a significant radiation risk if the water evaporates and the contents are exposed to the atmosphere.
Reports from Tokyo Electric Power Co., the plant's operator, indicate the electrical power supply to the No. 5 and No. 6 reactors was restored Saturday. Electric cables from outside power sources were connected to reactor 2 on Saturday, and work was also under way to return electricity to 1, 3 and 4, although it was unknown how much time it would take. Tepco hopes to restore the electricity Sunday. Tepco has said there is a possibility the tsunami-damaged water pumps may not work even after power is restored.
What is the situation at the spent fuel pools?
According to Tepco, the six reactors at the Fukushima plant store a total 4,546 fuel assemblies in the spent fuel pools. A typical nuclear reactor uses several hundred fuel assemblies when generating energy.
The spent fuel rods are stored in a square pool approximately 10 meters by 10 meters, and are submerged in about 1,400 tons of water that absorbs radiation and keeps down the temperature released by the fuel.
At present, stored fuel in the pool of Reactor No. 4 is releasing the greatest amount of energy, an estimated 2 million kilocalories per hour, which would increase the temperature of the pool's water by 2 degrees each hour.
Of the 1,331 fuel assemblies stored in the pool of the No. 4 reactor, 548 are only partially consumed and are likely to be generating more heat than the spent fuel.
At this pace, and with the cooling system disabled, it would take roughly 1 1/2 weeks before all water contained in the pool evaporates.
Reactors 1, 2 and 3 store 292, 587 and 514 fuel assemblies in their pools, while the No. 5 and No. 6 pools store 946 and 876 fuel assemblies.
The temperatures in the spent fuel pools at the No. 5 and No. 6 reactors, which at one point rose to nearly 70 degrees, began to drop Saturday as two of the backup diesel generators have been connected and began to cool the fuel pools at both reactors.
Spraying water at the pools in the No. 3 and No. 4 reactors is under way, with efforts temporarily focused on No. 3 after reports indicated there was still a safe amount of water in the pool at No. 4.
However, it is feared that the spent fuel assemblies in the No. 3 reactor's pool may have been damaged after columns of steam were spotted coming from the pool, indicating the water has been evaporating.
What is the worst-case scenario?
The fuel rods that generate energy in nuclear reactors are long rods in a zirconium alloy cladding. If the attempts to cool the reactors fail and the water that covers the fuel rods recedes, exposing the rods, the temperature begins to rise, as is the case with the Fukushima reactors.
If the temperature becomes too great, the fuel rods could melt, releasing highly radioactive fuel, a process more commonly known as a nuclear meltdown.
In the worst-case scenario, the radioactive fuel would burn through the reactor's protective containment layers and release large amounts of radiation into the atmosphere.
Once outside the reactor, the radioactive fuel could seep through the ground until it hits the water table, which would eventually contaminate crops and other produce.
This could prove disastrous as radioactive particles are considered much more dangerous when ingested.