Question:
I have a few questions regarding the nuclear meltdown. First question, has the meltdown happened yet? Second question, how can the nuclear meltdown affect Japan?
Answer:
While details of the damage to the nuclear fuel at the site are not known, it appears that more total fuel damage has occurred during this accident than all previous reactor accidents combined.
The hydrogen explosions and release of radioactivity at Fukushima are evidence of rupture and burning of the fuel cladding in some of the reactors and spent fuel pools. There is speculation about fuel melting and relocating, but this can’t yet be confirmed.
The Fukushima accident has been rated at the highest level (7) of the International Atomic Energy Agency scale used to rank serious accidents; the Chernobyl accident also is rated as a 7.
Each level differs from the previous level by a factor of 10 increase in the amount of radioactive iodine-131 (I-131) released to the environment. Due to the failure of containment structures in the Chernobyl reactor, all of the I-131 released from fuel damaged in that accident went into the atmosphere, while containment in the Fukushima reactors has allowed only a fraction of the I-131 released from the fuel to escape.
The Three Mile Island nuclear accident was rated as a 5 on this scale.
In late March, workers discovered very highly contaminated water in the basements of the turbine buildings and in trenches under the reactor buildings of Units 1, 2, and 3. The amount is estimated at nearly 70,000 tons of water. The water outside the reactor buildings came primarily from a crack in building 2. That water was said to have a very high radiation level—1 sievert/hour at the surface of the water—which is high enough to cause acute radiation sickness after a short exposure. Some of that water flowed into the ocean for a period of at least a week before the leak was fixed. As a result, this water led to significant contamination of the ground around the reactors and of water off the coast.
Status of the Reactors
Currently all reactors and pools are being cooled, although normal cooling systems have not been restored. They will require active cooling for many months or years because of the high levels of radioactivity in the fuel they contain.
Workers are now able to inject fresh water directly into reactors 1, 2, and 3 and into spent fuel pools 1 and 2. However, they are still shooting water toward the spent fuel pools at Units 3 and 4 from a long pipe connected to a truck to keep water in the pools.
Over the past week or two, workers have been pumping nitrogen into the primary containment vessel of Unit 1. This concern about the presence of hydrogen implies there is also concern that fuel damage is still occurring. This measure is also being considered for the reactors at Units 2 and 3, but Unit 1 has had a higher temperature and pressure and is getting attention first.
From NY Times
Reactor 1
There was a partial meltdown of the reactor’s fuel assemblies (about 55 percent of the fuel was damaged, according to latest estimates) and radioactive materials have leaked into the environment, in large but unknown quantities. The steel reactor core may have been breached by molten fuel.
Reactor 2
There was a partial meltdown of the reactor’s fuel assemblies (about 35 percent was damaged, according to the latest estimates) and molten fuel may have breached the reactor’s steel core. An explosion has damaged part of the primary containment vessel around the core, allowing large amounts of highly radioactive water used to cool the reactor to leak out.
Reactor 3
The reactor used uranium and plutonium, which produces more toxic radioactivity. There was a partial meltdown of the reactor’s fuel assemblies (about 30 percent was damaged, according to the latest estimates) and the reactor containment vessel may have been damaged. The spent fuel pool may also have become uncovered.
Reactor 4
The reactor was empty at the time of the earthquake, but the fuel was in a spent fuel pool that may have been uncovered, causing a partial meltdown and the release of radioactive materials. An explosion and fire have damaged the building.
Reactor 5 and 6
The reactor is shut down and the building is not damaged. As power has been restored, concern about that this part of the facility has abated.
For more info, check links.
http://allthingsnuclear.org/post/5173479…
