Here is a great animation on what exactly is going on in the Fukushima Daiichi power plant.
More on the leaking radiation.
Compared to Chernobyl
In favor of Nuclear Energy
Exposure to Iodine-129 and Iodine-131
How do iodine-129 and iodine-131 get into the environment?
Iodine-129 and iodine-131 are gaseous fission products that form within fuel rods as they fission. Unless reactor chemistry is carefully controlled, they can build up too fast, increasing pressure and causing corrosion in the rods. As the rods age, cracks or wholes may breach the rods.Cracked rods can release radioactive iodine into the water that surrounds and cools the fuel rods. There, it circulates with the cooling water throughout the system, ending up in the airborne, liquid, and solid wastes from the reactor. From time to time, reactor gas capture systems release gases, including iodine, to the environment under applicable regulations.
Anywhere spent nuclear fuel is handled, there is a chance that iodine-129 and iodine-131 will escape into the environment. Nuclear fuel reprocessing plants dissolve the spent fuel rods in strong acids to recover plutonium and other valuable materials. In the process, they also release iodine-129 and -131 into the airborne, liquid, and solid waste processing systems. In the U.S., spent nuclear fuel is no longer reprocessed, because of concerns about nuclear weapons proliferation.
Currently, spent nuclear fuel remains in temporary storage at nuclear power plants around the country. If the nuclear waste repository at Yucca Mountain opens, it will provide permanent disposal for spent nuclear fuel and other high-level radioactive wastes. Wherever spent nuclear fuel is stored, the short-lived iodine-131 it contains will decay away quickly and completely. However, the long-lived iodine-129 will remain for millions of years. Keeping it from leaking into the environment, requires carefully designed, long-term safeguards.
The detonation of nuclear weapons also releases iodine-129 into the environment. Atmospheric testing in the 1950's and 60's released radioactive iodine to the atmosphere which has disseminated around the world, and is now found at very low levels in the environment. Most I-129 in the environment came from weapons testing.
http://www.epa.gov/radiation/radionuclides/iodine.html#use
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