科学技術システム研究室
 Science and Technology System Laboratory
慶應義塾大学大学院 システムデザイン・マネジメント研究科
GRADUATE SCHOOL OF SYSTEM DESIGN AND MANAGEMENT

Radioactive Iodine detected in Tokyo Water


Tokyo Metropolitan Gvt. told Tokyo citizens not to give babies water containing iodine with radioactivity of 210 Bq/kg-water. This was detected at 9:00, 23 March at the water supply plant at Kanamachi.

Do not worry


It is not a fear, because half-life time of radioactive iodine is as short as 8 days. Also detected radioactive level is so low.

Radioactive iodine is produced in a N-power reactor as a fission product. It decayed in to stable xenon-131, emitting beta-line (electron beam).

Capability of emitting beta-line is degraded by half in almost 8 days. It disappears immediately. So, this is not a threat.




Relation between N-reactor accident


Where does iodine-131 come?

Iodine-131 was produced during atomic reaction in the N-power station. While cooling capability had been lost, fuel skin made of Zr-alloy was reacted with water, and then hydrogen was produced and leaked into the building. Also, iodine-131 was estimated to be with hydrogen.

On explosion iodine-131 was emitted from the building and transported to air. This iodine-dust was solved into rain and fell down to the water source area of Tokyo Metro. Gov., i.e. along the Tonegawa-river and so on.


Let me remind you again. N-fuel is now contained in a pressurized vessel, a container and a container made of concrete and is cooled by water. Fission will no more take place, as long as it is water cooled. Radiation detected around the reactor shows stable level.


Also water was supplied into a container for used fuel. Situation will not be worse.




What does radioactivity level of 210 Bq/kg mean?


Tokyo Metropolitan Gov. detected radioactivity level of 210 Bq/kg. Is it dangerous or safe?


People enjoy bathing at a radioactive hot spring, such as Masutomi hot spring. According to data book "Rikanenpyou" edited annually by the National Astronomical Observatory of Japan radioactivity level is 11000 Bq/kg-water.


People enjoy bathing and even drink for his/her health. This is as stronger as that detected in this morning at the Kanamachi water supply plant.


For further learning radioactive iodine-131, see the following site: http://en.wikipedia.org/wiki/Iodine-131.




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