These observations indicate that the level of radioactivity has been decreasing at the site.
As reported earlier, a 400 millisieverts (mSv) per hour radiation dose observed at Fukushima Daiichi occurred between units 3 and 4. This is a high dose-level value, but it is a local value at a single location and at a certain point in time.
World Nuclear News – A close watch is being kept on the radiation levels to ascertain the status of containment. As a precaution Tokyo Electric Power Company has evacuated all non-essential personnel from the unit. Fifty of the company’s engineers remain to pump seawater into the reactor pressure vessel in an effort to cool it.
A message recieved from Tepco at about 5pm said the primary containment vessel around the reactor and secondary containment provided by the reactor building “show no significant change.”
The International Atomic Energy Agency did confirm that the fire had taken place in the used fuel storage pool. The Japan Atomic Industry Forum’s status report said the water was being supplied to make up for low levels.
Similar to the need to cool fuel in the reactor core, used fuel assemblies in cooling ponds require a covering of water to remove decay heat. The main differences being the amount of decay heat to be removed decreases exponentially with time and that fuel ponds are much less of an enclosed space than a reactor vessel. At the same time, ponds may contain several years of fuel.
JAIF reported that temperatures in the cooling ponds at units 5 and 6 are increasing, but the reason for this is not yet available.
About 150 persons from populations around the Daiichi site have received monitoring for radiation levels. The results of measurements on some of these people have been reported and measures to decontaminate 23 of them have been taken. The IAEA will continue to monitor these developments.