SOURCE: IAEA

Radiation Dose Rates Observed at the Site

The Japanese authorities have informed the IAEA that the following radiation dose rates have been observed on site at the main gate of the Fukushima Daiichi Nuclear Power Plant.

At 00:00 UTC on 15 March a dose rate of 11.9 millisieverts (mSv) per hour was observed. Six hours later, at 06:00 UTC on 15 March a dose rate of 0.6 millisieverts (mSv) per hour was observed.

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. The IAEA continues to confirm the evolution and value of this dose rate. It should be noted that because of this detected value, non-indispensible staff was evacuated from the plant, in line with the Emergency Response Plan, and that the population around the plant is already evacuated.

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.

Evacuation of the population from the 20 kilometre zone is continuing. The Japanese have asked that residents out to a 30 km radius to take shelter indoors. Japanese authorities have distributed iodine tablets to the evacuation centres but no decision has yet been taken on their administration.

Background on Radiation

A person’s radiation exposure due to all natural sources amounts on average to about 2.4 millisievert (mSv) per year. A sievert (Sv) is a unit of effective dose of radiation. Depending on geographical location, this figure can vary by several hundred percent.

Since one sievert is a large quantity, radiation doses are typically expressed in millisievert (mSv) or microsievert (µSv), which is one-thousandth or one millionth of a sievert. For example, one chest X-ray will give about 0.2 mSv of radiation dose.

Source: New York Times – The warnings were stark and issued repeatedly as far back as 1972: If the cooling systems ever failed at a “Mark 1” nuclear reactor, the primary containment vessel surrounding the reactor would probably burst as the fuel rods inside overheated. Dangerous radiation would spew into the environment.

Now, with one Mark 1 containment vessel damaged at the embattled Fukushima Daiichi nuclear plant and other vessels there under severe strain, the weaknesses of the design — developed in the 1960s by General Electric — could be contributing to the unfolding catastrophe.

G.E. began making the Mark 1 boiling-water reactors in the 1960s, marketing them as cheaper and easier to build — in part because they used a comparatively smaller and less expensive containment structure.

Source: ABC News -Fukushima: Mark 1 Nuclear Reactor Design Caused GE Scientist To Quit In Protest

Thirty-five years ago, Dale G. Bridenbaugh and two of his colleagues at General Electric resigned from their jobs after becoming increasingly convinced that the nuclear reactor design they were reviewing — the Mark 1 — was so flawed it could lead to a devastating accident.

Questions persisted for decades about the ability of the Mark 1 to handle the immense pressures that would result if the reactor lost cooling power, and today that design is being put to the ultimate test in Japan. Five of the six reactors at the Fukushima Daiichi plant, which has been wracked since Friday’s earthquake with explosions and radiation leaks, are Mark 1s.

“The problems we identified in 1975 were that, in doing the design of the containment, they did not take into account the dynamic loads that could be experienced with a loss of coolant,” Bridenbaugh told ABC News in an interview. “The impact loads the containment would receive by this very rapid release of energy could tear the containment apart and create an uncontrolled release.”