16 Mar (NucNet): Figures from a radiation monitoring post near the main gate of the Fukushima-Daiichi nuclear power plant show the latest reading was 1,900 microsieverts per hour (microSv/hr) at 11:20 Japan time (03:20 central European time; CET) and that there was a peak indication of 6,400 microSv/hr at 10:45 Japan time.
Toshihiro Bannai of Japan’s Nuclear and Industrial Safety Agency (NISA) gave the figures to NucNet in a telephone call at midday CET.
In most countries, the natural background radiation level is in the range of 0.2 to 0.5 microSv/hr, or about 2 to 4 millisieverts per year (mSv/yr).
The annual dose limit for controlled nuclear workers or medical personnel is 20 mSv/yr and for the general public 1 mSv/yr. This means if a person had remained at the Fukushima-Daiichi site boundary without any protection today between 10:00 and noon, the annual dose limit would have been exceeded in less than half an hour.
Mr Bannai also said authorities are taking every measure to keep the temperature low of spent fuel in the storage pool at unit 4. He said fire engines had been brought in to help pump water containing boric acid over the spent fuel.
He said the spent fuel pool had not reached “a critical configuration” and the last temperature reading from the pool was 84 degrees Celsius at 0400 local time on March 14.
Earlier, small fires had broken out inside the reactor building at unit 4 where damaged fuel in the spent fuel pool was leading to the uncontrolled release of radioactive substances.
The Japan Atomic Industrial Forum (JAIF) said reactor building integrity at unit 4 was “severely damaged”, but primary containment vessel integrity was not affected. Unit 4 was undergoing a scheduled inspection and was already shut down when Friday’s earthquake struck.
Units 1, 2 and 3 at the plant are still being cooled using seawater. JAIF said core and fuel integrity is damaged at all three units while containment integrity damage is suspected at units 2 and 3.
At Units 5 and 6, which were already shut down at the time of the quake, the temperature of spent fuel in the fuel pools has been increasing, according to JAIF. Measures such as ventilating the reactor building to prevent the accumulation of hydrogen – which is what led to earlier explosions in units 1, 2 and 3 – are being taken.
>>Related reports in the NucNet database (available to subscribers)
Authorities Order Evacuation Around Fukushima-Daiichi Nuclear Plant (News in Brief No. 52, 11 March 2011)
EC Calls For Information On Back-Up Power Systems At European NPPs (News In Brief No. 66, 15 March 2011)
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