THE NUCLEAR COMMUNICATIONS NETWORK
Japan Update / Brief No. 68 / 16 March 2011
Efforts Continue To Cool Spent Fuel At Unit 4
16 Mar (NucNet): Small fires have broken out inside the reactor building at unit 4 of the Fukushima-Daiichi nuclear power plant in Japan, where damaged fuel in the spent fuel pool is 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.
JAIF said the level of cooling water in the unit 4 spent fuel pool was low and workers were preparing to inject water in an effort to keep the fuel cool.
Plant operator Tokyo Electric Power Company (Tepco) said it was keeping the area “under surveillance”.
The International Atomic Energy Agency (IAEA) said there had been two fires. The latest was seen at 21:45 central European time (CET) on 15 March, but as of 22:15 CET of the same day, “the fire could no longer be observed”, a statement said.
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.
There have been uncontrolled releases of radioactivity at unit 2 because the primary containment vessel is damaged in the suppression pool sector.
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 is 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.
Radiation Measurements
Tepco has published readings from two monitoring posts near the plant’s main gate, but there have been marked fluctuations depending on events such as controlled venting of containments and the prevailing wind direction.
On 13 March, measurements were about 20 to 120 microsieverts per hour
(microSv/hr) on 13 March; about 40 to 160 microSv/hr on 14 March; about 60 microSv/hr on 15 March.
However, according to the government’s chief cabinet secretary the values have now increased and reached more than 1,000 microSv/hr today at 10:00 local Japan time.
On the plant site itself, various monitoring posts and mobile measuring equipment produced much higher levels with a peak of 400 millisieverts per hour (mSv/hr) between units 3 and 4 on 15 March.
According to samples taken outside the plant site, contamination is not at an unacceptable or alarming level, and the 20 km evacuation zone remains in force together with an order for people living between 20 and 30 km from the plant to stay indoors.
Notes on Radiation
In most countries, the natural background radiation level is in the range of 0.2 to 0.5 microSv/hr (including the natural radon background radiation in buildings) 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, but may reach 50 mSv in an exceptional year when the 5-year average is not higher than 20 mSv, and 100 mSv in case of declared emergencies according to recommendations by the International Commission on Radiological Protection.
THE NUCLEAR COMMUNICATIONS NETWORK
Japan Update / Brief No. 69 / 16 March 2011
NISA Releases Details Of Fukushima-Daiichi Radiation Readings
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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