THE NUCLEAR COMMUNICATIONS NETWORK
Japan Update / Brief No 72 / 18 March 2011
Radiation Levels Above Normal, But Falling, Say Authorities
18 Mar (NucNet): Radiation levels on the site and at the site boundary of the Fukushima-Daiichi nuclear power plant in Japan are slowly falling, but are still far above normal levels, according to figures from Tokyo Electric Power Company (Tepco) and Japan’s Nuclear and Industrial Safety Agency (NISA).
At a monitoring post near one of the gates, readings have fallen from 310 microsieverts per hour (microSv/hr) yesterday to 264 microSv/hr (at 12:00 Japan time on 18 March), they said.
Readings from a mobile monitoring post on the plant site have not been updated today, but final readings from yesterday (17 March) at 20:00 Japan time showed 3,600 microSv/hr.
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.
Unit Status: 18 March 06:00 Japan Time
Unit 1: Cooling with seawater continues.
Unit 2: Cooling with seawater continues. White smoke has been discharged from the reactor building, where damage is suspected to the containment vessel. The cause of the smoke has not been confirmed.
Unit 3: Cooling with seawater continues. After dropping water four times from helicopters yesterday, five tanks of water have been injected from the ground to the spent fuel pool sector using a water-cannon truck. The results are being evaluated.
Unit 4: Work continues to stabilise the reactor, which was shut down at the time of the earthquake and has no fuel in it. Work also continues to stabilise the spent fuel pool, which is almost full of spent fuel. The plan is to fill this pool with cooling water from water cannon and from the unit’s normal pumps as soon as power is re-established onsite. The International Atomic Energy Agency (IAEA) said the temperature of the spent fuel at unit 4 was 84 degrees Celsius on 14 March at 04:08 Japan time.
There are no newer readings.
Units 5 and 6: The status of these reactors and of their fuel pools remains stable as long as the emergency power diesel engine for unit 6 – the only one that is working – remains operational and supplies power to the pool cooling system and essential water cooling components. Tepco said it hopes the situation can be improved with restoration of the plant’s main external power supply. The IAEA said the temperature of the fuel in the unit 5 fuel pool was 65.5 degrees Celsius at 03:00 Japan time on 18 March.
At unit 6 it was 62 degrees Celsius.
External power supply: A connection has been laid from the Tohoku high-voltage line to the site and at 06:30 Japan time today it was being checked. Final connection must wait until water injection can be interrupted. The IAEA said engineers had also begun to lay an external grid power line cable to unit 2 at the plant. The operation was continuing as of 05:30 Japan time. They plan to reconnect power to unit 2 once the spraying of water on the unit 3 reactor building is completed.
The latest status update from the Japan Atomic Industrial Forum is
online:
>>Related reports in the NucNet database (available to subscribers)
THE NUCLEAR COMMUNICATIONS NETWORK
Japan Update / Brief No. 73 / 18 March 2011
Fukushima-Daiichi Events Provisionally Rated INES Level 5
18 Mar (NucNet): The reactor core damage and loss of all cooling function at units 3 and 4 of the Fukushima-Daiichi nuclear power plant, and the abnormal rise of radiation dose rates at the plant site boundary due to releases by unit 1 following an earthquake and tsunami on 11 March 2011, have been provisionally rated at Level 5 on the International Atomic Energy Agency’s International Nuclear and Radiological Event Scale (INES).*
Level 5 on the scale means an “accident with wider consequences”.
The event at unit 4, where there was loss of coolant to the spent fuel pool, has provisionally been rated as Level 3, which means “serious incident”.
Incidents at units 1, 2 and 4 of the Fukushima-Daini nuclear plant have provisionally been rated as Level 3.
In a report to the IAEA on units 1, 2 and 3 at Fukushima-Daiichi, Japan’s Nuclear and Industrial Safety Agency (NISA) said electricity rooms and seawater pump rooms were flooded when the tsunami struck, and the only cooling function available was from a turbine driven pump.
The operation of this pump increased the pressure in the reactor’s suppression chamber and the pump had to be stopped. This resulted in the loss of all cooling function and the declaration of a state of emergency, the report says.
In an effort to cool units 1, 2, and 3, seawater was injected into the reactor vessels using fire pumps. The gas in the containment vessels was also vented. When venting the containments, the radiation dose rate at the boundary level of the site exceeded the limit of 0.5 millisievert per hour, and for this reason the initial Level 4 classification for unit 1 was revised upwards.
The report says the explosions at the reactors on 12 to 14 March were believed to have been caused by hydrogen gas.
It adds that the behaviour of the pressure of the reactor vessel and the containment vessel, and the behaviour of the rector vessel’s water level, were “complicated” and some measurements could not be taken because of failures of measurement equipment.
* The International Nuclear and Radiological Event Scale
The International Nuclear and Radiological Event Scale (INES) was developed by the IAEA and the OECD in 1990 to better communicate and standardise the reporting of nuclear incidents or accidents to the public.
Events are classified on the scale at seven levels: Levels 1–3 are called “incidents” and Levels 4–7 “accidents”. The scale is designed so that the severity of an event is about 10 times greater for each increase in level on the scale. Events without safety significance are called “deviations”
and are classified Below Scale / Level 0.
Chernobyl rated as 7 (Major Accident) on the scale and Three Mile Island rated 5 (Accident with Wider Consequences).
7 Major Accident
6 Serious Accident
5 Accident With Wider Consequences
4 Accident with Local Consequences
3 Serious Incident
2 Incident
1 Anomaly
0 Below Scale/No Safety Significance
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