12 Mar (NucNet): Radioactivity at the boundary of the Fukushima-Daiichi nuclear power plant in northern Japan has exceeded statutory limits and the incident has been rated as level 4 on the International Atomic Energy Agency’ International Nuclear and Radiological Event Scale (INES).*
The Japanese authorities did not give any radiation measurements in their INES report to the IAEA, but plant operator Tokyo Electric Power Co.
(Tepco) said earlier that radiation levels next to the machine building have increased from 0.07 microsieverts per hour (microSv/hr) to 6.7 microSv/hr and around the building measurement have shown an increase from 0.07 microSv/hr to 5.3 microSv/hr. Normal background radiation levels would be 0.07 microSv/hr.
Tepco confirmed that it has successfully vented the containment of unit 1 at Fukushima-Daiichi and was preparing to vent units 2 and 3, which also shut down automatically when the 8.9-magnitude earthquake hit yesterday.
Venting is designed to reduce pressure in the containment. It is not yet known why the pressure increased.
Noriyuki Shikata, deputy cabinet secretary for public relations and director of global communications at the prime minister’s office, said Tepco’s efforts to depressurise unit 1’s containment was successful. “Additional measures are now being taken tonight using sea water and boric acid,” he said, commenting on methods being used to cool the plant’s reactors and containments.
He said an explosion earlier today at the plant was caused by accumulated hydrogen combined with oxygen in the space between containment and outer structure. He said there was no damage to the containment.
* 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.
INES explains the significance of events from a range of activities, including industrial and medical use of radiation sources, operations at nuclear facilities and transport of radioactive material.
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
>Related reports in the NucNet database (available to subscribers)
Japanese Safety Agency Reports ‘No Incidents So Far’ Following Quake (News in Brief No. 50, 11 March 2011)
Tepco Confirms Venting Of Unit 1, ‘Reactor Not Affected’ By Explosion (News in Brief No. 55, 12 March 2011)
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