The Fukushima nuclear incident in Japan was upgraded on April 12, 2011, to an INES (International Nuclear Events Scale) Level 7. Several journalists contacted us with questions about this rating, and here a local expert answers those questions.
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Tony Irwin is a visiting lecturer in nuclear technology at The Australian National University and the University of Sydney.
He worked for British Energy in the UK commissioning and operating 8 nuclear power plants and following the Chernobyl accident he helped review operating practices at Russian reactors. In 1999 he joined the Australian Nuclear Science and Technology Organisation (ANSTO) and was Reactor Manager during construction and operation of the OPAL research reactor. After retiring from ANSTO in late 2009, he became a visiting lecturer for Masters Courses in Nuclear Technology at the ANU and the University of Sydney.
“The increase in level of this event from INES level 5 to level 7 does not meant the event is getting worse! This upgrade is not a reflection of the current situation.
In fact, the situation has been improving since about 15 March.
The reason for the increase in level is that one of a series of criteria defined in the IAEA INES Manual and used by Nuclear regulators worldwide to determine an event level has reached the level 7 figure. This is the criteria for the activity released. The level 7 definition is:
” An event resulting in an environmental release corresponding to a quantity of radioactivity radiologically equivalent to a release to the atmosphere of more than several tens of thousands of terabecquerels of I-131 “. For consistent interpretation of the criteria internationally, the IAEA suggests 50,000 TBq (50 PBq) * for level 7.
The Japan Nuclear regulator (NISA) and the Nuclear Safety Commission of Japan (NSC) have both estimated the amount discharged. Since the highest estimate is 630 PBq, which is above the 50 PBq criteria, the event is now rated as level 7.
Level 7 is the highest rating on the INES scale and puts the Fukushima Daiichi Nuclear Event on the same scale as the Chernobyl event. However there are significant differences between the two events:
- The estimated release is currently one tenth of the Chernobyl discharge
- The Chernobyl release was as the result of an explosion that discharged radioactive material 15km high and the plume spread across Europe. The discharges to air at Fukushima have been from controlled venting of the containment with mainly local effects.
- The Chernobyl reactor was totally destroyed. The Fukushima reactors still have their primary containment structures.
- 43 workers died at Chernobyl as the result of very high radiation doses. There have been no worker deaths from radiation at Fukushima.
- Conservative action was taken to evacuate the area around Fukushima early so that there has not been public exposure to dangerous levels, unlike Chernobyl.
The INES level 7 is technically correct, but the health impacts of Chernobyl were much worse.
The IAEA may want to refine their INES scale following this accident.
* 1 Tera Bq (TBq) = 1012 Becquerels
1 Peta Bq(PBq) = 1015 Becquerels
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Dr Pradip Deb is a Senior Lecturer in Medical Radiations at the School of Medicals Sciences, RMIT University
“The increase of the level of Fukushima incident to level 7 is combined re-classification of the earlier individual assessments of the different level of incidents of different reactors. This does not mean that it will do any extra harm to the public life. Although this level 7 is the same level as Chernobyl incident, the Japanese Nuclear and Industrial Safety Agency estimated that the amount of radioactive material released in Fukushima is ninety percent less than Chernobyl.
The International Nuclear and Radiological Event Scale (INES) is a scale of nuclear incident and accidents to report to the public like earthquake scales.
Level 1: Anomaly. This level is when minor problems with safety components, breach of operating limits at a nuclear facility, loss or theft of low activity radioactive sources.
Level 2: Incident. 10 times higher than Level-1. Exposure rate more than 50 mSv/hour with significant contamination within the facility.
Level 3: Serious Incident. 10 times higher than Level-2. Exposure rate is more than 1 Sv/hour in an operating area with severe contamination. Low probability of significant public exposure.
Level 4: Accident with local consequences. 10 times higher than level-3. Fuel melt or damaged and release of significant quantities of radioactive material within an installation with high probability of public exposure. Level-4 is called for if at least one death from radiation and minor release of radioactive material so only local food controls are necessary.
Level 5: Accident with Wider Consequences. This level is 10 times higher than level-4 and is called for when the reactor core is severely
damaged and large amount of radioactive materials are released with a high probability of significant radiation exposure to the public. And also when several deaths from radiation exposure and planned radiation control is needed.
Level 6: Serious Accident. 10 times higher than level-5. This level is declared when
significant amount of radioactive material is released in the environment and planned controlling procedures need to be taken.
Level 7: Major Accident. This is 10 times higher than level-6 and the highest level of accident. This is declared when significant amount of radioactive material is released into the environment which can affect public health. Implementation of planned and extended radiation safety programs are needed at this level.
Level-5 is the highest contamination level as was declared earlier. Now after accumulating the total release of radioactive materials in this accident the level is re-classified as the highest level as a one whole accident. Due to yesterdays 7 scale earthquake, there was no extra radioactivity. The radiation level near the reactors are decreasing as it should be. So this new classification (level-7) should not add extra impact.”
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Dr John Price is a former member of the Safety Policy Unit of the National Nuclear Corporation UK, an Adjunct Associate Professor at Monash University, Australia, and now a private consultant
“Although the level has been raised to 7 today, it doesn’t mean the situation today is worse than it was yesterday. It means the event as a whole is worse than previously thought.
The INES levels are used to define a nuclear event as a whole, they aren’t a moment-by-moment measure. To a certain extent you can say that they are non-objective as it’s just a case of when the boxes are ticked rather than someone saying this is now a really big event.
The reason Fukushima Dai ichi can be placed in Level 7 is because of the INES definition:
- Major release of radioactive material with widespread health and environmental effects requiring implementation of planned and extended countermeasures.
The releases of radiation got worse due to several events during the first week (explosions at the plant etc), and it is measuring the extent of these first-week events that has increased the level now.
The 10000 TBq has now reduced to 1000 TBq presumably through decay. The total Bq being stated is presumably the total radioactivity that a Japanese organisation has calculated to have been released from the reactors for the incident up to this date. 1 Bq is one decay per second. It is impossible to determine what is happening to humans near the plant since the radio-active particles will settle. The human effect depends on where the human is in relation to the particles.
One problem is that the radioactive material is able to be spread by the wind. It appears that they are now scattering an “anti-scattering agent” (NISA update 9 April 0800) which is presumably to stop particles from leaving the plant site.”
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