You will have heard by now that the Nuclear and Industrial Safety Agency of Japan has given the accident at the Fukushima Dai-ichi Nuclear Power Plant a rating of 7 on the International Nuclear and Radiological Event Scale (INES).
Before you panic, consider the following.
The Rating of the International Nuclear and Radiological Event Scale (INES) on the events in Fukushima Dai-ichi Nuclear Power Station (NPS), Tokyo Electric Power Co. Inc. (TEPCO), caused by the Tohoku District – off the Pacific Ocean Earthquake is temporarily assessed as Level 7, considering information obtained after March 18th. However, the amount of discharged radioactive materials is approximately 10 percent of the Chernobyl accident which was assessed on the same level.
Although Level 7 is the highest level of INES rating, it is estimated that the amount of discharged radioactive materials to the environment in the current stage is approximately 10 percent of the Chernobyl accident, which was assessed on the same level in the past.
Source: http://www.nisa.meti.go.jp/english/files/en20110412-4.pdf (Ministry of Economy, Trade, and Industry Press Release, April 12, 2011)
Also, from the International Nuclear and Radiological Event Scale (INES) User’s Manual:
= Major release of radioactive material with widespread health and environmental effects requiring implementation of planned and extended countermeasures.
= 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.
= This corresponds to a large fraction of the core inventory of a power reactor, typically involving a mixture of short and long lived radionuclides. With such a release, stochastic health effects over a wide area, perhaps involving more than one country, are expected, and there is a possibility of deterministic health effects. Long-term environmental consequences are also likely, and it is very likely that protective action such as sheltering and evacuation will be judged necessary to prevent or limit health effects on members of the public.
The simplest approach to rating actual consequences to people would be to base the rating on the doses received. However, for accidents, this may not be an appropriate measure to address the full range of consequences. For example, the efficient application of emergency arrangements for evacuation of members of the public may result in relatively small doses, despite a significant accident at an installation. To rate such an event purely on the doses received does not communicate the true significance of what happened at the installation, nor does it take account of the potential widespread contamination. Thus, for the accident levels of INES (4–7), criteria have been developed based on the quantity of radioactive material released, rather than the dose received.
The reason for using quantity released rather than assessed dose is that for these larger releases, the actual dose received will very much depend on the protective action implemented and other environmental conditions. If the protective actions are successful, the doses received will not increase in proportion to the amount released.
The final rating of an event needs to take account of all the relevant criteria described above. Each event should be considered against each of the appropriate criteria and the highest derived rating is the one to be applied to the event.
The scale should not be confused with emergency classification systems, and should not be used as a basis for determining emergency response actions.
So here is what I understand from this.
1. There is no change from yesterday to today in terms of the dose of radiation that I am receiving. (See, for example, the real-time measurements being taken at one of the local research institutes.) The reason this change was announced today was because the amount released hit a certain threshold over the period of a few weeks, not because something happened today to provoke this announcement. There has been no escalation of the crisis at the plant today. All sources (that I am aware of) are saying that the situation at the plant is not deteriorating.
2. The point of INES is to make sure people (non-experts) have some way of knowing whether an incident is serious or not. The highest possible level of the INES is to be applied so the seriousness can be conveyed. The highest level (7) defines the situation in Fukushima as a “major accident”. It certainly is a major accident. Three reactors and four spent fuel pools needing to be cooled simultaneously under the current conditions (problems with power, constant threat of aftershocks/tsunami) classifies as a major accident on any scale. The highest level also says that “protective action such as sheltering and evacuation” is “very likely”. That has also happened in this case, although not in Tsukuba.
So, to be clear, this rating is a statement of the seriousness of the event, not the impact of the event. Under any kind of scale, the Fukushima event is serious. This should not be interpreted to mean that two events that are both classified at the same level (i.e. Chernobyl and Fukushima) will necessarily have the same impact.