[ RadSafe ] An objective nuclear accident magnitude scale for quantification of severe and catastrophic events

Milcon Research & Consulting milcon.randc at googlemail.com
Tue Dec 20 03:35:22 CST 2011

Physics Today reports


An objective nuclear accident magnitude scale for quantification of severe
and catastrophic events
December 12, 2011
By David Smythe

Introduction and summary

Deficiencies in the existing International Nuclear Event Scale (INES)[1]
have become clear in the light of comparisons between the 1986 Chernobyl and
2011 Fukushima Daiichi nuclear power plant accidents.[2-4] First, the scale
is essentially a discrete qualitative ranking, not defined beyond event
level 7. Second, it was designed as a public relations tool, not an
objective scientific scale. Third, its most serious shortcoming is that it
conflates magnitude with intensity.
I propose a new quantitative nuclear accident magnitude scale (NAMS). It
uses the earthquake magnitude approach to calculate the accident magnitude
M = log(20R), where R = off-site atmospheric release of
radioactivity, normalized to iodine-131-equivalent terabecquerels. In NAMS
the observed frequency-magnitude distribution of 33 well-quantified events
over the past 60 years follows an inverse power law, as with earthquakes,[5]
but NAMS highlights four exceptional accidents that are greater by 2-3
orders of magnitude than the next largest. These are, in decreasing order of
severity, Chernobyl, Three Mile Island, Fukushima Daiichi, and Kyshtym. Such
catastrophic accidents can be expected to occur every 12-15 years.

The problem with INES

The International Atomic Energy Agency (IAEA) developed the INES in 1990. It
is based in part on a loose analogy with the logarithmic
earthquake-magnitude Richter scale, in that one unit difference in event
level between 4 and 7 corresponds approximately to a factor of 10 in
amplitude. Despite its reference to decade threshold values for off-site
radionuclide release for discriminating between levels 4 through 7, the INES
is essentially a discrete qualitative ranking. A true location-specific
intensity scale measures exposure at a particular time and place due to an
accident. Figure 1 shows a popular representation of the scale as a pyramid.

continues at




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