The International Nuclear Event Scale (INES) is currently used to rank the severity of nuclear accidents. Since the Fukushima nuclear disaster in March of 2011, deficiencies of the INES have become more apparent. The INES is a subjective qualitative assessment of the seriousness of a nuclear accident. It functions more as a public relations tool than as n objective scientific scale. And, it confuses the magnitude of a nuclear event with the intensity.
David Smythe is Emeritus Professor of Geophysics at the University of Glasgow, Scotland. He has been active in assessing the geology and hydrology of proposed nuclear waste repositories in the United Kingdom. In an article published in December of 2011, Symthe proposed a new scale to replace the INES. He calls the new scale nuclear accident magnitude scales (NAMS).
The proposed new scale uses the earthquake magnitude approach where levels are defined in logarithmic jumps. In the earthquake Richter Scale, each level is ten times the magnitude of the level below it. In the NAMS, the magnitude of an event is equal to log(20R) where R is the off-site release of radioactivity calibrated in terms of equivalence to trillions of becquerels (TBq) of iodine-131.
Where the levels of INES are simple integers, the magnitudes of the NAMS are actual numbers corresponding to the radiation release. The constant 20 in the NAMS equation is meant to synchronize the NAMS with the INES. The upper levels of the ranges of the NAMS are equal to the levels of the INES. For instance, 500 TBq is equal to the upper limit of level 4 in the INES scale and also equal to a NAMS magnitude 4.
Where the INES scale only considers atmospheric contamination in levels 4 to 7, the NAMS scale is intended to only measure off-site atmospheric contamination. Water contamination is not considered in the NAMS scale although evaporation of contaminated water can contribute to atmospheric contamination. In the future, Smythe would like to add liquid contamination to the NAMS but it will be difficult because of the different ways that contaminated water can leave the site of an accident.
Smythe focused on accidents involving civilian nuclear power stations and civilian and military fuel reprocessing plants. He disregarded nuclear explosions and accidents on nuclear powered ships and submarines. Checked against 33 such quantified nuclear events during the past 60 years, a power law distribution is found. This means that most events are low in magnitude with a few events that are much greater. In the case of nuclear events, the highest magnitude events in descending order are Chernobyl, Three Mile Island, Fukushima and Kyshtym. It is estimated that there will be one such high magnitude event every twelve to fifteen years.
The NAMS cannot predict the impact of an accident or the dosage that populations may be exposed to. But it can give an immediate accurate assessment of released radioactivity once an event has occurred. This is superior to the vague qualitative levels of the INES.