The Nuclear Accident Magnitude Scale (NAMS) is a scale to measure the severity of a nuclear accident. It was proposed by the British nuclear safety expert David Smythe in 2011 as a response to the Fukushima Daiichi nuclear disaster, where the conventional International Nuclear Event Scale was used in a confusing manner. Furthermore, it wants to bridge the various shortcomings of INES.
As Smythe pointed out, the INES scale ends at 7; a more severe accident than Fukushima in 2011 or Chernobyl in 1986 cannot be measured by that scale. In addition, it is not continuous, not allowing a fine-grained comparison of nuclear incidents and accidents. But then, the most pressing item identified by Smythe is that INES conflates magnitude with intensity; a distinction long made by seismologists to describe earthquakes. In that area, magnitude describes the physical energy released by an earthquake, while the intensity focuses on the effects of the earthquake. In analogy, a nuclear incident with a high magnitude (e.g. a core meltdown) must not result in an intense radioactive contamination, as the incident at the Swiss research reactor in Lucens shows – but yet it resides in INES category 5, together with the Windscale fire of 1957, which has caused significant contamination outside of the facility.
Definition
In contrast to INES, which comes with 200-page user manual, the definition of the NAMS scale is rather straightforward:
NAMS = log10(20 × R)with R being the radioactivity being released in terabecquerels, calculated as the equivalent dose of iodine-131. Furthermore, only the atmospheric release affecting the area outside the nuclear facility is considered for calculating the NAMS, giving a NAMS score of 0 to all incidents which do not affect the outside. The factor of 20 assures that both the INES and the NAMS scales reside in a similar range, aiding a comparison between accidents. It is noteworthy that an atmospheric release of any radioactivity will only occur in the INES categories 4 to 7, while the NAMS is does not have such a limitation.