One lesson stemming from the Chernobyl accident was the need for prompt dissemination to the public of the safety significance of an event at a nuclear in­stallation. A similar need in other areas is filled by an appropriate scale, for ex­ample, the Richter scale for earthquakes and the Beaufort scale for winds.

Tn 1ЧЧ0 the International Atomic Enemv A2"encv (IAEA) introduced.1 sewn-

level scale designed to allow prompt classification of such events. The levels, their descriptions, and detailed criteria are shown in Figure 5.26. Three criteria are applied:

Levels 3-7 relate to the extent of releases of radioactivity off-site.

Levels 2-5 relate to the extent of on-site contamination or exposure.

Levels 1-3 relate to the extent to which the defense-in-depth philosophy has been challenged.

Each of the incidents described in this chapter has been evaluated using the INES scale to provide a best estimate of the incident. The resulting classification is given in Table 5.2.

The International Nuclear Event Scale For prompt communication of aafety significance

Table 5.2 • The International Nuclear Event Scale (for prompt communication of safety significance)

Level Descriptor Criteria Examples

• External release of a large fraction of the Chernobyl, USSR

reactor core inventory typically involving a 1986

mixture of short-and long-lived radioactive fission products (in quantities radiologicaly equivalent to more than tens of thousands terabecquerels of iodine-131).

• Possibility of acute health effects. Delayed health effects over a wide area, possibly involving more than one country. Long-term environmental consequences.

• External release of fission products (in quantities radiologically equivalent to the order of thousands to tens of thousands

of terabecquerels of iodine-131). Full imple­mentation of local emergency plans most likely needed to limit serious health effects.

• External release of fission products (in Windscale, UK

quantities radiologicaUy equivalent to the 1957

order of hundreds to thousands of terabec­querels of iodine-131). Partial implementation of emergency plans (e. g., local sheltering and/or evacuation) required in some cases to lessen the likelihood of health effects.

• Severe to large fraction of the core ^Three Mile Island, USA

due to mechanical effects and/or melting. 1979

• External release of radioactivity resulting in a dose to the most exposed individual off­site of the order of a few millisieverts:"

Need for off-site protective actions generally unlikelyexcept possibly for local food control.

• Some damage to reactor core due to Saint Laurent, France

mechanical effects and/or melting. 1980

• Worker doses that can lead to acute health effects (of the order of 1 Sievert).b

• External release of radioactivity above authorized limits, resulting in a dose to the most exposed individual off-site of the order of tenths of a millisievert. a Off-site protective measures not needed. [3]

Table 5.2 continued

Level Descriptor Criteria Examples

• Incidents in which a further failure of safety Vandellos, Spain systems could lead to accident conditions, or 1989 a situation in which safety systems would be unable to prevent an accident if certain initiators were to occur.

2 Incident • Technical incidents or anomalies which,

although not directly or immediately affecting plant safety, are liable to lead to subsequent reevaluation of safety provisions.

1 Anomaly • Functional or operational anomalies which

do not pose a risk but which indicate a lack of safety provisions. This may be due to equipment failure, human error, or proce­dural inadequacies. (Such anomalies should be distinguished from situations where operational limits and conditions are not exceeded and which are properly managed in accordance with adequate procedures.

These are typically “below scale:’)

Below No safety

scale I significance

zero

Source: International Atomic Energy Agency, April 1990.

“ The doses are expressed in terms of effective dose equivalent (whole body dose).Those criteria, where appropriate, also can be expressed in terms of corresponding annual effluent discharge limits authorized by National authorities.

6These doses also are expressed, for simplicity, in terms of effective dose equivalents (Sieverts), although the doses in the ^ge involving acute health effects should be expressed in terms of absorbed dose (Grays).

Table 5.3 shows the ratings of the various incidents discussed in this chapter in terms of the INES scale. This table also shows how each of the safety princi­ples (the Three Cs—see Section 5.1) were met in each case and whether de­fense in depth was effective.

Table 5.3 a Nuclear Reactor Incidents

SAF^ETY PRINCIPLES International ^^E CS) Nuclear Control the Reaction Cool the Fuel Contain the Radioactivity Defense in Depth Event Scale Rating Light water-cooled reactors SL1 X — 4 Millstone 1 [?] 3 Browns Ferry 1 and 2 [./] [./] 3 ^lbree Mile lsland-2 X 5 Gina [?] 2 Mihama-2 [?] 2 Chemobyl X — X X 7 Heavy water-cooled reactors X — [?] 4 Lucens X 4 Gas-cooled reactors Wmdscale X X X 5 St. Laurent X 4 Hunterston B [?] 1 Hinkley Point B [?] 2 Liquid metal-cooled reactors EBR-1 X X 4 Enrico Fenni X 4 Safety principle violated Safety principle complied with