Radioactive contamination and waste may also arise from accidents. Acci­dents generate radioactive waste of volume and composition which depend on the material involved and the magnitude of the accident. The Interna­tional Nuclear and Radiological Event Scale (INES) was developed in 1990 by international experts convened by the IAEA and the OECD Nuclear

Energy Agency (OECD/NEA) with the aim of communicating the safety significance of events at nuclear installations (IAEA, 2008b). The INES facilitates understanding, using a numerical rating to explain the signifi­cance of nuclear or radiological events in a similar fashion to the Richter scale for earthquakes. INES applies to any event associated with the trans­port, storage and use of radioactive material and radiation sources. Such events can include industrial and medical uses of radiation sources, opera­tions at nuclear facilities, or the transport of radioactive material. Events are classified at seven levels (Fig. 1.6): Levels 1-3 are ‘incidents’ and Levels 4-7 ‘accidents’. These levels consider three areas of impact: people and the environment, radiological barriers and control, and defence in depth. The scale is designed so that the severity of an event is about ten times greater for each increase in level on the scale. Events without safety significance are called ‘deviations’ and are classified Below Scale/Level 0.

The partial core meltdown accident at Three Mile Island (TMI), Penn­sylvania, USA in 1979 was at Level 5 on the INES scale, while those at Chernobyl and Fukushima were Level 7. The proper management of the

1.6 The International Nuclear and Radiological Event Scale (from the IAEA website).

TMI accident meant that there were no person overexposures to radiation and no casualties, so keeping it at Level 5.

Table 1.6 shows the most significant accidents involving radioactive ma­terials. Accidents involving SRS are worryingly common. Over 2,300 cases have been reported of SRS found in scrap metal. A large number of cases have been reported of accidental melting of SRS with scrap metal in, for example, steel and aluminium foundries. The total number of cases of melting SRS with scrap metals exceeds 60 in 18 countries. In Algeciras, Spain in 1998, radioactive gases, aerosols and particles from melting SRS with scrap were released and detected all over Europe. Concentrations up to 2,000 Bq/m3 of 137Cs in the air were detected although the incident had minor consequences. Since 1983, 30 cases of melting of SRS with scrap metal occurred in the US, which required $8-10 million in each case to decontaminate and restore the metallurgical facilities. In 1987 a serious accident occurred in Goiania, Brazil with a 337Cs SRS left within a tele­therapy unit. The SRS was found by two scavengers who took the unit home, removed the source from the unit and ruptured the source capsule. This caused significant contamination of people and the surrounding envi­ronment. Four severely exposed people died and the health of many others was seriously affected. More than 112,000 people were monitored for radia­tion exposure, of which nearly 300 showed 137Cs contamination. The emer-

gency response and clean-up effort of houses, buildings and land lasted six months.

The Chernobyl accident in 1986 was due to lack of care in operation and disregard for standard safety procedures. The resulting steam explosion and fire released about 5% of the radioactive reactor core into the atmosphere. Some 31 people were killed in the first few weeks after the accident, and there have since been other deaths from thyroid cancer due to the accident. An authoritative UN report in 2000 concluded that there is no scientific evidence of significant radiation-related health effects to most people exposed to radiation during or after the accident.

The most recent accident was that of 11 March 2011 at Fukushima in Japan. A major earthquake, followed by a 15 m tsunami caused the deaths of over 20,000 people and led directly to the shutdown of three reactors and eventually to significant escape of radioactive material to the environ­ment. Three of the Fukushima Daiichi reactor cores were severely damaged in the first three days, releasing high levels of radioactive materials into the land, sea and air environments. The Japanese authorities announced an official ‘cold shutdown condition’ in mid-December, as reactor tempera­tures had fallen to below 80°C at the end of October 2011. According to the Japanese government, the total amount of radioactivity released to date is approximately one-tenth that released during the Chernobyl disaster However, the full extent and level of radioactive contamination remain unclear.