The International Nuclear and Radiological Event Scale uses a seven-level ranking scheme to rate the significance of nuclear and radiological events: levels 1-3 are “incidents,” and 4-7 are “accidents,” with a “Level 7 Major Accident” consisting of “a major release of radioactive material with widespread health and environmental effects requiring implementation of planned and extended countermeasures.”
而国际核与辐射事件等级标准（International Nuclear and Radiological Event Scale）则将核与辐射事件按其严重性分为7级：1~3级为“事件”，4~7级为“事故”，其中第7级“重大事故”定义为“辐射物质严重泄漏并造成广泛的人体健康和环境影响，需要实施有计划以及持续的应对措施”。