The accident at the three reactor units at Fukushima Daiichi showed weaknesses in the plant coping capability for beyond design basis accidents caused by extreme external events. The weaknesses included plant design features, accident management procedures and guidance, and offsite emergency response. As a result, significant changes to plant coping capability have been made to light water reactors worldwide to enhance the coping capabilities for beyond design basis accidents. However, the response in the United States has been significantly different from that in Europe in a number of ways. In the United States, the regulator and the industry convened separate expert panels to review the Fukushima accident and make recommendations for enhancements. On the regulatory side, a series of three Orders were issued and that required the implementation of certain enhancements (Mitigation strategies, hardened vents for certain BWRs, spent fuel pool level indication) to ensure adequate protection for the health and safety of the public. Other enhancements were subject to the “Backfit Rule” which requires that changes to regulatory requirements be shown to be cost beneficial using accepted methodologies. Simultaneously, the industry took independent steps to develop a diverse and flexible coping strategies (known as FLEX) and other enhancements. The focus in the United States was clearly on enhancements to guarantee continued core, containment and spent fuel pool cooling in the event of beyond design basis accidents, particularly those resulting from extreme external events.
In Europe, the regulatory agencies ordered the development and completion of “Stress Tests” for each reactor site. These Stress Tests were focused on identifying the capability of the plant and its staff to respond to increasingly severe external events. The Stress Tests not only examined the ability to maintain core, containment and spent fuel pool cooling but also the ability to mitigate the consequences of accidents that progress to core damage (i.e., a severe accident). Regulatory requirements were then issued by the national regulators that addressed the weaknesses identified from the Stress Tests. While many of the enhancements to the plant coping capability were similar to those in the United States, significant hardware enhancements were also required to reduce the consequences of core damage accidents including hydrogen control and containment filtered venting.
Finally, most European regulators also include severe accident management guidance (SAMG) as a regulatory requirement. In the United States, SAMG will be maintained as a voluntary industry commitment that is subject to regulatory oversight review.
Analysis of Fuel Criticality during Severe Accidents
