Ensuring the safety of nuclear installations: Lessons learned from the Fukushima Daiichi accident

Experts found that a higher level of safety could be incorporated into existing nuclear power plants by adhering to more demanding requirements.

By Carley Willis

The Fukushima Daiichi nuclear accident reinforced the importance of having adequate national and international safety standards and  guidelines in place so that nuclear power and technology remain safe and continue to provide reliable low carbon energy globally.

By recognizing the lessons learned from the 2011 accident, the IAEA has been revising its global safety standards to ensure that Member States continue to receive up-to-date guidance of high quality.

“The Fukushima Daiichi accident has left a very large footprint on nuclear safety thinking, which manifested itself in a distinct shift from the prevention of design basis accidents to the prevention of severe accidents and, should an accident occur, the practical elimination of its consequences,” said Greg Rzentkowski, Director of the IAEA’s Division of Nuclear Installation Safety.

New safety measures

Following the accident, through a review of relevant standards, including the IAEA safety standard on design safety, experts found that a higher level of safety could be incorporated into existing nuclear power plants by adhering to more demanding requirements for protection against external natural hazards and by enhancing the independence of safety levels so that, even if one layer fails, another layer is unimpacted and stops an accident from happening.

While requirements for protection against natural hazards have always been included in the design of nuclear reactors, these have been strengthened since the accident. In general, the design requirements now take into account natural hazards of an estimated frequency above 1 in 10 000 years, as opposed to 1 in 1000 years used previously.

The independence of reactor levels in the defence in depth concept, which ensures that the various levels of defence in a plant act as independently as possible and thereby provide for effective implementation of safety functions, was also strengthened. The need for this independence can particularly be seen in the protection of reactors against common cause events. For example, in the case of a tsunami, back-up safety systems should be located at an elevation sufficiently high to be protected from potential flooding and ensure their operability when systems designed for normal operation have failed.

Implementing improved safety measures

Incorporating these new safety standards into the design of existing reactors was subsequently tested through comprehensive safety assessments and inspections. The assessments took into account the design features of installations, safety upgrades and provisions for the use of non-permanent equipment to demonstrate that the probability of conditions that may lead to early or large releases is practically eliminated.

“New power plants are designed to account for the possibility of severe accidents,” said Javier Yllera, a senior Nuclear Safety Officer at the IAEA. “Different safety improvements have been implemented at existing power plants, together with accident management measures.”

Safety assessments or ‘stress tests’ implemented in the European Union following the Fukushima Daiichi nuclear accident focused on the assessment of natural hazards such as earthquakes and flooding, and on the behaviour of power plants in cases of extreme natural events and severe accidents. The overall objective was to analyse the robustness of reactors to such events and, if necessary, increase it. The margins of the safety of reactors were analysed and possible improvements were identified. The implementation of those stress tests remained the responsibility of Member States, and resulted in many design and operation enhancements in Europe.

As an example, the French Nuclear Safety Authority (ASN) initiated an assessment of the country’s 56 nuclear power reactors as well as the 2 EPR reactors under construction. The ASN then prescribed the implementation of both fixed and mobile equipment that could potentially prevent a large release, including high-resistance diesel generators and pumps able to function in extreme scenarios such as major earthquakes or flooding. The availability of alternative sources of water for cooling was also prescribed under the same conditions. In addition, the ASN required a back-up plan including rapid action force groups that can be on site within 24 hours with light equipment and within three days with heavy equipment, using transportation means such as helicopters, and that can operate in a severely disrupted environment.

“One of the lessons learned from the Fukushima Daiichi accident is that disruptions caused on and off site by extreme natural hazards can pose major problems,” said Philippe Jamet, former Commissioner of the ASN and Chairman of the Board of the European stress tests. “In the case that an accident does happen, there must be adequate means of transportation to reach the site and trained personnel to work in challenging conditions.”