This month marks the fifth anniversary of the Fukushima nuclear accident that was caused by a devastating earthquake and tsunami on the east coast of Japan.
The OECD Nuclear Energy Agency has released a report outlining the lessons learned since Fukushima. It also deals with what work still needs to be done, not just to further strengthen safety in and around nuclear power plants – but also in the event of another so called ‘Act of God’ such as an earthquake.
As NEA director-general William Magwood says: “The great lesson of Fukushima is that we cannot predict every natural event… but plants have been made more resilient and better prepared for the unexpected than they were five years ago”.
The report states that prior to Fukushima, nuclear operators and regulators had carried out detailed assessments of the potential internal hazards and risks of a plant, but spent less time considering external risk factors – such as natural disasters.
As a result, post-Fukushima, NEA member countries “have used the latest data technology to identify and consider plausible combinations of sequential and consequential events”.
The NEA stresses that the potential consequences of an earthquake had indeed been considered by the Japanese – they just never expected that such an earthquake would be accompanied by a tsunami: “The accident did not reveal any unknown initiators, sequences or consequences. However, the combination and the severity of initiating events had never occurred before, and the evolution of the accident in three different units simultaneously was also new.”
It is the need to plan for such catastrophic natural events that form the basis for much of the report.
Ensuring that heat can be removed from the core in the event of an extreme natural event has led some countries to consider bunkered safety systems, designed to resist such extreme natural events.
Many countries have now installed or are considering strategic placement of portable equipment that can be quickly positioned and provide emergency cooling. Some countries have enhanced the decay heat removal function by providing an alternate path for cooling water supply, arranging for passive cooling or identifying an alternate heat sink.
Fukushima demonstrated that the ability of nuclear plants to maintain containment integrity could be challenged by severe natural hazards. In response, some NEA member countries have refocused their efforts on upgrading plants with respect to containment venting and hydrogen mitigation.
In other countries, new or improved filtered vents or filtering strategies are being implemented or considered for use during venting to limit containment pressure. Some are introducing a different hydrogen mitigation strategy or improving the existing one.
On the strengthening of regulatory frameworks, the NEA notes that there has been a particular emphasis on ensuring that “a clear and comprehensive legal framework exists to allow the operator of a nuclear plant – and its government if necessary – to quickly react and adapt to the specific circumstances of an event in order to ensure timely and financially adequate compensation to victims”.
Fukushima highlighted the challenges involved in managing the consequences of a large-scale accident. As time progressed, radiological and social consequences became increasingly evident, while decisional responsibilities were shifting from central to regional and local governments.
Even countries not directly affected by the accident spent significant resources on understanding the rapidly-evolving situation so they could help their expatriates in Japan, and address concerns over people, cargo and food arriving from Japan.
The Japanese government was challenged by the need to not just tackle the immediate accident but also “dedicate resources to formally and informally address questions from other countries and from international organizations”.
The NEA therefore concludes that “emergency management planning should take into account the Japanese experience in terms of the training and resources required to be appropriately prepared to manage the collection and flow of information”.
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