29 Feb (NucNet): Countries with nuclear programmes need to develop and improve their approach to the evaluation of external hazards because there has traditionally been a more thorough and detailed consideration of internal hazards in safety cases and analysis, says a report by the Nuclear Energy Agency on lessons learnt from the March 2011 Fukushima-Daiichi nuclear accident in Japan.
The report, published today by the Paris-based agency, says the accident did not reveal any “unknown initiators, sequences or consequences”. However, the combination and severity of initiating events had never occurred before, and the evolution of the accident in three different units simultaneously was also new.
The report says that since the accident regulatory authorities in NEA member countries have performed “diverse activities” that have led to the establishment of new requirements. The focus of these requirements has been the potential impact of external hazards such as floods and earthquakes, and plant improvements related to the diversity of equipment, improvements to the robustness of safety functions, and continuing efforts to improve organisational behaviour.
Specific actions have led to improvements to the robustness of electrical systems, enhancement of the robustness of the ultimate heat sink, protection of the reactor containment system, and protection of spent fuel in spent fuel pools.
The ability to quickly provide “diverse equipment and assistance” from onsite and offsite emergency preparedness facilities has been improved. Safety culture has been improved and safety research is continuing, the report says.
The report also makes it clear that regulatory frameworks in the nuclear industry need to be strengthened and the independence of regulatory bodies enforced.
“The principal of regulatory independence, in particular the effective separation between the functions of the regulatory body and those of any other organisation concerned with the promotion or use of nuclear energy, is fundamental and requires vigilance to ensure it is maintained,” the report says.
A 2012 report by Japan’s government said there had been a lack of regulatory independence in Japan at the time of the accident and collusion between regulators and operators. The independent Nuclear Regulation Authority was established in September 2012 to meet the need for clear separation of regulation from promotion.
The same report identified other contributory factors in the accident such as a flawed safety culture, organisational failures and weaknesses, weak emergency preparedness and response, and poor onsite and offsite disaster handling.
Today’s NEA report says the accident demonstrated the challenges involved when 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 government to regional and local governments, and to affected individuals.
The International Atomic Energy Agency said in a September 2015 report that at the time of the accident, separate arrangements were in place to respond to nuclear emergencies and natural disasters at national and local levels. There were no coordinated arrangements for responding to a nuclear emergency and a natural disaster occurring simultaneously.
Another report, the official report of the Fukushima Nuclear Accident Independent Investigation Commission, said it was a manmade disaster – the result of collusion between the government, the regulators and plant operator Tepco and the lack of governance by these three parties.
The commission said it had verified that on 11 March 2011, the structure of the Fukushima-Daiichi nuclear plant was not capable of withstanding the effects of the earthquake and the tsunami. Nor was the plant prepared to respond to a severe accident.
“In spite of the fact that Tepco and the regulators were aware of the risk from such natural disasters, neither had taken steps to put preventive measures in place. It was this lack of preparation that led to the severity of this accident,” the commission concluded.
NEA director-general William Magwood said: “Much work is still before us to address new lessons, including how to effectively deal with more complex issues such as the human aspects of nuclear safety reflected in safety culture, training and organisational factors.”
On 11 March 2011 an earthquake caused damage to the electric power supply lines to the Fukushima-Daiichi nuclear site on the northeast coast of Japan, and a tsunami caused substantial destruction of the operational and safety infrastructure on the site. The combined effect led to the loss of offsite and onsite electrical power.
This resulted in the loss of the cooling function at the three operating reactor units as well as at the spent fuel pools. Despite the efforts of the operators to maintain control, the reactor cores in Units 1, 2 and 3 overheated, the nuclear fuel melted and the three containment vessels were breached.
Hydrogen was released from the reactor pressure vessels, leading to explosions inside the reactor buildings in Units 1, 3 and 4 that damaged structures and equipment and injured personnel. Radionuclides were released from the plant to the atmosphere and were deposited on land and on the ocean. There were also direct releases into the sea.
The NEA report, ‘Five Years After the Fukushima-Daiichi Accident: Nuclear Safety Improvements and Lessons Learnt’, is online: http://bit.ly/1RzYLSj
The NEA is a specialised agency within the Organisation for Economic Co-operation and Development (OECD) and operates as a forum for sharing information and promoting cooperation. It has 31 member countries in Europe, North America and Asia-Pacific.
