There has been a misconception since the early days of nuclear power that human error or mechanical failure, in other words risk factors within the plant itself, are the most significant variables regarding possible radiological release to the environment. In fact, the greatest threat to a plant´s operation may lie outside its walls.
Nuclear power plants all over the world are exposed to natural hazards, such as hurricanes, floods, fires, tsunamis, volcanoes and earthquakes. With safety always a key concern, engineers, safety specialists and architects also have to take extreme natural forces into consideration.
The importance of external events in the selection of the site, the design, construction and operation of nuclear power plants became apparent when an earthquake with its epicenter in Romania caused superficial damage to the Kozloduy Nuclear Power Plant in nearby Bulgaria in 1977.
A distinct new emphasis on external hazards in nuclear safety consideration followed the earthquake that hit the Kashiwazaki-Kariwa Nuclear Power Plant in July 2007, the largest to ever affect a nuclear facility.
When the earthquake hit the Kashiwazaki-Kariwa nuclear power plant four reactors shut down automatically. Water containing radioactive material was released into the sea, though without an adverse effect on human health or the environment.
"That event was significant in the field of nuclear safety and there are many lessons to be learned from it," says Mr. Antonio Godoy, Acting Head of the IAEA´s International Seismic Safety Centre.
One is the fact that the plant´s design seismic hazard inputs were underestimated, as the tremors were two-and-a-half times the level of design inputs. Although the structure, systems and components that are important to safety were actually built robust enough to withstand the earthquake, the 2007 event was a wake-up call that reverberated around the globe.
In addition, the incident underlined the need for an international safety network concerned with natural hazards that did not exist at the time of the event. This led to a bold response from the international nuclear community and Godoy hopes that improvements since adopted by all nuclear power plants and lessons learned will serve as an example worldwide.
"The IAEA has revised the already well recognized set of safety standards on site selection and evaluation to analyze the impact of external hazards. It is a process of reflection on the best available and updated knowledge," says Godoy.
The IAEA published a safety guide on seismic safety evaluation for existing nuclear installations in May 2009, and another guide on seismic hazard assessment is close to publication in 2010. All IAEA safety guides on natural hazards are being reviewed and revised accordingly.
"A new IAEA Safety Report is another project of the International Seismic Safety Centre. It will provide reference to make plans before earthquakes occur as well as actions to be taken in the aftermath such as plant instrumentation, shutdown criteria, plant walk-down and restart of reactors," he comments.
An International Workshop on Seismic Safety of Nuclear Installations will be held in Kashiwazaki in Japan from 17 to 19 March 2010. It is being organized by the IAEA International Seismic Safety Centre (ISSC) in co-operation with the IAEA Asian Nuclear Safety Network (ANSN). The workshop will focus on technical knowledge and research developments related to the occurrence and effects of extreme external events on nuclear power plant sites.
"The output we expect from the seminar is two-fold," Godoy says.
"One aspect is to formulate a future program to address current seismic safety issues which impact the design of new nuclear power plants. Another is to involve countries with ambitions in the field of nuclear technology which are following all these developments with interest."
Godoy believes the workshop to be an important stepping stone in a global plan for the protection of nuclear power plants against seismic events.
"Earthquakes often transcend national boundaries. In the future we want to establish a global network of knowledge and expertise, in all regions, constituted by the most qualified experts and scientists in geosciences and structural engineering," he says.
Expected participants are high-level experts from regulatory authorities, utilities, engineering and consultancy organizations, and academic and R&D institutions active within the subject areas covered by the workshop.
Hosted by the Nuclear and Industrial Safety Agency of Japan (NISA), the workshop will feature five open forums where up to fifty Member States will raise issues related to the protection of nuclear power plants against natural hazards with a particular focus on seismic activity.
Worldwide, it is estimated that around 20% of nuclear reactors are currently operating in areas of significant seismic activity.
The International Seismic Safety Centre (ISSC) was established on 29 September 2008 and presently is a section in the organizational structure of the Nuclear Installation Safety Division. It serves as a focal point on seismic safety for nuclear installations worldwide. The ISSC assists countries in the assessment of seismic hazards of nuclear facilities to mitigate the consequences of strong earthquakes.
The Asian Nuclear Safety Network (ANSN) was established in 2002. It aims to facilitate sustainable nuclear safety activities, to establish, improve and maintain nuclear safety infrastructures and to achieve a high level of safety of nuclear installations in the Asian region by cooperation amongst its Member States.