design, i.e. performance-based design, related to the second and third levels of
defense in depth should be more emphasized, to control abnormal operations or
accidents effectively when external events occur.
3.5 Accident Management
Provisions for management of severe accidents are required for the fourth level of
defense in depth. For nuclear power plants in Japan, accident management was
planned and introduced as a countermeasure for severe accidents around 2000 by
operators as voluntary basis without regulatory requirements. Reports on probabilistic risk assessment for internal events were published in 2002 to confirm the
effectiveness of introducing severe accident countermeasures. However, it has been
recognized among experts that the main source of risk is not from internal events but
from external events such as earthquakes. Therefore, a standardized method was
prepared and published as AESJ (Atomic Energy Society of Japan) standard for
seismic risk assessment by 2007. Risk assessment due to natural hazards, i.e. individual plant examination for external events, was not published for each specific
plant in Japan before the Fukushima Daiichi NPP accident in 2011. Accident
management was not yet reinforced to suppose natural hazards by continuous efforts.
3.6 Regional Disaster Prevention/Mitigation
It is of significant concern for the public whether they can survive, e.g. by successfully evacuating in case of a nuclear accident. For this purpose, we need to
provide information on the likelihood, timing, and possible amount of radioactive
material released for possible accidents. Because all units may suffer from identical
external events, multi-unit risk assessment is necessary considering the disturbance
of on-site and off-site activities related to mitigate the consequences of the accident,
as was observed in the hydrogen explosion in the Fukushima Daiichi accident.
Considering off-site emergency response, it is critically important that we recognize that off-site facilities may suffer damage from a nuclear accident due to
natural events occurring simultaneously. There are many kinds of possible interactions. A first point of consideration is the difficulty for local residents to evacuate
and also the difficulty for external organizations to provide support to the nuclear
site, due to the spatial distribution of damage to the infrastructure (see the examples
of damages for surface transportation around the Fukushima Daiichi NPP shown in
Figs. 1 and 2) and other factors. A second point of consideration is that the rescue
activity for people affected by the severe natural event may be disturbed because of
the forced evacuation due to nuclear accident, such as when a rescue team is forced
to leave the site. Additionally, people are discouraged from multiple damages
repeatedly by the natural event as well as by the nuclear accident.
Challenges for Nuclear Safety from the Viewpoint …
73
defense in depth should be more emphasized, to control abnormal operations or
accidents effectively when external events occur.
3.5 Accident Management
Provisions for management of severe accidents are required for the fourth level of
defense in depth. For nuclear power plants in Japan, accident management was
planned and introduced as a countermeasure for severe accidents around 2000 by
operators as voluntary basis without regulatory requirements. Reports on probabilistic risk assessment for internal events were published in 2002 to confirm the
effectiveness of introducing severe accident countermeasures. However, it has been
recognized among experts that the main source of risk is not from internal events but
from external events such as earthquakes. Therefore, a standardized method was
prepared and published as AESJ (Atomic Energy Society of Japan) standard for
seismic risk assessment by 2007. Risk assessment due to natural hazards, i.e. individual plant examination for external events, was not published for each specific
plant in Japan before the Fukushima Daiichi NPP accident in 2011. Accident
management was not yet reinforced to suppose natural hazards by continuous efforts.
3.6 Regional Disaster Prevention/Mitigation
It is of significant concern for the public whether they can survive, e.g. by successfully evacuating in case of a nuclear accident. For this purpose, we need to
provide information on the likelihood, timing, and possible amount of radioactive
material released for possible accidents. Because all units may suffer from identical
external events, multi-unit risk assessment is necessary considering the disturbance
of on-site and off-site activities related to mitigate the consequences of the accident,
as was observed in the hydrogen explosion in the Fukushima Daiichi accident.
Considering off-site emergency response, it is critically important that we recognize that off-site facilities may suffer damage from a nuclear accident due to
natural events occurring simultaneously. There are many kinds of possible interactions. A first point of consideration is the difficulty for local residents to evacuate
and also the difficulty for external organizations to provide support to the nuclear
site, due to the spatial distribution of damage to the infrastructure (see the examples
of damages for surface transportation around the Fukushima Daiichi NPP shown in
Figs. 1 and 2) and other factors. A second point of consideration is that the rescue
activity for people affected by the severe natural event may be disturbed because of
the forced evacuation due to nuclear accident, such as when a rescue team is forced
to leave the site. Additionally, people are discouraged from multiple damages
repeatedly by the natural event as well as by the nuclear accident.
Challenges for Nuclear Safety from the Viewpoint …
73