below the curve an acceptable region. If the system status is located within the
unacceptable region, one must make all efforts to reduce the risk. Safety management on a probabilistic concept of risk is actually more complicated. A tolerable
region will be introduced often between the unacceptable and acceptable regions
with an upper and lower risk limits. As low as reasonably practicable (ALARP)
principle is applied in this region. In addition, the risk profile of a complex system
cannot be treated with a risk limit presumed under the assumption of normal
distribution, because it shows a long tail and rare events may occur more frequently
than expected. Anyways, Fig. 1 shows the first-order approximation of the
risk-based approach of safety management.
Even if the system status is located in the acceptable region, however, it does not
mean that the risk has vanished. The risk that still remains after having satisfied the
risk limit is called the residual risk. We also have to deal with the residual risk after
having satisfied the risk limit. Occurrence of an unanticipated event often leads to
further reduction of the risk limit and then to elevation of safety regulation, but this
process is an endless loop. Management of the residual risk is performed by risk
retention and risk transfer, which are often out of the scope of ordinary safety
regulation, and the strategy of their application is to be established. In a social
setting, risk transfer is performed usually by insurance, and risk retention by disaster preparedness, compensation of damage, and so on. The method of risk
retention, however, for damage that exceeds the scale of commercial insurance is
disputable.
Renovating deterministic approaches following the defense-in-depth principle
will be a key. After Fukushima, the Nuclear Regulation Authority (NRA) of Japan
enforced new regulatory standards for commercial power reactors in July 2013. The
new standards request enhancement of design basis, protection against earthquakes
and tsunamis, and new requirements for severe accidents. In order to fulfil the
standards, Japanese utility companies are now taking remedial actions to their
plants and installing various safety measures such as follows [3, 4], and these
measures are in line with enhancement of defense-in-depth rather than introducing
new principles.
Fig. 1 Safety management
based on a probabilistic
concept of risk
How the Fukushima Daiichi Accident Changed …
37
unacceptable region, one must make all efforts to reduce the risk. Safety management on a probabilistic concept of risk is actually more complicated. A tolerable
region will be introduced often between the unacceptable and acceptable regions
with an upper and lower risk limits. As low as reasonably practicable (ALARP)
principle is applied in this region. In addition, the risk profile of a complex system
cannot be treated with a risk limit presumed under the assumption of normal
distribution, because it shows a long tail and rare events may occur more frequently
than expected. Anyways, Fig. 1 shows the first-order approximation of the
risk-based approach of safety management.
Even if the system status is located in the acceptable region, however, it does not
mean that the risk has vanished. The risk that still remains after having satisfied the
risk limit is called the residual risk. We also have to deal with the residual risk after
having satisfied the risk limit. Occurrence of an unanticipated event often leads to
further reduction of the risk limit and then to elevation of safety regulation, but this
process is an endless loop. Management of the residual risk is performed by risk
retention and risk transfer, which are often out of the scope of ordinary safety
regulation, and the strategy of their application is to be established. In a social
setting, risk transfer is performed usually by insurance, and risk retention by disaster preparedness, compensation of damage, and so on. The method of risk
retention, however, for damage that exceeds the scale of commercial insurance is
disputable.
Renovating deterministic approaches following the defense-in-depth principle
will be a key. After Fukushima, the Nuclear Regulation Authority (NRA) of Japan
enforced new regulatory standards for commercial power reactors in July 2013. The
new standards request enhancement of design basis, protection against earthquakes
and tsunamis, and new requirements for severe accidents. In order to fulfil the
standards, Japanese utility companies are now taking remedial actions to their
plants and installing various safety measures such as follows [3, 4], and these
measures are in line with enhancement of defense-in-depth rather than introducing
new principles.
Fig. 1 Safety management
based on a probabilistic
concept of risk
How the Fukushima Daiichi Accident Changed …
37