where, C i is the consequence and P i is the probability of occurrence of C i for the ith scenario. This definition of risk, however, represents only one aspect of risk,
which is complex in nature.
It is described in ISO 31000 [2] that organizations face internal or external
factors and influences that make it uncertain whether and when they will achieve
their objectives. The effect this uncertainty has on an organization’s objectives is
defined as “risk.” The objective of nuclear safety is to protect people, individually
and collectively, as well as the environment from the harmful effects of ionizing
radiations [3]. Therefore, the risks of nuclear power plant accidents are the effect of
uncertainty in the various predecessors of accidents, e.g., earthquakes, fires,
flooding and human errors, etc., on the objective of nuclear safety.
Analysis of risk is to attempt to envision what will happen if a certain course of
action, including inaction, is taken [4]. Therefore, risk is defined as an answer to the
following questions [4]:
• What can go wrong? (Scenario)
• How likely is it? (Likelihood)
• What might its consequences be? (Consequence)
The importance of the scenario, in addition to the consequence and the likelihood (or frequency), is more emphasized in this definition to describe the characteristics of risk. It is also emphasized that the risk information should include
information about what is within/out of scope and how uncertain the result of the
risk assessment is. The consequences that are assessed by risk assessment are not
limited to fatalities due to radiation exposure, but include other consequences
related to quality of life, e.g., environmental damage. It should also be noted that
the actual fatalities related to nuclear power plant accidents are not always limited
to radiation effects but may include other factors as discussed above, which should
be included in the scope of the risk assessment of nuclear power plants.
3.2 Risk-Informed Decision Making
The purpose of probabilistic risk assessment is not limited to discussions whether a
certain nuclear power plant is safe enough based on the estimated value of accident
occurrence probability. We can also identify the weak points of the plant and
contribute to an improved quality of decision making related to the introduction of
safety-enhancement measures. Examples of required decision-making qualities are
reasonability, accountability, openness, and transparency.
On the other hand, assessment of the risk due to external factors, e.g., earthquakes, fires, flooding, and aircraft impact, is hampered by an inherently large
uncertainty. Therefore, an integrated framework to deal with these kinds of risks,
i.e. a decision-making process under large uncertainty [5], is essential to ensure the
safety of nuclear power plants now and in the future. Key elements for the
Challenges for Nuclear Safety from the Viewpoint …
69
which is complex in nature.
It is described in ISO 31000 [2] that organizations face internal or external
factors and influences that make it uncertain whether and when they will achieve
their objectives. The effect this uncertainty has on an organization’s objectives is
defined as “risk.” The objective of nuclear safety is to protect people, individually
and collectively, as well as the environment from the harmful effects of ionizing
radiations [3]. Therefore, the risks of nuclear power plant accidents are the effect of
uncertainty in the various predecessors of accidents, e.g., earthquakes, fires,
flooding and human errors, etc., on the objective of nuclear safety.
Analysis of risk is to attempt to envision what will happen if a certain course of
action, including inaction, is taken [4]. Therefore, risk is defined as an answer to the
following questions [4]:
• What can go wrong? (Scenario)
• How likely is it? (Likelihood)
• What might its consequences be? (Consequence)
The importance of the scenario, in addition to the consequence and the likelihood (or frequency), is more emphasized in this definition to describe the characteristics of risk. It is also emphasized that the risk information should include
information about what is within/out of scope and how uncertain the result of the
risk assessment is. The consequences that are assessed by risk assessment are not
limited to fatalities due to radiation exposure, but include other consequences
related to quality of life, e.g., environmental damage. It should also be noted that
the actual fatalities related to nuclear power plant accidents are not always limited
to radiation effects but may include other factors as discussed above, which should
be included in the scope of the risk assessment of nuclear power plants.
3.2 Risk-Informed Decision Making
The purpose of probabilistic risk assessment is not limited to discussions whether a
certain nuclear power plant is safe enough based on the estimated value of accident
occurrence probability. We can also identify the weak points of the plant and
contribute to an improved quality of decision making related to the introduction of
safety-enhancement measures. Examples of required decision-making qualities are
reasonability, accountability, openness, and transparency.
On the other hand, assessment of the risk due to external factors, e.g., earthquakes, fires, flooding, and aircraft impact, is hampered by an inherently large
uncertainty. Therefore, an integrated framework to deal with these kinds of risks,
i.e. a decision-making process under large uncertainty [5], is essential to ensure the
safety of nuclear power plants now and in the future. Key elements for the
Challenges for Nuclear Safety from the Viewpoint …
69