The author made the original version of this table shortly after JCO. Category 1
corresponds to unsafe events of relatively a high frequency and low consequence
and they do not differ from work accidents in the ordinary industries. The risk of
these events can be evaluated statistically and remedial actions are taken in ergonomics and work management. In contrast, Category 3 includes design basis events
of a low frequency and high consequence, and they are unique to the nuclear
industry. The countermeasures for this category are evaluating their risks theoretically and installing some engineered safety features. Category 2 is a new type of
unsafe events that emerged in the past decades. This category includes complex
events of systemic or organizational accidents, and they sometimes exceed the
design bases.
A locus of interest will be the trade-off of safety measures with economy. Since
NPPs are protected from Category 3 events with engineered safety features, most of
which are out of service during the normal operation, their enhancement conflicts
with the plant economy. In contrast, safety enhancement measures for Category 1
events often contribute also to the improvement of efficiency and productivity of
works, and they can be compatible with the plant economy. It differs from the
natural image that safety and economy are in a trade-off relationship. Those for
Category 2 are located between the both, i.e., if safety enhancement measures using
engineered safety features are necessary, the investments are costs. Otherwise, they
Table 1 Three categories of unsafe events
Category 1
Category 2
Category 3
Manifestation
Work accidents or
single failures
Systemic or
organizational
accidents
Design basis events
or anticipated
incidents
Frequency
Relatively high
Extremely low
Very low
Scale of damage
Local and limited
Medium * devastating
Devastating
Primary victims
Interested people
Interested people and/or
third party
Interested people
and/or third party
Complexity of
scenarios
Simple
Complicated and
non-linear
Complicated but
linear
Variety of
scenarios
Diverse but
classifiable
Extremely diverse
Limited and finite
Quantitative risk
assessment
Statistically
possible
Impossible
Theoretically
possible
Safety goal
ALARP
a
ALARP
a
Absolute risk limit
Countermeasures
Quality assurance
and work
management
Systems approach
Engineered safety
features
Trade-off with
economy
Compatible
Partly compatible
Conflicting
Status in nuclear
industry
Already resolved
Unresolved
Already resolved
a ALARA As low as reasonably practicable
How the Fukushima Daiichi Accident Changed …
43
corresponds to unsafe events of relatively a high frequency and low consequence
and they do not differ from work accidents in the ordinary industries. The risk of
these events can be evaluated statistically and remedial actions are taken in ergonomics and work management. In contrast, Category 3 includes design basis events
of a low frequency and high consequence, and they are unique to the nuclear
industry. The countermeasures for this category are evaluating their risks theoretically and installing some engineered safety features. Category 2 is a new type of
unsafe events that emerged in the past decades. This category includes complex
events of systemic or organizational accidents, and they sometimes exceed the
design bases.
A locus of interest will be the trade-off of safety measures with economy. Since
NPPs are protected from Category 3 events with engineered safety features, most of
which are out of service during the normal operation, their enhancement conflicts
with the plant economy. In contrast, safety enhancement measures for Category 1
events often contribute also to the improvement of efficiency and productivity of
works, and they can be compatible with the plant economy. It differs from the
natural image that safety and economy are in a trade-off relationship. Those for
Category 2 are located between the both, i.e., if safety enhancement measures using
engineered safety features are necessary, the investments are costs. Otherwise, they
Table 1 Three categories of unsafe events
Category 1
Category 2
Category 3
Manifestation
Work accidents or
single failures
Systemic or
organizational
accidents
Design basis events
or anticipated
incidents
Frequency
Relatively high
Extremely low
Very low
Scale of damage
Local and limited
Medium * devastating
Devastating
Primary victims
Interested people
Interested people and/or
third party
Interested people
and/or third party
Complexity of
scenarios
Simple
Complicated and
non-linear
Complicated but
linear
Variety of
scenarios
Diverse but
classifiable
Extremely diverse
Limited and finite
Quantitative risk
assessment
Statistically
possible
Impossible
Theoretically
possible
Safety goal
ALARP
a
ALARP
a
Absolute risk limit
Countermeasures
Quality assurance
and work
management
Systems approach
Engineered safety
features
Trade-off with
economy
Compatible
Partly compatible
Conflicting
Status in nuclear
industry
Already resolved
Unresolved
Already resolved
a ALARA As low as reasonably practicable
How the Fukushima Daiichi Accident Changed …
43