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Description / Abstract:
Introduction
The Great East Japan Earthquake and Tsunami inflicted major loss
of life and destruction of property on the Nation of Japan, as well
as substantial devastation of its environment. Emergency response
capabilities were overtaxed and often overwhelmed. Furthermore, the
extraordinary natural forces unleashed on the East Japan coastal
areas led to a series of accident-initiating events that resulted
in the inability to cool the reactor cores in three operating units
of the Fukushima Dai-ichi Nuclear Plant, also referred to as
Fukushima. Loss of core cooling in Units 1, 2, and 3 led to core
degradation and fuel melting. Subsequently, continuing lack of core
cooling led to loss of the reactor coolant pressure boundary, loss
of containment integrity, and hydrogen explosions from zirconium
cladding-water reactions, followed by large radioactivity releases
to the environment from all three units. Radiological protection of
the public necessitated evacuation of populated areas up to 30 km
or nearly 19 miles from the plant.
Due to the current and expected absence of discernible radiation
health effects, radiological protection of public health and safety
appears to have been effective in Japan; however, the multi-unit
nuclearplant accident at Fukushima continues to have serious
impacts on socio-political, economic and energy-related issues in
Japan, as well as globally, and has received extensive Government
and media attention worldwide. The accident at the Fukushima plant
has already affected energy portfolios by skewing the importance of
nuclear electricity generation and its beneficial impacts on fuel
diversification, climate change initiatives, and stability of
electrical costs.
The ASME Presidential Task Force on Response to Japan Nuclear
Power Events (ASME Task Force) is convinced that global and
thoughtful solutions to the issues raised by the Fukushima Dai-ichi
nuclear accident are essential to continue benefitting from use of
nuclear power, to expand its use, and to address critical
environmental and energy portfolio issues. The ASME Task Force is
proposing an extended safety framework that would add complementary
improvements to the existing nuclear safety infrastructure in a
systematic manner, strengthening safety and accident response to
external and internal events. The proposed improvements are focused
on prevention or minimization of major impacts on public health,
the environment, and socio-political-economic issues from large
accidental releases of radioactivity. To achieve these objectives,
the ASME Task Force supports development of a new safety construct
for nuclear power generation, as described in this report, which
enhances the existing safety framework using an all-risk
approach.
The term "construct" denotes the conjunction of component parts
that, working together, achieve a desired outcome. For nuclear
power plants, the existing and evolving safety construct
incorporates a set of elements, including plant design, physical
systems, structures, and components, safety regulations, quality
assurance, and procedures and practices for plant operation and
maintenance, accident management, and emergency preparedness.
The term "all-risk," as used in this report, refers to
consideration of all credible hazards in developing probabilistic
risk assessments (PRAs), assessing defense-in-depth, and developing
accident management strategies. Risk is the combination of the
probability of an adverse event and its consequences. Protection of
public health and safety and the environment are and will continue
to be the most important consideration for nuclear safety.
Complements to the existing and evolving safety construct would
further strengthen the protection of public health and safety. The
additional consequence of concern in the new safety construct is
extensive disruption of society from a radioactivity release to the
environment.
"All risks" should be considered to include rare yet credible
events and potential accident scenarios that could threaten the
safety of a nuclear power plant. Accident scenarios can be
initiated by either internal or external hazards from natural or
man-made causes, during all modes of plant operation.
Addressing rare yet credible events with extreme consequences
should be limited by appropriate risk considerations. In this
regard, it is important to rely on the rule of law, using what the
Courts have determined is acceptable for nuclear power safety:
"The level of adequate protection need not, and almost
certainly will not, be the level of zero risk." [1]
Events that are not credible need not be considered. It is the
intention of the ASME Task Force that very-low-probability events,
such as extreme floods, large scale tornadoes, and other natural
phenomena that are unprecedented but conceivable at a given site,
should be considered. Of particular concern are initiating events
that could lead to cliff-edge effects, whereby for a small
incremental increase in severity, the consequences
disproportionally increase. In those cases, systems and planned
actions should be in place to provide core cooling and prevent a
large release of radioactivity.