Page 30 - Nuclear Plant Journal Digital Edition: January - February 2013
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30
Nuclear Plant Journal, January-February 2013
Exploring
Fukushima...
and continues to securitize and improve
the procedures, guidelines and training
provided to the managers, operators and
staff.
With respect to the accident
conditions, such as hydrogen generation,
passive autocatalytic recombiners, called
PARs, are being widely implemented
now in Europe and they are also being
considered for some reactors in the Far
East. These reactors also have the inerted
Mark I and Mark II containments, and the
PARs are being considered for consuming
any hydrogen that could leak into the
reactor building. Using AP1000 as an
example, it uses both passive recombiners
and igniters for hydrogen control in the
containment under accident conditions.
Let me mention a couple of areas
where the industry has continued to
improve the guidance material to be
used in response to severe accident
conditions.
The loss of AC and DC power
accident conditions that occurred at
Fukushima is an important sequence to
evaluate for every plant design. This
sequence has been studied for every plant
specific Probabilistic Risk Assessment
(PRA) performed for the operating plants
in the U.S. and also in the NRC reference
plant evaluations for different reactor
designs. It is most unfortunate when a
disaster like the 2011 Tohoku earthquake
and tsunami occurs. The loss of life in
the coastal cities is staggering and the
extent of damage due to flooding at the
Fukushima site along with the delays in
implementing the accident management
guidelines is troubling. Nevertheless,
when such unfortunate events occur, they
should be explored to the nth degree, to
find out everything that can be used to
be proactive in preventing, or mitigating
damage to the reactor site. Such
insights and understanding represents
advancements in the technology basis for
usingnuclear power togenerate electricity.
In particular, these insights help us to be
aware of important precursors, just like
the precursors that were learned from the
accident at Three Mile Island. Because
there were three reactors involved in
the Fukushima accidents, the plant
experiences related to challenges to the
reactor cores and the containments, as
well as the core configurations and actions
that ultimately cooled the fuel should be
clearly understood and documented. We
must learn all that we can from these
unfortunate events to help prevent these
in the future.
In some respects the industry has
come full circle. Unit I of Fukushima
had components called “Isolation
Condensers,”. Dresden and Oyster Creek
have these in their designs. These are
piped to the reactor vessel, so steam
formed in the core can be ducted into the
condensers, with the condensate returning
to the core, and heat is transferred to the
environment. So it is a close system that
can remove heat for a long time without
having any need to inject water into
the reactor. The only water that needs
to be added is to the secondary side of
the condenser, which is at atmospheric
pressure.
As BWRs progressed, the isolation
condensers were replaced, and they
put in what’s called a “Reactor Core
Isolation Cooling System” which is a
small steam driven pump that uses steam
from the vessel to power the pump and
that injects water into the core with the
steam generated by the decay heat in the
core being condensed in the containment
condensation pool. It is noted that this
system also worked very well in cooling
the reactor core for Fukushima Unit 2. But
the advantage of the Isolation Condenser
is that it operates by two-phase natural
circulation with steam being condensed
and the condensate returning to the
reactor core. Moreover, the secondary
side is effectively boiling water at
atmospheric pressure and sustained
cooling only requires that water be added
to the secondary side..
The ESBWR, the newest GE-Hitachi
BWR design, has a number of isolation
condensers. It is a return to the more
passive original design, and the key
aspect being that the ultimate heat sink is
the atmosphere.
The AP1000 design also has heat
removal from the outer surface of a steel
shell containment thus the atmosphere
is the ultimate heat sink. Under accident
conditions, the RCS is depressurized and
water is passively added to keep the core
covered. The steam generated in the core
flows into the containment, condenses
on the walls of the containment and
drains into the In-containment Refueling
Water Storage Tank (IRWST) where it
is eventually drained back to the core to
sustain long term cooling.
If the AP1000 design was subjected
to the Fukushima accident sequence,
that occurred to take away all the power,
everything, the plant would have survived
and the core would not have been
damaged. Now, that’s a huge step forward.
A paper
1
was presented on November 12,
2012 that talked about Fukushima and
the BWR designs. The author was very
specific that if the isolation condenser
had been used correctly, they would
have never lost Unit 1 at Fukushima.
Accident management has a very simple
perspective. For each important system,
understand in what ways the function
could be challenged and then find ways
to address the challenge. In Fukushima
Unit 1, there was a challenge because
there were DC valves that controlled
that flow path to the isolation condenser.
Once those valves closed, the operators
were unable to get those valves open.
There was no manual bypass. From the
practical point of view, the industry has
come full circle in that regard to what was
part of the original designs.
One last point, there are many
essential accident management insights
that come from Three Mile Island
accident. If you want to teach someone
what they should know about accident
management, just look at the specific
insights that came out of the Three Mile
Island event. Three-fourths of what one
needs to know is demonstrated in that one
accident.
While we are only beginning to
assemble the insights form the Fukushima
accidents, I am confident that the same
thing is true in case of Fukushima as they
were in Three Mile Island.
1
Levy, S., 2012, “How Fukushima Daiichi
Could beAvoided”, Paper presented in the
ANS Embedded Topical Meeting, ANS
Winter Meeting, San Diego, California.
Contact: Robert Henry, Fauske &
Associates, 16W070 83
rd
Street, Burr
Ridge, Illinois 60527; telephone: (630)
887-5201, fax: (630) 986-5481, email:
.
