Page 29 - Nuclear Plant Journal Digital Edition: January - February 2013
Basic HTML Version
Nuclear Plant Journal, January-February 2013
29
local inventories stored with each reactor.
Eventually, they had to use seawater. It
is the only resource available, and it was
the right thing to do. It took some time to
make that decision to use seawater to cool
the reactor cores because seawater would
attack the steel as well.
Adding seawater obviously provided
the cooling capability, but it has another
aspect to it. There are dissolved materials
in seawater that is mostly sodium
chloride and other salts. If the water is
vaporized, the salt begins to concentrate
and will eventually precipitate from
solution. As such, it takes up volume, and
it interacts with the core debris as well.
From a practical point of view, given the
time available to the operators and the
resources available, adding seawater was
the only thing they could do and it was
exactly the right thing to do. The salt water
provided the cooling and the fact that it
had some internal dissolved material,
basically salt, probably also accelerated
the cooling of the core material compared
to the cooling rate with pure water.
Whether or not the reactor vessel, the
wall of the reactor vessel was breached
and molten core debris came out into the
containment, it is yet to be determined
for sure, but the analysis that the utility,
Tokyo Electric Power has done suggests
that they have had failures in at least one
of the vessels and maybe all the three
that have discharged core debris onto
the floor. However, they have not yet
received specific proof that core debris
is on the containment floor through the
robotic investigations that have been
conducted. Such investigations are really
just at the starting point of getting into
the containment to determine where the
core debris is located. Currently, they
are reasonably sure that in unit one there
is core material on the floor. For the
other two units, the analyses performed
suggest that the reactor vessels did fail but
they still are seeking proof of the vessel
status. Eventually, TEPCO can use that
information to assess the actions they will
take in the future to clean up the reactor
sites, disassemble the units, remove the
fuel from the containments and transport
the remaining debris away from the site.
If hydrogen generated during the
accident is released to the inerted con-
tainment building, there is insufficient
oxygen to burn the hydrogen. However,
if the hydrogen should leak out of the
containment into the reactor building that
surrounds the containment, it now could
be a combustible mixture because the re-
actor building is not inerted.
Given what they know about unit
one, the pressure in the containment
was quite high, maybe approaching
about 8 bars absolute at the time, so it is
conceivable that hydrogen could have
leaked through the drywall seal. To refuel
a BWR, the closure head for the drywell
and the reactor vessel upper head are both
removed and the reactor is refueled from
the top. After the refueling is complete,
the reactor vessel upper head is placed
back on the vessel and bolted in place
and the drywall head is placed over the
top of the vessel head and also bolted
in place to complete the containment.
There is a drywell head seal between the
upper part of the drywell and the drywell
head that could leak if the containment
is pressurized to as much as 8 to 10 bars
absolute. A leak through the drywall
head seal would discharge hydrogen to
the reactor building. The explosion that
occurred in the Unit 1 reactor building,
a little more than a day after the tsunami
was certainly hydrogen and I think it is
agreed now that it likely leaked through
the drywall seal and perhaps through a
couple of other places as well. Regardless
of the leakage locations, there was more
than enough hydrogen accumulated to
destroy the upper level of the reactor
building in the explosion.
Hydrogen has a smaller molecule
than helium and helium is used for leak
detection, so hydrogen could begin
leaking through some small pathways.
If the seal is leaking hydrogen, the gas
could interact with the seal potentially
to cause the leak to grow. Furthermore,
the fact that the containment was at a
high pressure means that the bolts for the
drywell head could have been stretched
slightly to open a leakage path out
through the seal into the reactor building.
Such a path could have leaked hydrogen,
nitrogen, and steam into the reactor
building–the nitrogen and the steam
tend to inert the hydrogen but the steam
could be condensed. That is one way for
hydrogen to concentrate in a locale that
eventually results in an explosion. Clearly,
what happened in the top of unit one was
an explosive combustion event that blew
away all of the siding for the upper level
of the reactor building.
Most of the current operating plants
that have in-core instrumentation that is
either some form of Traveling In-Core
Probes or fixed in-core instrumentation
like that used in the TMI-2 reactor. All of
these have the potential to open a flow path
from the reactor core to the containment
under severe accident conditions.
That’s all been part of the documented
information for the Severe Accident
Management Guidelines (SAMG).
If the core temperatures increase
to a range of 1500 Centigrade or
higher (1700 Centigrade is roughly the
melting temperature of stainless steel
and Inconel) as a result of an accident,
the instrument thimbles would fail and
these flow paths would open, at least
temporarily. Those responsible for
managing the accident have guidelines
that call attention to the possibility that
hydrogen could be released from the core
to the containment. If the containment
is inerted, such as the Mark I and Mark
II BWRs, the hydrogen is not going
to be combustible in the containment.
However, those managing the accident
response also need to be aware of the
leakage through the drywall head seal
that apparently happened in Fukushima
Unit I and maybe the other units as well.
The BWRs operating in the US, in the
Far East, as well as those in Europe, have
the in-core instrumentation that is similar
to that used in the Fukushima designs.
2.
What can be done in the current
plants to avoid an accident similar to
Three Mile Island or Fukushima?
Most importantly, it is essential to
understand, prioritize and use all of the
ways in which the decay heat can be
removed fromtheReactor Coolant System
as well as the ways in which water can be
added to the RCS to ensure that the core
is continually submerged in water. In
short, have a thorough understanding of
how to prevent core damage events. This
is the direction that the nuclear industry
has pursued since the TMI-2 accident
