Page 28 - Nuclear Plant Journal Digital Edition: January - February 2013
Basic HTML Version
Exploring
Fukushima
to the nth
degree
By Robert Henry, Fauske & Associates.
Robert Henry
Robert Henry is currently the Emeritus
Senior Vice President of Fauske &
Associates. Previously he was Senior
Vice President and co-founder of Fauske
& Associates, LLC (FAI) in 1980. He
was responsible for developing the
understanding of pressurized and boiling
light water reactors during severe
accident conditions. This knowledge
base has been integrated into the MAAP
code, widely used by the domestic and
foreign nuclear industry.
He has published more than 150
articles in the areas of nuclear safety
and engineering and has authored, six
U.S. patents as well as a book, “TMI-
2: An Event in Accident Management
for Light-Water-Moderated Reactors”
published by the American Nuclear
Society (ANS). He is a member of the
ANS and in 1985, he received the Tommy
Thompson Award: the highest honor
the ANS gives in the field of reactor
safety. In 1990, Dr. Henry also received
an Award for Outstanding Engineering
Accomplishment from the College of
Engineering, University of Notre Dame.
An interview by Newal Agnihotri,
Editor of Nuclear Plant Journal at
the American Nuclear Society Winter
Meeting in San Diego, California on
November 13, 2012.
1.
Provide a possible scenario of
what happened to the fuel at Fukushima
Daiichi nuclear power plant.
The whole accident begins when
you lose the capability to keep the core
covered with water. Once the water level,
the water inventory in the vessel becomes
so low that the fuel begins to uncover,
then the amount of decay heat that is
below the water level boils away thereby
removing the energy generated, but the
rest of the fuel (the uncovered region) is
not sufficiently cooled and this leads to
a continued heating up of the exposed
fuel. The lower the water level gets, the
higher the temperature of the fuel pellets
and the Zircaloy cladding. When you
reach temperatures in the range of maybe
a thousand centigrade - something like
1,800 Fahrenheit - you begin to have the
onset of significant cladding oxidation,
solely because that cladding is a zirconium
alloy, which is a reactive metal. It reacts
with steam in the core to give zirconium
dioxide and hydrogen and a considerable
amount of energy is also released. The
higher the temperature, the higher the
rate of Zircaloy oxidation, hence, the
amount of chemical energy released
increases exponentially as the water level
decreases. Decay heat basically overheats
the fuel, but it is principally the chemical
energy release as a result of the oxidation
that causes the temperature to increase
rapidly into melting, relocation, and
eutectic formations between the Zircaloy
and the uranium dioxide fuel.
When the core materials begin to
melt, the core geometry changes. It
becomes much different than what it was
designed to be. When I wrote my book
on Three Mile Island, one thing that I
found very interesting is that the initial
failure of the pressure boundary of the
reactor vessel was the failure of the in-
core penetrations for the instruments that
extended up through the entire height of
the core. Those particular instruments, the
little tubes that contained the self-powered
neutron detectors and the thermocouple
to measure the core outlet temperature,
also had a calibration channel that was at
the containment pressure. That was made
of stainless steel, which has the lowest
melting temperature in the core. Once
these began to fail, flowpaths were opened
from the reactor cooling system out to the
containment that was being driven by
2,000 psi. High temperature steam and
hydrogen were being discharged through
these instruments and this can cause
considerable ablation (melting of the
instrument thimble materials).
Those behaviors are also relevant to
the Fukushima reactors because they also
have in-core instrumentation thimbles
made of stainless steel that extend upward
through the central region of the core.
These instruments are used to measure
the local power distribution in the core.
Some of the plants have Travelling In-
core Probes called TIPs, which can be
traversed all the way to the top of the core,
Once that the TIP tube is melted, there is
a flow path opened to the containment,
and to the drywall. Once again, what
would be venting through these flow
paths would be high temperature steam
and hydrogen. Both of these can be very
abrasive to materials if they are at a very
high temperature.
This is the first part of how the
accident begins to release hydrogen
and radioactive materials to the
containment. If nothing else, melting
of the core constituents is changing the
core geometry through compaction so it
makes becomes less coolable, than it was
in the form of fuel pins. The fact that this
is being driven by a chemical reaction
between the steam and the Zircaloy fuel
pin cladding means that the core melting
and relocation occurs in a timeframe of
tens of minutes: 10 minutes, 20 minutes,
and 30 minutes. In essence, the core has
no recognizable fuel pin geometry after
that point in time. Nevertheless, water
needs to be added to submerge and cool
the core. The rate at which the core
cools (coolability) depends on the core
configuration. However, the accident
management actions are independent of
the core configuration and these are to
submerge the core in water.
In Fukushima, once the core damage
events started - which were different
timings for unit one, unit two, unit three,
the rate at which the water was added
was insufficient to keep the core covered.
The operators also had a very difficult
time getting and keeping a sufficient
water inventory because they used up the
28
Nuclear Plant Journal, January-February 2013
