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March 30, 2011
NUCLEAR POWER -- THE CORE ISSUES
Arshad M Khan
Before 2:46 PM on Friday, March 11, 2011, Japan's nuclear power industry
had an unmatched record of safety. Then, suddenly that record became a
shambles. An earthquake of magnitude 9.0 continued for five minutes, the
highest since records were kept. Within minutes, it was followed by a
tsunami of unprecedented height and devastation. The reactors shut down
as planned but the cooling systems were compromised. Power failed as did
backup diesel generators to run the pumps circulating cooling water. The
plants carried eight hours of battery backup as a last resort -- greater than
usual at U.S. stations -- but the tsunami cut a wide swath in northeastern
Japan and power became impossible to restore in that time. The event now
has become at least as significant as Three Mile Island if not worse.
In a nuclear reactor, boron control rods serve as brake and accelerator.
Raising them allows the reaction to proceed; lowering them slows it down.
Fission heats the fuel and the surrounding medium. This is pumped through
heat exchangers producing steam to drive turbines connected to alternators
Problems arise if fuel rods cannot be cooled. Unlike, say, pulling an iron out
of a fire where the iron cools immediately, nuclear fuel rods stay hot
(because of decaying radioactive waste products) for days, weeks, even
years. This means once a reactor has been started, it must always have
sufficient coolant to cover the fuel in the reactor. Moreover, the spent fuel
that is stored in tanks on site must also remain covered in water. After a
while, this spent fuel can be encased in dry casks. It is a safer but more
expensive means of storage, so most operators leave the fuel in the holding
tanks until they are out of capacity. In France and Germany, the spent fuel,
when safe to move, is reprocessed and the waste stored at disposal sites.
At Fukushima, much of the spent fuel was still in the holding tanks, and in the
U.S., almost all is stored that way because no agreement can be reached as
to where the waste is to be stored -- the NIMBY problem because the waste
stays radioactive for thousands of years.
So, the central problems with respect to nuclear power are safe disposal of
the waste, and how to keep the reactor safe. The latter requires effective
shut down procedures, which failed at Chernobyl and the reactor blew up. It
also means maintaining coolant flow to prevent meltdown -- the cause of the
problems at Three Mile Island and Fukushima.
In addition, the spent fuel tanks need water covering the fuel rods
replenished as it evaporates from the substantial heat that continues to emit
from radioactive decay. If the rods are exposed, it can lead to their
zirconium cladding being compromised, reacting and emitting hydrogen,
which in contact with air just needs a spark to cause an explosion. This, too,
has happened at Fukushima. If exposed rods start to melt and are stacked
too closely together, there is the additional serious danger of resuming a
nuclear fission reaction, uncontrolled and with horrendous consequences.
The spent fuel at some U.S. plants are stacked even more densely than at
Fukushima because none of it has been ever removed for processing or
storage at a waste disposal facility. And the GE Mark 1 reactor plants (like
Fukushima) designed for a twenty-year life have had their licenses
extended, and have been operating for over forty.
After some years, when it was finally safe to examine the reactor at Three
Mile Island, they discovered three-quarters of the core had suffered
meltdown, much greater than their estimate of ten percent. The uranium fuel
melts at 2750 to 2800 degrees C; at 3000 degrees C, everything melts forming
a coagulated mass known as corium. Heated enough, this can breach the
pressure vessel. If mistaken attempts are made to cool it without adequate
precautions, the resulting steam can lead to an explosion scattering highly
radioactive fission products over a wide area.
Another issue that is frequently raised with nuclear power is energy
balance. How long does it take for a nuclear station to generate the power
used up in building it including the construction of its components and the
mining and crafting of its fuel. It is a question for debate because many
contend it is unable to recover the energy in its planned lifetime.
In the U.S. there are 104 nuclear stations of which 23 are of the same design
as Fukushima. Many of these are storing twice the spent fuel at Fukushima.
Originally designed to store about five years of used fuel rods before they
were removed for processing and for storage at a safe permanent facility,
these are piling up at the plants, packed more and more densely in
increasingly dangerous proximity because Congress is unable to agree on a
suitable disposal site. Yucca mountain, once thought to be the answer is
now off the slate. It is also no longer large enough to accommodate all the
current and expected nuclear waste.
To take a case in point, the Zion, Illinois nuclear station, closed in 1993, still
contains 1100 tons of spent fuel over half the 2000 tons at Fukushima. When
water levels at the latter dropped exposing the spent fuel, the heat damaged
the zirconium cladding, which in turn reacting with water vapor released
hydrogen resulting in an explosion that blew off the roof. An accident at Zion
polluting Lake Michigan would affect the water supply of millions around it.
According to the National Energy Institute, dry cask storage, which renders
the spent fuel relatively safe, is contemplated. But it is 18 years since it was
padlocked, so the question to ask of Zion's owner is why the almost two
There are 7,800 tons of spent fuel in Illinois at seven storage sites. It
comprises 28,425 fuel assemblies in pools and 5156 in dry casks. In light of
Fukushima, it may come as a shock to residents of the Chicago area to
realize there are 11 spent pools within 40 miles. In the U.S. overall, the 104
operating reactors are storing more than 60,000 metric tons of spent fuel,
mostly in pools. Of the reactors, 23 are of similar design to Fukushima --
many well past their planned 20-year life after multiple extensions. In a test
not so long ago, the Riverbend plant in Louisiana was determined to have an
86.5% chance of a blackout scenario (i.e. losing all power as at Fukushima)
under certain conditions. We have plants over fault lines in earthquake
prone zones such as Diablo Canyon in California. While upgraded to a 7.5
magnitude earthquake, the magnitude 9.0 that affected Fukushima was 32
times more powerful. Just as the size of the tsunami there was unforeseen,
the unforeseen is what places such plants under untenable risk.
Fukushima itself has not been resolved. Radiation from it has now been
detected as far away as the United Kingdom and as near as the rainwater in
Massachusetts. The story continues to get worse, and the operators there
are caught on the horns of a dilemma. Water leakage from Reactor #3 is now
reaching the ocean. Continuing to pour water into the reactor will
contaminate the ocean -- but not doing so would expose the fuel rods to air
leading to a meltdown and a possible explosion of active fuel.
Given the unpredictability of natural phenomena and the possible
horrendous consequences of a nuclear accident, one is forced to the
conclusion that only judgment clouded by assurances from vested interests
can postulate nuclear power as a "safe, reliable, clean form of energy."