How safe is Diablo Canyon's nuclear waste?
Leroy N. Soetoro
diablo-canyon.html
Japan�s Fukushima disaster has prompted PG&E to accelerate transfer of
Diablo Canyon�s spent fuel out of its cooling pools � though not to the
lowest capacity for which the pools were originally designed.
Seventy-five percent of the highly radioactive fuel used by the nation�s
nuclear plants remains today in cooling pools at those facilities.
At Diablo Canyon nuclear power plant, the amount is higher � 81 percent of
spent fuel sits in two pools. In response to the nuclear disaster in
Japan, PG&E plans to reduce the amount of used fuel assemblies it keeps in
its pools at Diablo Canyon. But it has no plans to reduce them to their
original and safest low-density configuration.
The safety of the spent fuel storage pools at the nation�s 104 nuclear
reactors has come under renewed scrutiny in the wake of the March 11
nuclear disaster in Japan. In addition to the crippled reactors at the
Fukushima Dai-ichi plant, a spent fuel pool also contributed to the
crisis. Workers there had to take emergency steps to keep it full of water
to avoid a catastrophic fire.
Nuclear watchdog and public safety groups say the first step that should
be taken to increase the safety of U.S. nuclear plants is to transfer
spent fuel to dry-cask storage, leaving only the newest assemblies that
must cool off in the pools for five years for safety reasons.
�In protecting America from nuclear catastrophe, safely securing the spent
fuel by eliminating highly radioactive, crowded pools should be a public
safety priority of the highest degree,� wrote Robert Alvarez in May in a
report for the Institute for Policy Studies, a liberal Washington, D.C.,
think tank.
Operators at California�s two nuclear plants � Diablo Canyon and the San
Onofre Nuclear Generating Station near San Clemente � say they have no
plans to take that step. The federal Nuclear Regulatory Commission does
not require it.
�The NRC has determined that storage and management of used fuel in either
an engineered pool or in sealed canisters as part of an independent fuel
storage installation are technologies that protect public health and
safety,� said Gil Alexander of Southern California Edison, the utility
that operates San Onofre.
According to the Nuclear Energy Institute, an industry group, about 75
percent of the nation�s spent fuel is stored in crowded pools, with the
remaining 25 percent in dry casks. California�s two nuclear plants mirror
this national trend. Diablo Canyon has 81 percent of its spent fuel still
in the two pools, and San Onofre has 68 percent of its spent fuel in its
two pools.
Dangerous end product
Spent fuel � a form of high-level nuclear waste � is one of the most
hazardous known materials. Direct exposure to its intense radiation would
kill a person within minutes, and it stays dangerous for tens of thousands
of years.
Studies have shown that spent fuel in densely loaded pools could catch
fire if the water drained out. Conversely, loosely stored fuel has enough
space between the assemblies to prevent a fire.
Critics charge that spent fuel pools lack some of the safety features that
protect a nuclear power plant�s reactors. For example, at Diablo Canyon,
the spent fuel pools are robust concrete-and-steel-lined structures, but
they are located outside the containment domes that house the reactors.
One of the main purposes of the containment domes is to prevent the spread
of radiation in the event of an accident. According to a 2004 study by the
National Academy of Sciences, smoke from a fire in a pool could spread
�radioactive aerosols hundreds of miles downwind under appropriate
atmospheric conditions.�
Used fuel assemblies are stored in pools for five years before they can be
transferred to dry casks. These casks are huge, sealed steel-and-concrete
containers.
Sixteen loaded casks sit bolted to a thick concrete slab behind the Diablo
Canyon plant.
Most nuclear experts say that dry-cask storage is safer than pools because
it is passive and does not require pumps and other equipment to keep it
cool. Heat that builds up in each cask is slowly released into the air.
The hardened casks are also a less attractive target for terrorists. It
would be much easier for terrorists to drain a spent fuel pool than breach
a dry cask.
In 2008, Gregory Jaczko, then a board member of the NRC and now its
chairman, admitted that �the most clear-cut example of an area where
additional safety margins can be gained involves additional efforts to
move spent nuclear fuel from pools to dry-cask storage.�
In spite of this, the NRC�s official policy remains that the spent fuel
pools are sufficiently safe. The agency maintains that there is no
pressing safety or security reason to mandate earlier transfer of fuel
from pool to cask.
Stored at Diablo
At Diablo Canyon, the spent fuel pools contain more than five times the
number of assemblies than the reactors. The utility plans to reduce the
density of the pools by about 45 percent over the next five years, to
about 600 assemblies per pool.
�It is important to try to create a buffer,� said Jearl Strickland, Diablo
Canyon�s used fuel manager. �We want to show people that we are serious
about reducing the risk.�
However, there are no plans to return the pools to their original low-
density configuration, called open racking, of 270 assemblies per pool.
Doing so would not allow the utility to load the spent fuel casks with the
ideal mixture of older and new used fuel, Strickland said.
The dry casks are filled using a storage technique with newer, hotter fuel
surrounded by older fuel in the casks. If PG&E technicians reduced the
density to open racking, they would not have enough older fuel available
to load the spent fuel casks with the proper ratio of older and newer
fuel, Strickland said.
Concern about nuclear waste at Diablo Canyon is long-standing. In 2006,
the anti-nuclear group San Luis Obispo Mothers for Peace sued the NRC to
force the agency to look at the environmental effects of a radiation
release caused by a terrorist attack at Diablo Canyon. The agency
determined that the probability of such an incident is so low that it need
not be taken into account.
�The ongoing catastrophe in Fukushima illustrates the folly of that
position,� said Jane Swanson, a Mothers for Peace spokeswoman.
In the aftermath of the Japanese disaster, the NRC is requiring that all
utilities review and test their emergency procedures and report the
results. A recent NRC inspection found more than 20 problems with Diablo
Canyon�s emergency preparedness.
The most significant was a backup cooling pump that failed to start; it
has since been repaired. In spite of these problems, the NRC said plant
operators would still have been able to deal with a major emergency.
PG&E is correcting those problems. The utility also takes several
precautions to keep the spent fuel pools safe, Strickland said.
They center on ensuring that the pools are kept filled with water, which
is circulated continually to prevent it from boiling away. The plant�s
emergency diesel generators produce enough electricity to power the pumps
that keep cooling water circulating around the used assemblies, even in
the event of a blackout, as happened in Japan.
If the pumps fail, procedures are in place to use emergency fire pumps and
hoses to keep the water circulating, Strickland said. As a last resort,
water from two freshwater ponds behind the plant, containing a total of 5
million gallons, could be gravity-fed into the pools.
Additionally, the NRC requires that spent fuel in the pools be arranged in
a checkerboard fashion, with the newest and hottest assemblies surrounded
by older fuel. This reduces the chances that the fuel would catch fire if
the assemblies were ever exposed in an emergency.
No permanent solution
The nation got into this dilemma because its nuclear power plants were
built with the assumption that used fuel would be taken by the federal
government to be reprocessed into new fuel or stored in a central
underground facility at Yucca Mountain in Nevada. Neither of those
permanent solutions has come about, forcing each nuclear plant to store
its used fuel on-site for the foreseeable future.
Utilities solved this problem on an interim basis by installing new,
higher-density racks while they applied to build dry-cask storage
facilities. At one time, Diablo Canyon�s pools even had temporary racks
installed to accommodate the load.
Since then, Diablo Canyon�s dry-cask storage facility has opened, and 16
casks with 32 spent fuel assemblies each are stored there. The spent fuel
pools contain 1,072 and 1,104 assemblies. The pools can hold a total of
2,648 assemblies.
The establishment of a dry-cask facility at Diablo Canyon comes after a
nationwide trend, with about half of the nation�s nuclear plants using
them. Another 12 storage casks are due to be delivered to Diablo this
summer. The utility will start filling them in January. Ten more casks are
on order for delivery later in 2012, Strickland said.
One-hundred-thirty-eight casks will be needed to store all the spent fuel
the plant will generate in the 40 years of its initial operating license.
PG&E has applied to the NRC to extend the life of the plant by 20 years
starting in 2024 and 2025.
If approved, license renewal will cause the dry-cask facility to
eventually expand to 207 casks.
Graphic: Spent fuel storage
To zoom in, click the "+" in the toolbar below the graphic.
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More nuclear power plant coverage coming this week
Editor�s note:
We�ve long kept readers apace of developments at Diablo Canyon nuclear
power plant, and in recent months have taken close looks at seismic and
design issues, relicensing and emergency preparedness.
Today�s explanatory story on spent fuel pools at Diablo Canyon and at San
Onofre near San Clemente, the state�s other nuclear power plant, addresses
a top safety concern raised in light of the March 11 nuclear disaster in
Japan: How much spent fuel should be transferred to dry-cask storage and
how quickly?
David Sneed, who has covered the environment for The Tribune for the past
14 years, is widely regarded as an authoritative reporter on the nuclear
industry. He will report on the future of on-site storage nationwide � and
what that means long-term for sites like Diablo � on Sunday, June 26.
Beyond Sneed�s insightful coverage, the Associated Press has produced a
four-part investigative series called Aging Nukes. The first story � to be
published Monday � examines how regulators and the U.S. industry have
worked in concert to keep older plants operating, and the potential
problems that poses.
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