Scientific American  [Printer-friendly version]
January 1, 1998


By Charles D. Hollister and Steven Nadis

Although the notion troubles some environmentalists, the disposing of
nuclear refuse within oceanic sediments merits consideration

On the floor of the deep oceans, poised in the middle of the larger
tectonic plates, lie vast mud flats that might appear, at first
glance, to constitute some of the least valuable real estate on the
planet. The rocky crust underlying these "abyssal plains" is blanketed
by a sedimentary layer, hundreds of meters thick, composed of clays
that resemble dark chocolate and have the consistency of peanut
butter. Bereft of plant life and sparsely populated with fauna, these
regions are relatively unproductive from a biological standpoint and
largely devoid of mineral wealth.

Yet they may prove to be of tremendous worth, offering a solution to
two problems that have bedeviled humankind since the dawn of the
nuclear age: these neglected sub-oceanic formations might provide a
permanent resting place for high-level radioactive wastes and a burial
ground for the radioactive materials removed from nuclear bombs.
Although the disposal of radioactive wastes and the sequestering of
material from nuclear weapons pose different challenges and
exigencies, the two tasks could have a common solution: burial below
the seabed.

High-level radioactive wastes--in the form of spent fuel rods packed
into pools at commercial nuclear power plants or as toxic slurries
housed in tanks and drums at various facilities built for the
production of nuclear weapons--have been accumulating for more than
half a century, with no permanent disposal method yet demonstrated.
For instance, in the U.S. there are now more than 30,000 metric tons
of spent fuel stored at nuclear power plants, and the amount grows by
about' 2,000 metric tons a year. With the nuclear waste repository
under development at Yucca Mountain, Nev., now mired in controversy
and not expected to open before 2015 at the earliest [see "Can Nuclear
Waste Be Stored Safely at Yucca Mountain?" by Chris G. Whipple;
SCIENTIFIC AMERICAN, June 1996], pressure is mounting to put this
material somewhere.

The disposition of excess plutonium and uranium taken from
decommissioned nuclear weapons is an even more pressing issue, given
the crisis that might ensue if such material were to fall into the
wrong hands. The U.S. and Russia have each accumulated more than 100
metric tons of weapons-grade plutonium, and each country should have
at least 50 metric tons of excess plutonium, plus hundreds of tons of
highly enriched uranium, left over from dismantled nuclear weapons.
Preventing terrorists or "rogue states" from acquiring this material
is, obviously, a grave concern, given that a metric ton of plutonium
could be used to make hundreds of warheads, the precise number
depending on the size of the bomb and the ingenuity of the designer.

The Clinton administration has endorsed two separate methods for
ridding the nation of this dangerous legacy. Both entail significant
technical, economic and political uncertainties. One scheme calls for
the surplus weapons plutonium to be mixed with radioactive wastes and
molded into a special type of glass (a process called vitrification)
or, perhaps, ceramic for subsequent burial at a site yet to be chosen.
The glass or ceramic would immobilize the radioactive atoms (to
prevent them from seeping into the surrounding environment) and would
make deliberate extraction of the plutonium difficult. But the matrix
material does not shield against the radiation, so vitrified wastes
would still remain quite hazardous before disposal. Moving ahead with
vitrification in the U.S. has required construction of a new
processing plant, situated near Aiken, S.C. Assuming this facility
performs at its intended capacity, each day it will produce just one
modest cylinder of glass containing about 20 or so kilograms of
plutonium. The projected cost is $1.4 million for each of these glassy
logs. And after that considerable expense and effort, someone still
has to dispose of the highly radioactive products of this elaborate

The second option would be to combine the recovered plutonium with
uranium oxide to create a "mixed oxide" fuel for commercial reactors--
although most nuclear power plants in the U.S. would require
substantial modification before they could run on such a blend. This
alternative measure of consuming mixed-oxide fuels at commercial power
plants is technically feasible but nonetheless controversial. Such
activities would blur the traditional separation between military and
civilian nuclear programs and demand heightened security, particularly
at mixed-oxide fabrication plants (of which none currently exist in
the U.S.), where material suitable for building a nuclear bomb might
be stolen. And in the end, mixed-oxide reactors would produce other
types of radioactive waste. Hence, neither of the schemes planned for
disposing of material from nuclear weapons is entirely satisfactory.

Pressing Problems

For the past 15 years, the operators of nuclear power plants in the
U.S. have been paying the Department of Energy in advance for the
eventual storage or disposal of their wastes. Even though there is no
place yet available to put this radioactive refuse, the courts have
ordered the DOE to meet its contractual obligations and begin
accepting expended fuel rods from nuclear utilities this year. It is
not at all clear what the DOE will do with these materials. One plan
supported by the U.S. Senate is to build a temporary storage facility
in Nevada near the Yucca Mountain site, but President Bill Clinton
opposes this stopgap measure. In any event, the mounting pressure to
take some action increases the likelihood of hasty, ill-considered
judgments. The best course, in our opinion, would be to do nothing too
drastic for now; immediate action should be limited to putting the
spent fuel currently residing in cooling ponds into dry storage as
needed and trying to stabilize the leaks in high-level-waste
containers at weapons sites, while scientists and engineers thoroughly
investigate all reasonable means for permanent disposal.

Although some ambitious thinkers have suggested that nuclear waste
might one day be launched into space and from there cast into the sun,
most people who have studied the problem agree that safety and economy
demand that the waste be put permanently underground. Curiously, the
search for a suitable nuclear graveyard has been confined almost
exclusively to sites on the continents, despite the fact that geologic
formations below the world's oceans, which cover some 70 percent of
the planet's surface, may offer even greater potential. The disposal
of nuclear weapons and wastes below the seabed should not be confused
with disposal in the deep-ocean trenches formed at the juncture of two
tectonic plates-a risky proposition that would involve depositing
waste canisters into some of the most geologically unpredictable
places on the earth, with great uncertainty as to where the material
would finally reside.

Sub-seabed disposal, in contrast, would utilize some of the world's
most stable and predictable terrain, with radioactive waste or nuclear
materials from warheads "surgically" implanted in the middle of
oceanic tectonic plates. Selecting sites for disposal that are far
from plate boundaries would minimize chances of disruption by
volcanoes, earthquakes, crustal shifts and other seismic activity.
Many studies by marine scientists have identified broad zones in the
Atlantic and Pacific that have remained geologically inert for tens of
millions of years. What is more, the clay-rich muds that would entomb
the radioactive materials have intrinsically favorable
characteristics: low permeability to water, a high adsorption capacity
for these dangerous elements and a natural plasticity that enables the
ooze to seal up any cracks or rifts that might develop around a waste
container. So the exact form of the wastes (for example, whether they
are vitrified or not) does not affect the feasibility of this
approach. No geologic formations on land are known to offer all these
favorable properties.

It is also important to note that disposal would not be in the oceans,
per se, but rather in the sediments below. Placing nuclear waste
canisters hundreds of meters underneath the floor of the deep ocean
(which is, itself, some five or so kilometers below the sea surface)
could be accomplished using standard deep-sea drilling techniques. The
next step-backfilling to seal and pack the boreholes-is also a routine
practice. This technology has proved itself through decades of use by
the petroleum industry to probe the continental shelves and, more
recently, by members of the Ocean Drilling Program, an international
consortium of scientific researchers, to explore deeper locales.

We envision a specialized team of drillers creating boreholes in the
abyssal muds and clays at carefully selected locations. These
cylindrical shafts, some tens to hundreds of meters deep, would
probably be spaced several hundred meters apart to allow for easy
maneuvering. Individual canisters, housing plutonium or other
radioactive wastes, would then be lowered by cable into the holes. The
canisters would be stacked vertically but separated by 20 or more
meters of mud, which could be pumped into the hole after each canister
was emplaced.

As is the case for disposal within Yucca Mountain, the waste canisters
themselves would last a few thousand years at most. Under the seabed,
however, the muddy clays, which cling tenaciously to plutonium and
many other radioactive elements, would prevent these substances from
seeping into the waters above. Experiments conducted as part of an
international research program concluded that plutonium (and other
transuranic elements) buried in the clays would not migrate more than
a few meters from a breached canister after even 100,000 years. The
rates of migration for uranium and some other radioactive waste
elements need yet to be properly determined. Still, their burial
several tens to 100 meters or more into the sediments would most
likely buy enough time for the radioactivity of all the waste either
to decay or to dissipate to levels below those found naturally in

The Seabed Working Group, as the now defunct research program was
called, consisted of 200 investigators from 10 countries. Led by the
U.S. and sponsored by the Nuclear Energy Agency of the Organization
for Economic Cooperation and Development, the project ran from 1976 to
1986 at a total cost of about $120 million. This program was an
outgrowth of a smaller effort at Sandia National Laboratories that was
initiated in response to a suggestion by one of the authors
(Hollister), who conceived of the idea of subseabed disposal in 1973.

As part of the international program, scientists extracted core
samples of the seabed and made preliminary environmental observations
at about half a dozen sites in the northern Atlantic and Pacific
oceans. The collected sediments showed an uninterrupted history of
geologic tranquillity over the past 50 to 100 million years. And there
is no reason to believe that these particular sites are extraordinary.
On the contrary, thousands of cores from other midplate locations
since examined as part of the Ocean Drilling Program indicate that the
sediments that were studied originally are typical of the abyssal
clays that cover nearly 20 percent of the earth. So one thing is
clear: although other factors may militate against subseabed disposal,
it will not be constrained by a lack of space.

Reviving an Old Idea

The Seabed Working Group concluded that although a substantial body of
information supports the technical feasibility of the concept, further
research "should be conducted before any attempt is made to use seabed
disposal for high-level waste and spent fuel." Unfortunately, the
additional investigations were never carried out because the U.S.--the
principal financial backer of this research--cut off all funding in
1986 so that the nation could concentrate its efforts on land-based
disposal. A year later the federal government elected to focus
exclusively on developing a repository at Yucca Mountain--a
shortsighted decision, especially in view of current doubts as to
whether the facility will ever open. And even if the Yucca Mountain
repository does become operational, it will not be able to handle all
the high-level wastes from military and commercial sources that will
have accumulated by the time of its inauguration, let alone the 2,000
or more tons of waste each year the nuclear industry will continue to
churn out.

At some point, policymakers are going to have to face this reality and
start exploring alternative sites and approaches. This view was
precisely the conclusion expressed in a 1990 report from the National
Academy of Sciences, which said that alternatives to mined geologic
repositories, including subseabed disposal, should be pursued-a
recommendation that remains absolutely valid today.

Fortunately, most of the experiments needed to assess more fully both
the reliability and safety of subseabed disposal have been designed,
and in many cases prototype equipment has already been built. One
important experiment that remains to be done would be to test whether
plutonium and other radioactive elements move through ocean-floor
clays at the same rates measured in the laboratory. And more work is
required to learn how the heat given off by fuel rods (caused by the
rapid decay of various products of nuclear fission) would affect
surrounding clays.

Research is also needed to determine the potential for disturbing the
ecology of the ocean floor and the waters above. At present, the
evidence suggests that mobile, multi-cellular life-forms inhabit only
the top meter or so of the abyssal clays. Below a meter, there appear
to be no organisms capable of transporting radioactive substances
upward to the seafloor. Still, scientists would want to know exactly
what the consequences would be if radioactive substances diffused to
the seafloor on their own. Researchers would want to ascertain, for
instance, exactly how quickly relatively soluble carriers of
radioactivity (such as certain forms of cesium and technetium) would
be diluted to background levels. And they would want to be able to
predict the fate of comparatively insoluble elements, such as

So far no evidence has been found of currents strong enough to
overcome gravity and bring claybound plutonium particles to the ocean
surface. Most likely the material would remain on the seabed, unless
it were carried up by creatures on the sea bottom. That prospect, and
all other ways that radioactive materials might rise from deep-sea
sediment layers to surface waters, warrant further investigation. The
transportation of nuclear waste on the high seas also requires careful
study. In particular, procedures would need to be developed for
recovering lost cargo should a ship carrying radioactive materials
sink or accidentally drop its load.

Engineers would probably seek to design the waste containers so that
they could be readily retrieved from the bottom of the ocean in case
of such a mishap or, in fact, even after their purposeful burial.
Although subseabed disposal is intended to provide a permanent
solution to the nuclear waste crisis, it may be necessary to recover
material such as plutonium at some point in the future. That task
would require the same type of drilling apparatus used for
emplacement. With the location of the waste containers recorded at the
time of interment, crews could readily guide the recovery equipment to
the right spot (within a fraction of a meter) by relying on various
navigation aids. At present, no non-nuclear nation has the deep-sea
technology to accomplish this feat. In any event, performing such an
operation in a clandestine way would be nearly impossible. Hence, the
risk that a military or terrorist force could hijack the disposed
wastes from under the seabed would be negligible.

All Eggs in One Basket

The overall cost of a concerted program to evaluate subseabed disposal
might reach $250 million--admittedly a large sum for an oceanographic
research endeavor. But it is a relatively modest price to pay
considering the immense benefits that could result. (As a point of
comparison, about $2 billion has already been spent on site evaluation
at Yucca Mountain, and another billion or two will probably be needed
to complete further studies and secure regulatory approval. No actual
construction, save for exploratory tunneling, has yet begun.) Yet no
nation seems eager to invest in any research at all on subseabed
disposal, despite the fact that it has never been seriously challenged
on technical or scientific grounds. For example, a 1994 report by the
National Academy of Sciences that reviewed disposal options for excess
weapons plutonium called subseabed disposal "the leading alternative
to mined geologic repositories" and judged implementation to be
"potentially quick and moderate to low cost." But the academy panel
stopped short of recommending the approach because of the anticipated
difficulties in gaining public acceptance and possible conflicts with
international law.

Convincing people of the virtues of subseabed burial is, admittedly, a
tough sell. But so is the Yucca Mountain project, which is strongly
opposed by state officials and residents of Nevada. Subseabed disposal
may turn out to be easier to defend among the citizenry than land-
based nuclear waste repositories, which are invariably subject to the
"not in my backyard" syndrome.

In any case, subseabed disposal is certain to evoke significant
opposition in the future should the idea ever go from being a remote
possibility to a serious contender. Oddly, the concept has recently
come under direct fire, even though no research has been done in more
than a decade. A bill introduced last year in the House of
Representatives contains a provision that would prohibit the subseabed
disposal of spent nuclear fuel or high-level radioactive waste as well
as prevent federal funding for any activity relating to subseabed
disposal--apparently including research. The intent of part of this
bill is reasonable: subseabed disposal should be illegal until
outstanding safety and environmental issues can be resolved. But it
makes absolutely no sense to ban research on a technically promising
concept for the disposal of weapons plutonium and high-level nuclear

Subseabed disposal faces serious international hurdles as well. In
1996, at a meeting sponsored by the International Maritime
Organization, contracting parties to the so-called London Dumping
Convention voted to classify the disposal of nuclear material below
the seabed as "ocean dumping" and therefore prohibited by
international law. This resolution still awaits ratification by the
signatory nations, and the outcome may not be known for several years.
But regardless of how that vote goes, we submit that "ocean dumping"
is a wholly inappropriate label. It makes as much sense as calling the
burial of nuclear wastes in Yucca Mountain "roadside littering."

Yet even assuming that the nations involved uphold the ban, the bylaws
of the London convention would allow for subseabed disposal to be
reviewed in 25 years, an interval that would provide sufficient time
to complete a comprehensive appraisal of this disposal method. The 25-
year moratorium could be wisely spent addressing the remaining
scientific and engineering questions as well as gaining a firmer grasp
of the economics of this approach, which remains one of the biggest
uncertainties at present. In our most optimistic view, the legal
infrastructure already established through the London convention could
eventually support a program of subseabed disposal on an international

A parallel effort should be devoted to public education and
discussion. Right now there seems to be a strong aversion among some
environmental advocates to any action at all to address the nuclear
waste problem--and a solution that involves the oceans seems
particularly unpalatable. But it makes no sense to dismiss the
possibility of disposal in stable sub-oceanic formations--which exceed
the land area available for mined repositories by several orders of
magnitude-simply because some people object to the concept in general.
It would be much more prudent to base a policy for the disposal of
nuclear waste, whose environmental consequences might extend for
hundreds of thousands of years, on sound scientific principles.

Barring a miraculous technical breakthrough that would allow
radioactive elements to be easily transformed into stable ones or
would provide the safe and economic dispatch of nuclear wastes to the
sun, society must ultimately find somewhere on the planet to dispose
of the by-products of the decades-long nuclear experiment. Americans
in particular cannot responsibly pin all hopes on a single, undersized
facility in a Nevada mountainside. They owe it to future generations
to broaden their outlook and explore other possibilities, including
those that involve the thick, muddy strata under the sea.

Further Reading

G.R. Heath in Science, Vol. 213, pages 1321-1326; September 18, 1981.

Research Council. National Academy Press, 1994.

THE SUB-SEABED SOLUTION. Steven Nadis in Atlantic Monthly, pages
28-39; October 1996.

Today, Vol. 50, No. 6, pages 24-29; June 1997.


Charles D. Hollister and Steven Nadis began regular discussions about
subseabed disposal of nuclear wastes in 1995. Hollister, who is a vice
president of the corporation of Woods Hole Oceanographic Institution,
has studied deep-sea sediments for the past three decades. He
continues to do research in the department of geology and geophysics
at Woods Hole. Nadis graduated from Hampshire College in 1977 and
promptly joined the staff of the Union of Concerned Scientists, where
he conducted research on nuclear power, the arms race and renewable
energy sources. He then wrote about tranportation policy for the World
Resources Institute. Currently a Knight Science Journalism Fellow at
the Massachusetts Institute of Technology, Nadis specializes in
writing about science and technology.