August 26, 1998
Reactor-Grade Plutonium Can be Used to Make Powerful and
Reliable Nuclear Weapons: Separated plutonium in the fuel
cycle must be protected as if it were nuclear weapons.
by
Richard L. Garwin(1)
Senior Fellow for Science and Technology
Council on Foreign Relations, New York
Draft of August 26, 1998
FAX: (914) 945-4419; Email: rlg2 at watson.ibm.com
As access to technology advances throughout the world, the
barrier to the acquisition of nuclear weapons by terrorists
or nations is more and more the barrier to weapon-usable
fissionable material-- traditionally high-enriched uranium
or "weapon-grade" plutonium. Even a modest nuclear weapon
delivered by aircraft, missile, ship, or truck can threaten
the lives of 100,000 people. Therefore it is important to
understand whether reactor-grade plutonium from the nuclear
fuel cycle-- typically 65% fissile (by thermal neutrons)
compared with 93% fissile for weapon-grade material-- can
readily be used to create nuclear weapons. Unfortunately,
the answer is that it can be so used. The conclusion,
therefore, is that separated reactor-grade plutonium must be
guarded in just the same way as if it were weapon-grade
plutonium if it is not to contribute greatly to the spread
and possible use of nuclear weaponry.
The facts required to judge the utility of reactor-grade
plutonium (R-Pu) for use in nuclear weapons were first made
widely available in 1993 by J. Carson Mark.(2) The isotopic
composition of reactor-grade plutonium as compared with
weapon-grade Pu (W-Pu), results in four differences between
R-Pu and W-Pu:
1. The "bare sphere" critical mass for R-Pu is about 13 kg,
vs. 10 kg for W-Pu (both alpha-phase metal of density 19.6
g/cc).
As regards the usability of R-Pu to make nuclear
weapons, the larger critical mass for R-Pu means that
about 30% more R-Pu metal is needed than W-Pu to build
a weapon.
2. The alpha-particle radioactivity of R-Pu contributes 10.5
watts of heat per kg R-Pu, vs. 2.3 W/kg for W-Pu.
The greater heat evolution (68 watts for half a
bare-sphere critical mass of R-Pu, vs. 11 watts for
half a bare-sphere critical mass of W-Pu) means that
the thick high-explosive that surrounds the plutonium
and any additional metal shells in a simple implosion
weapon will overheat if R-Pu is substituted for W-Pu.
Mark estimates the amount of aluminum heat conductor
that would suffice to cool the R-Pu. So-called
In-Flight Insertion devices that were used in early
nuclear weapons would allow adequate cooling of the
plutonium until it is inserted into the high explosive
a few minutes before detonation.
3. The continuing neutron emission from spontaneous fission
of Pu-240 contributes 360 neutrons per second per g of R-Pu,
vs. about 66 neutrons per second per g of W-Pu.
According to Mark, as the fissionable material is being
compressed so that it becomes critical, a neutron
injected at the worst possible time would cause the
earliest model of implosion weapon to have an explosive
yield between 1 and 2 kilotons (that is, between 1000
tons and 2000 tons of high explosive such as TNT)
rather than the full yield of some 20 kilotons when
neutron injection is optimally timed to occur near the
time of maximum criticality. In contrast, in 1972 the
U.S. Government officially revealed that the U.S.
possessed more advanced nuclear weapons whose yield
would not be diminished by the injection of a neutron
at no matter what instant of time. With this type of
design, the spontaneous neutrons from R-Pu would in no
way diminish the reliability or the expected yield.
4. A mass of R-Pu provides greater radiation exposure to a
person than does W-Pu. At a distance of 1 meter from an
unshielded 6-kg mass of each, the radiation field is 30
millirem per hour for R-Pu, vs. 5 millirem per hour for
W-Pu.
The greater external radiation from a weapon component
of R-Pu compared with that of W-Pu means that the dose
of 5 rem long deemed acceptable for a radiation worker
would be received in 160 hours one meter from a bare
core of R-Pu, vs. 1000 hours for a core of W-Pu.
These facts are interpreted by various bodies as follows:
Mark 1993:
"The difficulties of developing an effective design of the
most straightforward type are not appreciably greater with
reactor-grade plutonium than those that have to be met for
the use of weapons-grade plutonium."
CISAC(3) 1994:
"In short, it would be quite possible for a potential
proliferator to make a nuclear explosive from reactor-grade
plutonium using a simple design that would be assured of
having a yield in the range of one to a few kilotons, and
more using an advanced design. Theft of separated plutonium
whether weapons-grade or reactor-grade, would pose a grave
security risk."
American Nuclear Society Special Panel Report(4) 1995:
"We are aware that a number of well-qualified scientists in
countries that have not developed nuclear weapons question
the weapons-usability of reactor-grade plutonium. While
recognizing that explosives have been produced from this
material, many believe that this is a feat that can be
accomplished only by an advanced nuclear- weapon state such
as the United States. This is not the case. Any nation or
group capable of making a nuclear explosive from weapons-
grade plutonium must be considered capable of making one
from reactor- grade plutonium."
U.S. Department of Energy(5) 1997:
"Proliferating states using designs of intermediate
sophistication could produce weapons with assured yields
substantially higher than the kiloton-range made possible
with a simple, first- generation nuclear device." and
"The disadvantage of reactor-grade plutonium is not so much
in the effectiveness of the nuclear weapons that can be made
from it as in the increased complexity in designing,
fabricating, and handling them. The possibility that either
a state or a sub-national group would choose to use
reactor-grade plutonium, should sufficient stocks of
weapon-grade plutonium not be readily available, cannot be
discounted. In short, reactor-grade plutonium is
weapons-usable, whether by unsophisticated proliferators or
by advanced nuclear weapon states. Theft of separated
plutonium, whether weapons-grade or reactor-grade, would
pose a grave security risk."
As an author of the 1994 CISAC report, I helped formulate
the statement that I quote above. What should the reader
believe? Individuals are often skeptical of official
statements, and it is often said "Those who know, don't
speak; and those who speak, don't know." But that is not
the case with the members of CISAC, all of whom endorsed
this statement; they both know and speak. It is
particularly to be noted that among the Committee are the
following physicists who are knowledgeable about nuclear
weapons and who reviewed a secret study done for CISAC by
the Los Alamos National Laboratory and the Lawrence
Livermore National Laboratory-- the United States' two
nuclear weapon design laboratories. Besides myself, these
include John P. Holdren, Michael M. May, and W.K.H.
Panofsky. May is a former director of the Lawrence
Livermore National Laboratory.
WHY IS THERE SKEPTICISM ABOUT THE WEAPON UTILITY OF
REACTOR-GRADE PLUTONIUM?
The United States has long opposed the spread of nuclear
weapons and nuclear weapons technology to additional states,
and especially to terrorists. This position is that adopted
by almost all of the nations of the world, including Japan,
as embodied in the Non- Proliferation Treaty (NPT), which
entered into force in 1970. Now 185 nations have signed the
NPT, which makes it illegal to transfer nuclear weapons
technology from a nuclear weapons state, and also illegal
for a non-nuclear weapons state to acquire nuclear weapons.
At the same time, the NPT encourages the transfer of nuclear
technology for civil uses, and thus the technology of
nuclear reactors and fuel fabrication and reprocessing can
be communicated to any state that is a member of the NPT,
whether a nuclear weapon state or a non- nuclear weapon
state, provided that safeguards are in place inhibiting the
diversion of weapons-useable materials. According to the
NPT, it is illegal for the United States to explain to a
non-nuclear weapon state how to make a nuclear weapon, and
that is why details of how to fabricate a nuclear weapon
from reactor-grade plutonium cannot be published here or
communicated to any non-nuclear weapon state.
In 1976 the United States decided that releasing some
additional information about nuclear weapons would actually
aid in preventing their spread-- the purpose of the NPT.
The result was a 1976 briefing(6) by the Department of
Energy to nations with active nuclear power programs, so
that they should understand the utility of reactor-grade
plutonium in the fabrication of nuclear weapons, and thus
adopt measures to protect and account for plutonium in spent
fuel downloaded from nuclear power reactors. This was the
information published more fully in the 1993 article by
Mark.
The nations signing the NPT, and the nuclear power industry
worldwide, would be delighted if plutonium produced by
nuclear reactors that operate to generate electrical energy
were not usable to make nuclear weapons, but the facts are
otherwise, as explained in the previous paragraphs.
Nevertheless, some interpret their own wishes as the facts;
and beyond those who are confused in this fashion there are
advocates and publicists (either without the ability to form
their own judgment or who do not recognize the
responsibility to do so) who repeat arguments that-- if
true-- would cut one possible link between nuclear power and
nuclear weapons.
My colleague, Ambassador Imai was an International Member of
the American Nuclear Society Special Panel, which reported
as I have quoted above. The ANS panel included Harold M.
Agnew, who was present in December 1942 at criticality of
the first Fermi reactor, worked at Los Alamos to build the
atomic bomb, and was director of the Los Alamos National
Laboratory for 10 years. But Ambassador Imai has more
recently expressed doubts about the utility of reactor-grade
plutonium for making nuclear weapons. As I read closely his
remarks(7) I see that he suggests that the four
disadvantages of reactor-grade plutonium discussed above
mean that nuclear explosives made from this material could
not be reliable, might be "toy weapons", and that any nation
(as distinguished from terrorists and rogue states, which
could not arm themselves with nuclear weapons) wanting
nuclear weapons would "probably equip themselves with modern
weapons, mainly thermonuclear bombs" instead of "unreliable
bombs with reactor grade plutonium." But the impact of the
authoritative comments that I have quoted (and my own
view)(8) is that a nation could indeed make reliable fission
weapons (and hence the "primaries" for thermonuclear
weapons) by the use of reactor-grade plutonium.
The Summary of a February 1998 report(9) of the Royal
Society of Britain, chaired by Sir Ronald Mason, states of
the U.K. activity in reprocessing fuel from its nuclear
reactors and from those of foreign customers including
Japan:
"The existence of plutonium stocks, in whatever form,
is of concern on two counts: radiotoxicity and
proliferation risk. Whilst not underestimating
radiotoxicity risks, the chance that the stocks of
plutonium might, at some stage, be accessed for illicit
weapons production is of extreme concern. The current
stockpiling policy should not be maintained without
careful study of alternatives."
More directly, it observes:
"The surest anti-proliferation measure is to stop
reprocessing spent fuel and to reduce the quantity of
separated plutonium in store."
However I was troubled by the report's statement (p. 6):
"The critical mass of fissile plutonium (Pu-239 and
Pu-241) needed to sustain a chain reaction in
reactor-grade plutonium may be <an order of magnitude>
greater than for weapons-grade plutonium. The
reliability and yields of weapons constructed from
reactor-grade plutonium might also be reduced.
However, an experienced weapons designer could have
confidence in a weapons system based on reactor grade
plutonium <with 85% fissile content>. Reactor grade
plutonium, of known isotopic composition, must
therefore be regarded as a plausible target for
determined terrorist groups or states wishing to make
nuclear weapons."
As should be clear from my own analysis, the portions I have
surrounded by angular brackets "<...>" are incorrect. We
have noted that the bare-sphere critical mass of
reactor-grade plutonium extracted from highly irradiated
spent fuel from a normal pressurized water reactor or
boiling water reactor operating at 43,000 megawatt-days per
kg fuel is 13 kg-- only 30% (not the 100 kg which would be a
factor 10 or "an order of magnitude") greater than the 10-kg
critical mass of weapon-grade plutonium. And this is
reactor-grade plutonium with 66% "fissile content". The
point is that "non-fissile" Pu-240 is fissionable with the
fast neutrons that carry the chain reaction in plutonium
metal; in fact, even pure Pu-240 has a critical mass of 40
kg-- smaller than pure U-235-- for use in a nuclear weapon.
In a clarifying letter,(10) Sir Ronald Mason states that the
"order of magnitude greater" critical mass refers to
plutonium oxide, as compared with plutonium metal; he
writes also that he agrees the data I provide above on
critical mass, and notes further that the yield will depend
somewhat on the precise isotopic composition.
WHO IS CAPABLE OF USING REACTOR-GRADE PLUTONIUM TO MAKE
NUCLEAR WEAPONS?
None of the five nuclear weapon states (U.S., Russia,
Britain, France, and China) is believed to have in its
stockpile nuclear weapons made from reactor-grade plutonium.
In part this is due to their light- water power reactors
coming later than their nuclear weapons programs. But per
unit of heat removed generated in the reactor (which is a
limiting characteristic and cost of a plutonium production
program), plutonium is best obtained by reprocessing at low
burnup, and hence while it is still "weapon grade". And at
higher burnup, much of the Pu- 239 generated in the reactor
is fissioned and thus lost.
As Carson Mark made clear, the difficulties in making a
nuclear weapon with reactor-grade plutonium are not
different in kind than those involved in the use of
weapon-grade material. Made of reactor- grade plutonium, a
simple fission weapon a fraction of the time may have an
explosion yield of 1000 to 2000 tons of high explosive-- the
equivalent of 1000 truck bombs going off simultaneously at
one point, plus the effects of nuclear radiation; but it
would never have a lower yield, and a fraction of the time
it would have full design yield of 20 or 40 kilotons. As
for the "more sophisticated" designer, it is my own judgment
that not only the five nuclear weapon states, but also the
nuclear weapon establishments of India, Pakistan, and Israel
are capable of converting reactor-grade plutonium into
nuclear weapons that have similar yield and reliability to
those made with weapon- grade plutonium. (This paragraph
was drafted before the nuclear weapon test explosions by
India and Pakistan in May, 1998).
In conclusion, separated plutonium-- whether weapon grade or
reactor grade-- poses a similar danger of misuse in nuclear
weapons and must be provided similar physical protection,
control, and accountancy. This has been recognized by the
International Atomic Energy Agency (IAEA) from its
beginning-- all plutonium (except Pu-238 of isotopic purity
greater than 80%) is regarded as equally hazardous from the
point of view of diversion to nuclear weaponry.
WHAT SHOULD BE DONE ABOUT NUCLEAR WEAPONS AND ABOUT EXCESS
NUCLEAR WEAPON MATERIALS?
I am a member of the Committee on International Security and
Arms Control (CISAC) of the National Academy of Sciences,
which in July 1997 published a report(11) that strongly
urges the U.S. and Russia to reduce their nuclear weapons to
a level of 2000 total weapons, in contrast to the present
10,000 to 20,000 they now possess. If the weapons cannot be
dismantled and disposed of immediately, then they should be
demilitarized-- rendered incapable of being detonated before
the plutonium or uranium can be removed. And the excess
weapons should immediately be subject to bilateral and as
soon as possible to international (IAEA) accounting. It was
not the task of the CISAC to evaluate the risk of separated
civil plutonium being used in nuclear weaponry, but it is
clear that the CISAC analysis considers separated
reactor-grade plutonium, when it has been extracted from
spent fuel, as representing the same degree of hazard as
does weapon plutonium and that it should be subject to the
same measures of physical protection and accountancy.
We urge that the weapon-usable plutonium and high-enriched
uranium from dismantled weapons must be protected according
to the "stored nuclear weapons standard" and it is important
that the separated weapon materials be converted as soon as
possible to meet the "spent fuel standard". The nuclear
weapon materials in that form are then no more attractive a
source of weapon-usable material than the much larger amount
of Pu present in the unprocessed spent fuel from the 400
power reactors in the world today. Pakistan's nuclear
explosions are an urgent reminder that the excess
high-enriched uranium from dismantled nuclear weapons in
Russia and the United States is an ideal material from which
to make fission weapons, and that more must be done to
provide the conditions and resources to dilute this material
to the status of low-enriched uranium (less than 20% U-235)
so that it cannot be directly used to make a fission weapon.
An up-to-date and thorough presentation of the current
status of physical protection, the implication of the stored
weapons standard, and what steps could be taken to
strengthen global standards, is now available on the web and
is forthcoming in book form.(12)
I agree with Ambassador Imai(13) that Japan and the other
non-nuclear states of the NPT should play a more active role
in urging the U.S. and Russia to more rapid reductions in
their nuclear weaponry and to detailed consideration of the
elimination of nuclear weapons. The CISAC report considers
elimination (or, rather, prohibition) of nuclear weapons as
worthy of discussion, but argues that until agreement on
prohibition can be reached, it is both practical and
essential to make massive reductions in all nuclear weapons.
----------------
1 The author consulted for the Los Alamos National
Laboratory from 1950 to 1993, and since then for the
Sandia National Laboratories. Most of his work at Los
Alamos was involved with nuclear weapons design,
manufacture and testing. In recent years he has
reviewed for the Department of Energy matters related to
nuclear weaponry and particularly the Stockpile
Stewardship program, primarily as a member of the JASON
group of consultants to the U.S. Government. Some of
the JASON reports are available at
http://www.fas.org/rlg together with other recent papers
by the author. He is a member of the National Academy of
Sciences, the National Academy of Engineering, and the
Institute of Medicine. In 1997 he received the Enrico
Fermi Award from President Clinton and the Department of
Energy, "for a lifetime of achievement in the field of
nuclear energy." He is a member of the National Academy
of Sciences' Committee on International Security and
Arms Control, which in 1994 and 1995 published two
reports "The Management and Disposition of Excess
Weapons Plutonium". He served on the 9-person
Commission to Assess the Ballistic Missile Threat to the
United States, established by the U.S. Congress, that
issued its report in July 1998.
2 J. Carson Mark, "Explosive Properties of Reactor-Grade
Plutonium," Science and Global Security, 4, 111-128,
______________________________
1993. Mark headed the Theoretical Division at the Los
Alamos National Laboratory for decades; he died in 1997.
T-Division played a major role in nuclear weapons
design, and Mark was intimately involved in the design
of both fission weapons and thermonuclear weapon. Mark
had already in August 1990 prepared a report for the
Nuclear Control Institute (http://www.nci.org) titled
"Reactor-Grade Plutonium's Explosive Properties".
3 "The Management and Disposition of Excess Weapons
Plutonium," Committee on International Security and Arms
Control (CISAC) of the National Academy of Sciences,
National Academy Press, Washington, DC (1994). The full
text is available at
http://www.nap.edu/readingroom/enter2.cgi?0309050421.html.
See pages 32-33 for discussion of nuclear weapons from
reactor-grade plutonium.
4 "Protection and Management of Plutonium", American
Nuclear Society, Special Panel Report (August 1995).
See page 25.
5 "Final Nonproliferation and Arms Control Assessment of
Weapons-Usable Fissile Material Storage And Excess
Plutonium Disposition Alternatives", U.S. Department of
Energy, pp. 66-68, 13 January 1997. (Available at
http://twilight.saic.com/md/docs/ as "finalnew.pdf".
6 At a meeting of the Atomic Energy Forum/American Nuclear
Society.
7 Plutonium, 19, 3-5, Autumn 1997.
__________
8 Correspondence between Ambassador Imai and myself is
reproduced in Plutonium, 22, Summer 1998.
__________
9 "Management of Separated Plutonium", The Royal Society
(The UK Academy of Science), London (ISBN: 0 85403 514
1). Summary at http://www.royalsoc.ac.uk/st_pol24.htm.
10 Sir Ronald Mason, letter to R.L. Garwin, 2 June 1998.
11 "The Future of U.S. Nuclear Weapons Policy," report of
Committee on International Security and Arms Control,
National Academy of Sciences, Washington, D.C., National
Academy Press, June 1997. Text available at
http://www.nap.edu/readingroom/enter2.cgi?0309063671.html.
12 Full text at: www.ksg.harvard.edu/bcsia (click on
"Publications"). Matthew Bunn, "Security for
Weapons-Usable Nuclear Materials: Expanding
International Cooperation, Strengthening International
Standards," in Comparative Analysis of Approaches to
Protection of Fissile Materials: Proceedings of a
Workshop at Stanford California, July 28-30, 1997.
Livermore, CA: Lawrence Livermore National Laboratory,
Document Conf.-97-0721, 1998
13 "Japan Should Initiate Creation of International
Committee to have Specific Plan for the Elimination of
N-Weapons," Plutonium 21, 2-6 (Spring 1998).
_________
WHAT JAPAN CAN DO TO HELP ITS ECONOMY AND DEPLOY ADDITIONAL
NUCLEAR POWER.
o Safe and affordable energy is essential to a modern,
developed society, and electrical power from nuclear
reactors can be both safe and affordable. Japan should
continue to deploy light-water reactors as energy demand
requires, with a government role in ensuring safety of
operation.
o The normal operation of the uranium supply and
enrichment market is adequate for powering Japanese
reactors for several decades. However, a particular
opportunity is available to buy from Russia about 10,000
tons of low-enriched uranium that would be produced by
blending excess high-enriched uranium from Russian
nuclear weapons; this would be delivered as 4% U-235 and
would suffice to power all existing Japanese reactors
for ten years.
o For the long run, fuel for all of the world's reactors
could be supplied for centuries and even thousands of
years by uranium from seawater-- a field in which Japan
has played a leading role. Recent Japanese work (May
1998) projects a cost of $100 per kilogram of uranium
from seawater, in comparison with something like $20/kg
of uranium from ore. But the seawater resource in
enough to operate 10,000 power reactors for 1000 years
(without breeding). Even at $200/kg, uranium from
seawater would be cheaper than reprocessing spent fuel
and recycling plutonium and uranium. Uranium at $200/kg
would increase the cost of nuclear energy by about 0.4
cents per kWh.
o As of December 1996, there was in Japan about five tons
of civil unirradiated plutonium, and about 15 tons of
civil unirradiated Japanese plutonium in foreign
countries. In addition, in spent civil reactor fuel in
Japan there was almost 50 tons of plutonium. We note
that ten tons of civil plutonium would suffice to make
more than 1000 nuclear weapons. As with civil and
military plutonium anywhere in the world, these Japanese
stocks must be protected and safeguarded if they are not
to contribute to the acquisition of nuclear weapons by
other nations and sub-national groups.
o Finally, the interests of the Japanese consumer of
electrical energy and of the producer of electrical
energy would be well served by the availability of a
mined geologic repository, whether it is used for the
vitrified fission products from the reprocessing plants
at La Hague, France, or at Sellafield, Britain, or from
reprocessing in Japan. Furthermore, the repository
could equally well hold spent fuel in appropriate
disposal casks, as is planned in the United States. It
seems to me highly desirable to have competitive,
commercial, mined geologic repositories in various
countries of the world, with the repositories and the
waste forms (including spent fuel) regulated by the
International Atomic Energy Authority. Many areas of
the world, as well as the nuclear energy industry
itself, would benefit from the availability of such
repositories, which might be built in China, in the
United States, in Australia and Africa.