Security Studies Program & Department of Nuclear Engineering, Massachusetts Institute of Technology
Under what circumstances would
What political/legal route to weapons might it take?; in particular, what are the safeguards implications of a decision to withdraw from the NPT and utilize transferred nuclear technology?
What technical options would
What are the implications of the
1. Selig S. Harrison, "Japan and Nuclear Weapons", in Japan's Nuclear Future: The Plutonium Debate and East Asian Security, S. S. Harrison, Ed., Carnegie Endowment for International Peace, Washington, DC, 1996, pp. 7-44.
2. Yuri Kase,
"The Costs and Benefits of
3. Lillian Hoddeson
et al., Critical Assembly: A Technical History of
The official policy of the post World War II Japanese government has always been to disavow any intention to acquire nuclear weapons. The stated rationale is that Japan, as the only victim of nuclear attack, is especially sensitive to the immense human suffering caused by their use, and that it is protected from nuclear threats by other countries by the US nuclear umbrella. Moreover, Article 9 of the Japanese Constitution renounces war as a sovereign right of the nation, and prohibits the maintenance of military forces as well as other war potential.
Nevertheless, the intensive and protracted debate in Japan over whether and on what terms it should sign and ratify the Nuclear Non-Proliferation Treaty (NPT) - it signed in 1970 and ratified in 1977 - reflected the fact that some Japanese leaders wanted to keep a nuclear weapons option open. Indeed, in 1967 the Prime Minister E. Sato commissioned a secret study to assess the costs and benefits of nuclearization from the technical, economic, strategic, political, and diplomatic perspectives.
recently, speculation about possible Japanese nuclearization
has increased due to several factors including: (1)
the other hand, a nuclear-armed
it would take a geopolitical sea change" for
Politically, Japan could legally withdraw from the NPT by giving three months advance notice, in accordance with Article X, par. 1, instead of violating the treaty by acquiring nuclear weapons while still a non-weapons state party to it. But the agreements for nuclear cooperation that Japan has with its major nuclear suppliers, the US, France, and Canada, all provide for "back-up" safeguards on transferred materials and technology in the event that Japan withdraws from the NPT.
In particular, the Agreement for Cooperation Between Japan and the US Concerning Peaceful Uses of Nuclear Energy (November 4, 1987) requires both back-up safeguards on and the right to require the return of "any material, nuclear material, equipment or components transferred [by the US to Japan or vice versa] pursuant to this agreement or any special fissionable material produced through the use of such items if either party terminates or materially violates a safeguards agreement with the Agency [i.e., the IAEA]." (Article 9, par. 2 and Article 12, par. 1).
these constraints would count for much if
TECHNICAL WEAPONS OPTIONS
the direct use of reactor-grade plutonium (RGPu),
1. Producing weapons-grade plutonium (WGPu) in power reactors by shutting them down more frequently for refueling, thus reducing the fuel burnup, and increasing the Pu-239 content of the contained plutonium;
2. Upgrading RGPu
or producing WGU from natural or low-enriched uranium (LEU) by a laser
3. Using its commercial centrifuge plant to produce WGU instead of LEU;
4. Separating the plutonium produced in the natural uranium blankets of its Monju experimental breeder reactor which has a higher Pu-239 content than WGPu;
WGPu in a production reactor; and one
option not considered by
6. Using the stocks of WGPU and WGU - several hundred kg of each - in its fast critical assemblies.
one or more of these options, e.g., (3) or (6), might be more attractive than
using RGPu, depending on how many weapons would be
required in what time frame, and the perceived need for
testing. However, in the following we consider whether
The degree to which the obstacles to the use of RGPu
can be overcome depends on the sophistication of the state or group attempting
to produce a nuclear weapon. At the lowest level of sophistication, a potential
proliferating state or sub-national group using designs and technologies no
more sophisticated than those used
in first generation nuclear weapons could build a nuclear weapon from RGPu that would give an assured, reliable yield of one or a
few kilotons, and a probable yield much greater than that. At the other
end of the spectrum, advanced nuclear weapons states such as the
So the question is: where is Japan on the weapons sophistication spectrum?; also how much can one say/has been said/is worth saying on an unclassified basis with regard to how the problems involved in using RGPu can be minimized using designs more advanced than the "Fat Boy" implosion bomb ? [Fuchs' sketch]
In my judgment,
1. After the discovery of the high rate of spontaneous fission of Pu-240 by E. Segre in Spring 1944, the scientists at Los Alamos realized that only the implosion design would work for plutonium, and then only if high-purity plutonium was used and/or it was in the form of a shell rather than a solid pit. While "Fat Boy" did use high-purity plutonium, " the hollow implosion device, as conceived then, could not have been built with the wartime technology of high explosive and fissile material", and a solid pit configuration, the "Christy gadget", was chosen instead. (Ref. L. Hoddeson et al., p. 446, footnote .# 9).
2. In 1956, British scientists realized that the plutonium primaries in their thermonuclear designs were vulnerable to predetonation from external neutron sources. "They came to a startling conclusion - that there was a high risk that British nuclear weapons could be disabled by an intense flash of radiation from a nearby nuclear detonation, that is, from a defensive enemy warhead launched for this purpose. Fission weapons with plutonium cores were particularly vulnerable because of plutonium's susceptibility to pre-detonation... To make an immune warhead required knowledge of certain techniques imperfectly understood at Aldermaston at the time. One was boosting, the addition of thermonuclear material to a fission bomb in order to enhance the efficiency of the fission process and so increase the yield." (Ref: L. Arnold, Britain and the H-Bomb, Palgrave, New York, 2001, p. 177.)
Without going into details, a
study of Japanese work in such areas as high-explosive technology, inertial
fusion, and production and handling of hydrogen isotopes leads me to the
conclusion that they are capable of solving the problems involved in using RGPu in weapons,
specifically predetonation. In sum, I would agree
with the following assessment by Richard Garwin:
As for the more sophisticated designer, it is my judgment that not only the five nuclear weapons 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 to those made with weapons grade plutonium; they would not suffer at all from the high neutron generation rate of reactor grade plutonium
The question then is not whether