Blog Post - Nuclear Security Matters

Eliminating Potential Bomb Material from Japan’s Fast Critical Assembly

| Mar. 24, 2014

Today, the United States and Japan announced that Japan would eliminate all the plutonium and highly-enriched uranium at its Fast Critical Assembly (FCA) at Tokai-mura.  This is a tremendous step forward for nuclear security; for terrorists, this would be some of the best material that exists in any non-nuclear-weapon state.  The material includes 331 kilograms of plutonium, most of it weapons-grade, and 214.5 kilograms of weapons-grade HEU.  (The FCA also includes over a ton of material just at the 20 percent U-235 mark that defines HEU.)   The weapons-grade HEU is enough for four simple terrorist “gun-type” bombs or a larger number of trickier-to-build implosion bombs.  The plutonium amounts to more than 40 bombs worth of material.

(See here for a discussion of how hard it would be for terrorists to build these two types of crude bombs if they got the HEU. The number of plutonium bombs is based on the eight-kilogram “significant quantity” figure the International Atomic Energy Agency uses, though the U.S. government has acknowledged that “hypothetically” four kilograms “is sufficient for one nuclear explosive device” [see p. 23 here]).

Most of the FCA material is in metal form, in pocket-sized squares so non-radioactive researchers handle them with no more than rubber gloves – perfect for stealing and making into a bomb.  Indeed, the HEU is essentially exactly the same material you would get if you broke into the HEU storage building at Y-12 in the United States – but with far less security in place (though security has been improved in recent years, with the addition of armed guards from the national police after the 9/11 attacks and some significant improvements more recently).

Units 1 and 2 at the Tokai Nuclear Power Plant in Japan. (Saibo/Wikipedia)

In a recent report on progress in nuclear security and remaining gaps, we made the point that during the four-year effort, all of the places in non-nuclear-weapon-states where there was enough high-quality HEU for a gun-type bomb had either been eliminated or had significant security upgrades implemented; this announcement adds to that successful record.

The United States and Britain sent the FCA material to Japan decades ago for scientific experiments, taking measurements of the nuclear properties of different materials and modeling designs for future fast-neutron nuclear reactors.  It was never secret — it has been under IAEA safeguards from the beginning to ensure it was only used for peaceful purposes, and the facility was open to international researchers.  The FCA material includes the main stocks of so-called "weapon-grade" plutonium or HEU in Japan, which is the highest quality material for making nuclear bombs.  But unfortunately terrorists could also make bombs from reactor-grade plutonium, and there are some nine tons of that stored in Japan as well – and there will be more if Japan starts up its giant Rokkasho plutonium reprocessing plant this fall.  (See here for recent stories on Japan’s plutonium from the Center for Public Integrity, here for a discussion of the issue at Brookings in which I took part, and here for an interesting set of proposals for addressing Japan’s plutonium future from the International Panel on Fissile Materials.)

Some readers may remember Technical Area 18 (TA-18) at Los Alamos – a site that used to have similar fast critical assemblies, but whose location was so difficult to defend that it failed security tests again and again, and the National Nuclear Security Administration finally moved the HEU to the highly secure Device Assembly Facility in Nevada.  Years ago, I was speaking to a senior Congressional staffer about the dangers at Japan’s FCA and related facilities elsewhere, while the controversy over TA-18 was boiling, and he said: “My god, you mean to tell me there are TA-18s all over the world?”  Not quite – this was one of the only ones outside the nuclear weapon states – but it’s a good thing for the world that the HEU at TA-18 is gone, and that the HEU and plutonium at the FCA soon will be.

Of course, nuclear security is not perfect by any means in the United States either, as 2012’s nun intrusion at Y-12 makes clear.  But once the material gets here from Japan, it will be one part of larger stocks that already require substantial security measures, rather than off by itself, with lower security, posing an additional risk.  The joint statement says the HEU will be downblended to LEU, and the plutonium will go to whatever disposition plan the United States ultimately manages to decide on.

To help convince Japan to agree, the United States agreed to extend the time during which Japan can keep sending its research reactor fuel to the United States for treatment or disposal, rather than having to come up with ways to handle it on its own.  (In the joint statement, this is referred to briefly as “the United States will continue to accept research reactor spent fuel from several Japanese facilities that utilize LEU.”)  The FCA is not closing down – instead, Japan and the United States will “work together to design new enhancements to the FCA,” allowing it to study transmutation of nuclear waste without using HEU or plutonium This is a bit technically challenging, as getting nuclear reactions to work in a fast neutron spectrum such as that used in the FCA requires having a lot of fissile atoms close together – which is the reason such facilities have traditionally used HEU or plutonium.

The FCA, as important as it is, is only one site in the larger struggle to minimize dangerous stocks of HEU.  (See our recent report, our 2012 report specifically on consolidating stocks at fewer sites, and other resources on consolidation at Nuclear Security Matters for more.)  Once the plutonium and HEU from the FCA are shipped out, there will be only two sites left in non-nuclear-weapon states with enough really high-quality HEU for a gun-type bomb – in Belarus and South Africa.  Political disputes have so far blocked agreement on removing those two stocks, though a substantial amount of HEU was removed from Belarus before the agreement to eliminate that stock broke down, and major security upgrades have been completed at both sites.  Neither country needs these HEU stocks anymore, and there’s still hope they can be convinced to eliminate them.

In nuclear weapon states, many sites with enough HEU for a gun-type bomb remain – particularly in Russia, which has most of the world’s HEU-fueled pulse reactors and critical assemblies, types of facilities that often have hundreds of kilograms or even tons of weapon-grade HEU at a single site.  It would be in Russia’s own interest to close or convert most of these facilities, to get the science and testing they need with far less cost and risk.  But there’s a long way to go to convince Russia and the other states with nuclear weapons to scale back the number of places where the essential ingredients of nuclear bombs are stored.

For more information on this publication: Belfer Communications Office
For Academic Citation: Bunn, Matthew.Eliminating Potential Bomb Material from Japan’s Fast Critical Assembly.” Nuclear Security Matters, March 24, 2014,

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