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I read from here that

Japan does not possess any programs for the development of weapons of mass destruction (WMDs), but it is the only non-nuclear weapon state in possession of a full nuclear fuel cycle and has advanced WMD-relevant industries.

This seems to give me a naive impression that Japan has the capability of making nuclear weapons in a short period of time (say in weeks, if they want to). Can anyone clarify this?

For example, I am not sure if the making of atomic bombs need significantly more complicated industrial system than building nuclear plants, as Japan currently have.

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    Not my DV, but "a full nuclear fuel cycle". Not sure what that means. I recall that Germany is considered to have similar capabilities, even if their HEU reactors are nominally research only. nonproliferation.org/civilian-heu-germany Commented Mar 17, 2022 at 18:42
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    An interesting question, but I'm not sure it is political. You seem to be asking about the technical ability of Japan to produce a nuclear weapon. That is not a matter of politics, but of technology. But exactly how long it would take Japan to convert their uranium/plutonium and rockets to a functional nuclear ICBM is spectulation.
    – James K
    Commented Mar 17, 2022 at 19:26
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    @Fizz Full nuclear fuel cycle means that they have a processing industry capable of making new fuel, processing spent fuel. Generally this implies know-how of enriching uranium and separating different elements from spent fuel. Countries without this fuel cycle relies on purchasing processed reactor fuels from countries who have that know-how.
    – vidarlo
    Commented Mar 17, 2022 at 21:16
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    @vidarlo: I see the difference seems to be that Japan has a sizeable number of centrifuges en.wikipedia.org/wiki/Japan_Nuclear_Fuel_Limited Commented Mar 17, 2022 at 21:28
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    It takes quite a bit of engineering to produce a nuclear weapon and a delivery system, so it is unlikely to be accomplished in a few weeks time. Of course, enriched nuclear fuel could be used to make a dirty bomb, but I supposed that this is not what is meant by the OP.
    – Morisco
    Commented Mar 18, 2022 at 9:28

4 Answers 4

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Probably. According to an unnamed official cited by the NBC some years back:

But according to a senior Japanese government official deeply involved in the country’s nuclear energy program, Japan has been able to build nuclear weapons ever since it launched a plutonium breeder reactor and a uranium enrichment plant 30 years ago.

“Japan already has the technical capability, and has had it since the 1980s,” said the official. He said that once Japan had more than five to 10 kilograms of plutonium, the amount needed for a single weapon, it had “already gone over the threshold,” and had a nuclear deterrent.

Japan now has 9 tons of plutonium stockpiled at several locations in Japan and another 35 tons stored in France and the U.K. The material is enough to create 5,000 nuclear bombs. The country also has 1.2 tons of enriched uranium.

Germany's HEU stockpile, albeit officially owned by EU (apparently) was pretty similar. The Japanese (on-shore) plutonium stockpile seems sans pareil though in countries that are not nuclear weapons holders, but seemingly similar to what India has.

From the same NBC article:

Technical ability doesn’t equate to a bomb, but experts suggest getting from raw plutonium to a nuclear weapon could take as little as six months after the political decision to go forward. A senior U.S. official familiar with Japanese nuclear strategy said the six-month figure for a country with Japan’s advanced nuclear engineering infrastructure was not out of the ballpark, and no expert gave an estimate of more than two years.

One also has to think in terms of delivery capabilities. Japan has a space program, but still I think developing SLBMs, which (if I recall correctly) is how France or the UK have much of their deterrent placed, can take years. Building silos in Japan (like the US, China and Russia have, and France once had) is probably a non-starter in Japan due to geography, population density etc.


There's a bit more interesting background story how Japan ended up with such a large stock of plutonium. The TLDR version could easily make the plot for movie:

  • Japan started a ($17 billion) fast breeder reactor program, but its Monju prototype suffered a sodium-leak fire and was decommissioned.
  • Fuel sent for MOX reprocessing in the UK (at BNFL) came back with falsified (copy-pasted) data, which caused a scandal and loss of confidence, with environmental groups protesting that all MOX was thus not safe. The MOX program was set back some 12 years compared to its original schedule; the first operational/commercial MOX fuel was used in 2009.
  • Then Fukushima happened in 2011, shutting down much of the country's nuclear power plants in the aftermath. Of the 16-18 LRWs that were planned to use MOX, only 3 were operational in 2018.
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  • Also worth noting that Japan is home to a large corps of high energy physicists with all of the scientific knowledge and most of the engineering expertise to build nuclear weapons as well as a very advanced manufacturing sector.
    – ohwilleke
    Commented Mar 18, 2022 at 1:40
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    You recall correctly regarding the UK and French SLBMs. The submarines can launch Harpoon, so something like a tube-launched Tomahawk would not be a stretch. Also ALCM and bombers.
    – o.m.
    Commented Mar 18, 2022 at 5:35
  • The plutonium the japanese have is expended fuel. As such, is a mix of Pu-240, Pu-239 and other byproducts of radioactive decay. As far as I know nobody has tried to purify expended fuel to separate the Pu-239 used in nukes from the Pu-240 - too costly. The usual process is enriching U-233 with U-235 (which is already hard enough) and then use it in a breeder reactor to obtain Pu-239 with as few Pu-240 as possible (minimum theoretical purity is 95%, but you should aim for 97-98%). So the amount of plutonium Japan has stored is irrelevant to its nuke-readability.
    – Rekesoft
    Commented Mar 18, 2022 at 11:55
  • @Rekesoft: Separating PU-239 from Pu-240 is basically impractical, due to the small mass ratio, and the safety problems. Reactor-grade plutonium is not irrelevant. It makes weapon design harder, but not impossible. Commented Mar 18, 2022 at 15:11
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    @Rekesoft: interestingly one 2014 article said "The U.S. wants Japan to return 331 kilos of weapons grade plutonium – enough for between 40 and 50 weapons – that it supplied during the Cold War." So apparently they did have some weapons grade Pu. Looks like they sent that one back reuters.com/article/us-japan-nuclear-plutonium-idUSKCN0WO0R5 Commented Mar 31, 2022 at 18:19
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Being able to manufacture nuclear weapons in a short period of time is called nuclear latency. Japan is the best example of nuclear latency in the world. To quote Wiki,

Japan is considered a "paranuclear" state, being a clear case of a country with complete technical prowess to develop a nuclear weapon quickly, or as it is sometimes called "being one screwdriver's turn" from the bomb, as Japan is considered to have the materials, expertise and technical capacity to make a nuclear bomb at will.

Japan is not the only nuclear latent state in the world. Wiki names several more, including Germany, Taiwan, South Korea, and Canada.

Being able to make nuclear weapons is definitely not the same as being able to make nuclear power plants, although the latter is a stepping stone towards the former. This is why news coverage of Iran's nuclear program focuses on "weapons grade" material. For comparison, 4-5% enriched uranium is good enough for nuclear power, while military-grade weapons need 90%+ enrichment.

Both uranium and plutonium were used to make bombs before they became important for making electricity and radioisotopes. The type of uranium and plutonium for bombs is different from that in a nuclear power plant. Bomb-grade uranium is highly-enriched (>90% U-235, instead of up to 5%); bomb-grade plutonium is fairly pure Pu-239 (>90%, instead of about 60% in reactor-grade) and is made in special reactors.

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  • "Pu-239 (>90%, instead of about 60% in reactor-grade) and is made in special reactors" Pu-239 can also be produced in fast breeder reactors, provided you burn the fuel in short cycles. It is doable, but expensive and time consuming.
    – FluidCode
    Commented Mar 27, 2022 at 14:38
  • For the record, the assertion that bomb grade uranium is >90% U-235 is potentially misleading; the little boy bomb was about 80% U-235, for example. Commented Aug 7, 2023 at 18:54
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What is described here is a virtual nuclear arsenal. The non-proliferation treaty acknowledged that a civilian infrastructure could be abused to build weapons. Under the NPT, non-weapon-states have reporting requirements to prevent this from happening. Japan is not alone in this status, but probably the largest one.

During and after the 1991 Gulf War, the flippant comment was that Japan and Germany were closer to the bomb than Iraq ever got, despite not having a weapons program at all.

Any significant power would have to study and understand the design of nuclear weapons, if only to evaluate intelligence reports of other countries' projects. Say there is a report of Iranian or North Korean designs -- do they want to rely on what Washington, Paris, or Beijing say, or do they want their own experts to look at the intelligence?

Many significant powers also have nuclear power plants and either the capability to produce enriched uranium, or even stocks of "waste" plutonium.

What they need are engineers to put the material and the plans together, and a delivery system. A number of NATO powers have certified fighters to drop US bombs, and there were plans to have them fire nuclear artillery shells as well. The NATO view was that the NPT did not apply in wartime.

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  • This is why there is a missile ban on Iran; delivery system
    – paulj
    Commented Mar 28, 2022 at 12:56
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Making a bomb in weeks would not be possible. I'll skip the technical discussion about a Uranium 235 bomb because it requires a lot of work and the military capabilities are not so good, it's heavy.

Japan currently might have some spent fuel from their experimental fast breeder reactor, but Plutonium coming from civilian reactors is a mix of isotopes too difficult to use for a bomb, so extracting Plutonium from that spent fuel would not make sense.

So, assuming they have all the capabilities to process correctly the spent fuel, in order to produce a bomb they would have to restart some reactors and produce more Plutonium, that would take a long time.

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