Japan's Nuclear Weapon Breakout Capability

The term "Nuclear Weapon Breakout Capability" does not necessarily mean a country has an operational nuclear weapon system immediately available (in the open or usually hidden). Capability, particularly considering Japan's Hiroshima-Nagasaki memory, is different from peaceful will or intention. 

Japan's 1960s Nuclear Ambitions

During the administration of Japanese Prime Miniser Sato in the 1960's, it was reported that Japan secretly studied the development of nuclear weapons. In those years Sato argued that Japan needed nuclear weapons to match those of China, but the US opposed the idea. The Johnson administration pressed Japan to sign the Nuclear Non-Proliferation Treaty, ending, for then, Japan's nuclear ambitions.

On June 17, 1974, Japanese Prime Minister Tsutomu Hata told reporters that "it's certainly the case that Japan has the capability to possess nuclear weapons but has not made them." This remark aroused widespread concern in the international media at the time.

"Nuclear Weapon Breakout Capability" means a country, like Japan, already has, or within a limited time (maybe less than 2 years) the capability to put together the 3 major ingredients of a nuclear weapons system. 

These ingredients include:

1. A nuclear delivery system - often land based long range ballistic missiles - like Japan's large MX missile size and shaped Epsilon satellite deployment rocket. The Epsilon, ideally for an ICBM, has three solid fuel stages. See an Epsilon launch 20 seconds into the video below.

2. Nuclear explosives - like Japan's literally tonnes of separated Uranium and Plutonium. Japan has spent many years and $Billions building and maintaining the Tokai Plutonium Reprocessing plant (apparently still open) for economically unconvincing reasons. Japan's Hitachi gained control of SILEX laser enrichment of Uranium technology in the 1990s.

and

3. A nuclear device design which might be in components or at least on paper or computer file. Simple (76 year old) gun type device designs may have been placed on paper and/or computer file by scientists in the  Japanese Atomic Energy Research Institute (JAERI) during its 1956-2005 existence. This is before JAERI merged into the Japanese Atomic Energy Agency (JAEA) in 2005. Such designs may not belong to a specific JAEA section, may attract no budgeted cost and are secret, but the designs existence are known to a few JAEA scientists. Two or three Ministry of Defense officials may be aware of the designs existence in JAEA's files.

Putting Japan's capability in perspective - even South Africa in the 1970s (a country with vastly less money and high tech resources than Japan) was on the point of testing nuclear weapons in the late 1970s. South Africa (with a bit of collaboration with countries like Israel, France and Taiwan) actually "developed a small finite deterrence arsenal of gun-type fission weapons in the 1980s. Six were constructed and another was under construction at the time the program ended.^[8]"

FURTHER REFERENCES

Some geo-strategic context

[2017] Japan has large stockpiles of plutonium from civilian uses and already possesses uranium enrichment and plutonium reprocessing technologies. Estimates of Japan’s breakout time range from six months to several years. Japan’s alliance with the United States has thus far deterred it from developing nuclear weapons because it knows it can rely on the US for defense. However, North Korea’s progress in its nuclear program could drive Japan to reconsider. A nuclear Japan would threaten China’s desired hegemony in the region and force it to proceed with greater caution in its actions in the South China and East China seas.

Epsilon satellite booster quite ICBM suitable

A 2019 Epsilon rocket launch 20 seconds in, boosting a satellite into space.

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The extent to which the US assisted JAXA's Epsilon Project is unclear. Japan Aerospace eXploration Agency (JAXA) is Japan’s NASA equivalent. Space agencies have dual military-civilian use technology and dual-use career personnel. Epsilon's 3 solid fuel stages make it ideal for quick ICBM launch and it doesn't rely on strap-on boosters making silo basing an option.