Recent news reports say that Japan failed to disclose the existence of about 640 kilograms of unused plutonium—enough to make about 80 nuclear bombs—in its annual reports to the International Atomic Energy Agency (IAEA) in 2012 and 2013. This has raised Chinese concerns about Japan’s plutonium program.

Japanese officials claim that this under-reporting was an honest error of interpretation of the rules, because the material in question was part of the plutonium-uranium mixed oxide (MOX) fuel stored in a reactor that happened to be offline during this period.

But some Chinese policymakers and strategists question whether such under-reporting was an honest mistake, and wonder if it was a deliberate effort at concealment, as it is relatively straightforward to separate out the plutonium in MOX fuel that is “fresh” (i.e., not needing further reprocessing) and use it in a nuclear weapon. And in any case, the IAEA requires a report on all fresh, unirradiated MOX fuel.

China’s Foreign Ministry spokeswoman, Hua Chunying, highlighted this issue at a June 9 daily press briefing: “Japan’s long-term storage of sensitive nuclear materials has outweighed Japan’s needs and aroused the serious concern of the international community… We expect Japan to respond to the concerns of the international community, take practical action at an early date, and address the imbalance between its demand and supply of sensitive nuclear materials.”

A long-running concern. MOX is an end product of nuclear reprocessing, a technology that recovers fissionable plutonium from spent nuclear fuel. MOX is an alternative to the low-enriched uranium commonly used as fuel in the light-water thermal reactors that generate electrical power. Because of the ease with which the plutonium in MOX can be extracted, however, it can contribute to nuclear proliferation and the threat of nuclear terrorism.

In fact, even before the Nuclear Security Summit this March, Chinese authorities were already expressing serious concerns about Japan’s huge surplus of plutonium stocks. China’s Foreign Ministry spokesman, Qin Gang, told reporters on March 11 that Japan’s existing stockpiles of separated plutonium are “far exceeding … [the] normal needs” of a nuclear power program, a concern only reinforced by Japanese plans to separate more such materials. Chinese policymakers suspect that the real intention behind Tokyo’s plutonium recycling program may be to develop a nuclear weapon.

Beijing argues that while Tokyo did agree at the March summit to turn over to Washington 331 kilograms of weapons-grade plutonium and roughly 215 kilograms of weapons-grade highly enriched uranium, this represents just a tiny part of Japan’s stores. Beyond the unreported plutonium, Japan still stores nine metric tons of separated plutonium at home and 35 metric tons in France and the United Kingdom. (Japan has been shipping its spent nuclear fuel to Europe for reprocessing at facilities there, under a reprocessing agreement that was at its peak in the 1990s. After completing this procedure, the materials are shipped back to Japan in the form of MOX fuel.)

These plutonium stocks are large enough to manufacture more than 5,500 bombs, using an average of 8 kilograms of plutonium per bomb. Furthermore, once Japan’s Rokkasho nuclear reprocessing facility starts operating—a priority of Japan’s prime minister, Shinzo Abe—then about eight tons of plutonium each year, or enough to build almost 1,000 bombs, would be added to Japan’s already immense plutonium stockpile. According to NBC News, “Once the Nuclear Fuel Reprocessing Center at Rokkasho opens, the size of this stockpile could double in five and a half years.”

In addition, nuclear-weapons experts point out that Japan has already produced roughly 50 kilograms of “supergrade plutonium” in the natural uranium blankets of its Joyo and Monju prototype fast-breeder reactors. Once separated, it would be enough to build more than 10 bombs. In addition, because this fuel is of better quality than the usual weapons-grade plutonium, it would be easier to make smaller bombs.

An accounting. Rokkasho’s reprocessing facilities are, of course, under international safeguards. But given the natural variations and fluctuations in any manufacturing process, it is nearly impossible to account for every last bit of plutonium produced; accordingly, measurements that are just a single percentage point off might “miss” up to 10 bombs’ worth of plutonium at such a large plant. Indeed, Tokyo admitted as much to the IAEA in 2003, when it reported to the agency that more than 200 kilograms of plutonium—or enough for 25 bombs—produced at its Tokai pilot reprocessing plant could not be accounted for. Supposedly, this was due to a measurement error that was three times higher than normal, or an error rate of three percent instead of one percent. Tokyo later gave some explanations, but they did not convince China’s strategists.

With this experience in mind, Chinese experts worry that once it is operating, Rokkasho’s reprocessing facilities could provide Japan with a steady and undetectable supply of plutonium, enough for 10 nuclear bombs per year.

Japan’s rationale for plutonium recycling. Tokyo defends its plutonium-recycling program by saying that it is intended solely for civilian purposes, such as providing reprocessed plutonium as fuel for its nuclear-powered reactors.

Indeed, this is a laudable goal. However, efforts in the past several decades show that building and maintaining a fast breeder reactor that produces more plutonium than it consumes is an endeavor that is much more costly and unreliable than working with water-cooled reactors. And cheap uranium fuel will not be a problem, at least in this century. Consequently, a breeder reactor that is commercially viable will not be feasible before 2050, if ever. In fact, Japan’s Monju prototype breeder reactor operated for only a few months in 1995, and it is now slated mainly for other research uses.

And any plans that Japan might have had to recycle its stockpile of plutonium into MOX fuel for use in light-water reactors came to grief with the Fukushima accident. Moreover, selling MOX on the international market is unlikely, as more countries realize that there are few economic reasons to use MOX fuel, and no significant environmental benefits. Furthermore, despite the Abe government’s pro-nuclear tilt, the future of nuclear power generation in Japan remains highly uncertain. Without a clear prospect of using MOX fuel in the near future, it makes no sense, from the Chinese viewpoint, to continue separating more plutonium—except as a way of keeping open the option for nuclear weapons.

If Tokyo is really pursuing its plutonium-recycling program for purely civilian purposes, then it should take steps to address international concerns about its surplus plutonium, based on its 1991 pledge to the IAEA to follow the principles of a “no surplus plutonium policy.”

Proliferation worries. Japan is the only non-nuclear state that continues to reprocess its nuclear fuel. By pursuing this activity, it encourages its neighbors to do the same. This could lead to a broader race for building reprocessing technology—or even weapons—in east Asia, further destabilizing the region. Indeed, in its negotiations for a new Agreement of Nuclear Cooperation with the United States, Seoul has been insisting on the same right to reprocess as Japan.

Furthermore, many Chinese question Tokyo’s real intent in moving forward with reprocessing at a time when the current world trend is in the opposite direction. In fact, a recent communiqué from The Hague Nuclear Security Summit encourages nation-states “to keep their stockpile of separated plutonium to the minimum level…”

Closely related to this worry of China’s is the thought of Japan’s sophisticated and advanced space program, which includes successfully tested solid-propellant rocket technology, along with guidance and re-entry technology. Those technologies, Chinese experts believe, could be adapted to develop intercontinental ballistic missiles. Chinese nuclear-weapons specialists emphasize that Japan has everything technically needed to make nuclear weapons; the only thing lacking is the political will. Once committed, Japan could make its nuclear weapons and become a full-fledged nuclear state in a short time, as many Japanese officials have repeatedly admitted. A former prime minister of Japan, Tsutomu Hata, once told reporters in 1994 that it was certainly the case that Japan had the capability to possess nuclear weapons, even if it had not made them yet, according to Selig Harrison’s book, Japan’s Nuclear Future.

China argues that Tokyo has closely linked its plutonium-recycling program with defense from the very beginning of its nuclear program. As early as 1967, a high-level Japanese study commissioned by the government pointed out that the plutonium stocks resulting from its civilian nuclear power program would give Japan the option of making nuclear weapons. The strategy to keep the nuclear weapons option open was set out by the Ministry of Foreign Affairs in 1969 in an internal document that was leaked to the daily newspaper Mainichi Shimbun in 1994. The document said that “for the time being we will maintain the policy of not possessing nuclear weapons.” But regardless of joining the Nuclear Non-Proliferation Treaty (NPT) or not, “we will keep the economic and technical potential for the production of nuclear weapons, while seeing to it that Japan will not be interfered with in this regard.”

And during the signing and ratifying of the NPT in the 1970s, one of Tokyo’s preconditions was that Washington promise not to interfere with Japan’s pursuit of independent reprocessing capabilities.

What’s more, many Japanese high-level officials have argued publicly that Japan should have nuclear weapons, and that this development “would not violate its constitution.” A Japanese white paper on defense from 1970 made the same legalistic argument about the possession of small-yield, purely defensive nuclear weapons, but admitted that in view of the danger of adverse foreign reaction and possible war, a policy should be followed of not acquiring nuclear weapons “at present,” said the Japan Times—implying that this policy could be changed at some time in the future.

Many Chinese worry that as Japan’s politics move rightward, they could cause the country to seek its own nuclear weapons. In its eyes, Beijing has been seeing increasing signs that Tokyo could move toward possible re-militarization: continued visits by Japanese politicians to the Yasukuni Shrine (a memorial noted for containing the remains of over a thousand convicted World War II war criminals); the Japanese government’s refusal to apologize to its neighbors for the invasions and atrocities conducted on those countries’ soil by the Imperial Japanese Army during World War II; the approval of revisionist history textbooks that downplay the role of Japan as an aggressor state in that conflict; the stated intention of Japanese politicians to revise the “no war” clause in the constitution; and the recent territorial debates between the two countries.

Some Chinese fear that Tokyo may use some pretext someday to decide to have nuclear weapons, such as overstating the threat of a nuclear North Korea or the ongoing modernization of China’s nuclear force. They pay heed to the statements of individuals such as Japan’s then-Foreign Minister Kaban Muto, who in July, 1993, declared: “If North Korea develops nuclear weapons and that becomes a threat to Japan, first there is the nuclear umbrella of the US upon which we can rely. But if it comes down to a crunch, possessing the will that ‘we can do it’ is important.”

In fact, since North Korea conducted its nuclear tests in 2006, hawkish Japanese politicians have called the government to consider developing nuclear weapons.

Many Chinese experts fear that a nuclear Japan would destabilize and threaten the security situation not only in the region, but also the whole world. Surely, this would be in no one’s interest.

Tokyo should address these concerns over its reprocessing plans and its plutonium stocks. These worries, if left to fester, can have negative consequences for regional security in Asia. To reduce suspicions, Tokyo should take specific steps to abide strictly by its “no surplus plutonium policy.” It is time for Tokyo to stop reprocessing, and eliminate its surplus plutonium as soon as possible.