US government officials and nuclear experts are privately concerned about security at Japan's atomic plants - in particular the Rokkasho reprocessing facility, which will soon begin producing vast quantities of weapons-grade plutonium.
Their warnings, reported on Tuesday by the Washington-based Centre for Public Integrity (CPI), contrast with Washington's official line that it is unworried by the project, and Japan's stockpiles of nuclear fuel.
The report by the non-profit investigative journalism centre focused on Rokkasho and was headlined "Japan could be building an irresistible terrorist target, experts say".
The plutonium-powered fast-breeder reactor plant on the Pacific coast is to fully open in October, and was conceived as a way of creating a special fuel to feed Japan's nuclear plants.
When it is operating at full capacity, the plant will produce eight tonnes of plutonium a year. Experts say that would be enough to build 2,600 nuclear warheads, although Japan insists the plant is purely for energy production.
The US officials' reported comments to the CPI are in contrast to public reassurances from Washington that it is not concerned about Japan's stockpiles of plutonium. Those public statements were made before a board session of the International Atomic Energy Agency in response to expressions of concern raised by both China and Russia.
But behind the scenes, according to the CPI, the administration of US President Barack Obama has been lobbying hard to convince Japan that terrorists might see Rokkasho's new stockpile of plutonium as an attractive target and to increase security at the plant. Officials told the CPI that all their warnings have fallen on deaf ears.
US experts say the facility needs a more capable and armed force - replacing the elderly, unarmed security guards that presently check visitors' ID cards - as well as full background checks for the 2,400 workers at the plant.
"Some Japanese officials have told their American counterparts that the homogenous, pacifistic nature of their society makes nuclear conspiracies unlikely - a conclusion that US officials and independent experts categorically reject," the CPI report said.
"Other Japanese officials have insisted that in a nation where gun ownership is rare and privacy rights are zealously guarded, armed guards and background checks are unacceptable at even the riskiest sites," it added.
A US government official told the CPI that precautions typical of sensitive facilities in the US were "not there" in Japan.
The report cites a US embassy science officer who attended a security drill at the Mihama nuclear plant in 2006. The unnamed official sent a cable - published in 2011 by Wikileaks - describing the Japanese police presence as "a lightly armoured police vehicle with up to six police officers - some of them fast asleep".
Paul Dickman, an ex-chief of staff to the chairman of the US Nuclear Regulatory Commission, told the CPI of his dismay at a conversation with an official from the Tokyo Electric Power Company (Tepco) after the September 11 terrorist attacks.
Dickman said he asked the official why security measures were slow to be implemented at Tepco's Kashiwazaki-Kariwa plant, the largest nuclear plant in the world.
"[We] don't want to do them all at once because we don't want people to think we have been operating unsafely in the past," the Tepco official told him.
Other US officials said security training was inadequate and that drills were unrealistic, as they always followed the same choreographed script and had "no surprises thrown in".
The main aim of this was not to embarrass those involved in the drills, they said.