As Japan desperately fought this week to bring its damaged reactors at Fukushima under control, a new meltdown faced the world's nuclear industry and the energy plans of many nations.
Regardless of the outcome in Japan, the nuclear renaissance prompted by climate change is now under intense scrutiny and renewed opposition.
There is one simple proposition: if this can happen in an industrial and technological superpower, what greater risks lie with reactors in less advanced countries, including those on some the world's most active fault lines?
And beyond natural disasters, how vulnerable is nuclear energy to military attack or to terrorists?
The nuclear industry has countered these fears with extensive scientific reasoning and studies reporting the inherent safety of reactor design and their resilience to disaster, attack and sabotage.
But in the past few days there has been increasing evidence of unease even among the most powerful advocates of nuclear energy.
Luis Echavarru, head of the Organisation for Economic Co-operation and Development's nuclear agency, told the Spanish newspaper El Pais that new standards were needed for the reactor siting and emergency systems.
China, with 13 working reactors and 27 under construction, has suspended approvals for any new plants - including some on which work has started - pending a review of safety standards and energy planning.
Calls for similar reviews have also gathered pace in the United States, where President Barak Obama's endorsement of nuclear power is being challenged.
A series of scientists spent much of the week minimising the potential consequences of the Japanese crisis, based on information from officials in Japan who insisted there was little danger from radiation, and a 20km safety zone was more than adequate.
But US Nuclear Regulatory Commission chairman Gregory Jackzo told Congress the situation at Fukushima appeared to be far more serious than Tokyo had publicly acknowledged and radiation levels were extremely high
Washington has since advised Americans to remain at least 80km from the damaged reactors, a warning repeated by Australia's Radiation Protection and Nuclear Safety Agency as a precautionary measure in an "unstable situation".
But the crisis for the nuclear industry extends far beyond Fukushima. Stock prices for companies in the sector have plummeted with the prospect of long-term damage to the industry's credibility.
In the past decade nuclear energy has gained from soaring demand for electricity and the industry's argument that it is the only viable clean alternative to fossil fuels.
Its case has been endorsed by rapidly developing nations, especially in Asia, and by powerful political allies - last year, Apec Energy Ministers supported new nuclear power plant development "consistent with the commitment to safety, security and non-proliferation".
There are 62 nuclear plants in China, South Korea, Pakistan and India, plus about 25 research reactors.
Outside China, which will soon become the world's third-largest nuclear power producer behind the US and France, 19 new reactors are planned to come on line in Asia by 2016, including six in South Korea, five in India and four in Japan. Vietnam, Thailand and Malaysia intend to follow suit.
Many of the plants are or will be in the Pacific rim of fire, embracing more than 450 volcanoes and 80 per cent of the world's largest earthquakes.
The region's political and security structure is also becoming more competitive and less predictable. A recent assessment by Australian Strategic Policy Institute researchers Rod Lyon and Will Clegg warned that Salafist jihadi movements continued to threaten stability in Muslim countries, regional security in South Asia, and Western interests.
The nuclear industry rejects doomsday scenarios and says the risk factors for reactors are design flaws, poor management, inadequate regulation, natural disaster or attack.
The World Nuclear Organisation says the industry's safety record has been proven, with only two significant incidents in 50 years - Three Mile Island in the United State in 1979, with no human or environmental toll, and the 1986 Chernobyl explosion in the Ukraine. Casualty figures there are disputed - the industry says about 56 died; other studies put the toll, with subsequent cancers, at more than 2000.
The WNO also compares its safety record with other power producers. Between 1969 and 2000, it says, accidents killed more than 22,000 in coal energy production, 2000 in natural gas, and more than 30,000 in hydroelectric generation.
It also says safety has been improved with new designs based on multiple systems that account for about one quarter of the capital cost of reactors, and which take into account attack by terrorists.
The WNO says US studies concluded that even a fully fuelled Boeing 767-400 crashing at 560km/h would not damage modern reactors.
"Penetrating even relatively weak reinforced concrete requires multiple hits by high-speed artillery shells or 'bunker busting' ordnance, both of which are well beyond what terrorists are likely to deploy."
Even if terrorists managed to storm a nuclear plant and caused loss of cooling, the WNO says core melting and breach of containment "would not result in any significant radioactive releases".
A range of other studies reject the industry's assurances.
Fukushima has shaken confidence in the ability of reactors to survive severe natural disasters, and allegations of safety failings and cover-ups in Japan have raised concerns about standards, management and regulatory supervision in less-developed countries.
Design problems were cited in the US - where senior Nuclear Regulatory Commission engineer John Ma said the concrete shield of a new reactor could shatter "like a glass cup" under heavy stress - and in a 2008 International Atomic Energy Agency cable released by Wikileaks and reported in Britain's Daily Telegraph, warning that strong earthquakes posed a serious problem for Japan's nuclear plants.
Numerous studies also say that despite industry assurances, reactors are vulnerable to attacks in war or by terrorists.
The former head of the American Central Intelligence Agency's terrorism and weapons of mass destruction unit, Charles Faddis, warned on CNN.com that even US reactors were "woefully unprepared" for terror attacks.
Less well-protected research reactors could be more vulnerable than power plants.
Other military studies warn that nuclear reactors would become targets in a war.
Several have been attacked or targeted - Iraq's Osirak reactor near Baghdad was attacked by Iran in 1980 and destroyed by the US in the first Gulf war, and Israel bombed Syria's partially built Al Kibar facility in 2007.
India reportedly considered destroying Pakistan's Kahuta enrichment plant during the early 1980s, and Slovenia's Krsko reactor was shut down after Yugoslavia threatened to bomb it during the Balkan wars.
Fukushima has brought all these back into focus.