The accident, in which four maintenance staff were killed and seven others were injured, is only the latest in a list of the alarmingly frequent mishaps at Japanese nuclear plants, but could prove to be a telling blow.
Clearly chastened by the media and public backlash in the wake of the accident, in which a pipe failed and sprayed super-heated steam on the workers, Kansai Electric Power Co (Kepco) announced on 13 August that it was shutting down all eight nuclear plants that are presently in operation, for safety checks.
"The Japanese government has a long-term policy on nuclear power that is reviewed every five years," said Osamu Gotoh, director of planning at the Atomic Energy Commission of Japan.
"The policy will be reviewed next year and we have just started to gather information to be submitted - but we wonder about the impact of this very severe accident."
A spokesman for Kepco said the incident occurred at 3.28pm on 9 August and that automatic safety protocols immediately shut down the 826,000-kilowatt reactor, which went into service in 1976.
A task force headed by Kepco President Yosaku Fuji has been set up to examine the cause of the accident.
According to initial inquiries, a pipe running from the facility's secondary coolant system exploded, releasing high-temperature steam into the building.
"Kepco has been planning to put plutonium into its reactors, even though that has met some opposition from local people. After this latest incident, that opposition is certain to be stronger. People just don't trust nuclear power."
Nuclear expert Kazue Suzuki,
There was no release of radioactive material, but the four men suffered 50% burns to their bodies and died of suffocation as the steam burned their respiratory tracts.
"Right now, we are not sure how much influence it will have on the government's policy, that will depend on the results of the inquiry, but it will have to be noted," Gotoh said.
"The steam leaked from the second loop of the plant, which does not have any radiation in it, so we believe there was no leak of radiation outside of the plant," said Kazue Suzuki, an expert on nuclear power with Greenpeace Japan.
"But it is still already the most lethal in the history of Japan's nuclear industry," he said.
Raising more questions
In September 1999, Japan was shocked by a critical accident at a Japan Nuclear Fuel Conversion Co plant in the town of Tokai that led to three employees being exposed to massive doses of radiation as they transferred liquid uranium into a mixing tank by hand, violating government rules.
Two of the three died and hundreds of nearby residents suffered varying levels of exposure to the radiation.
The latest incident is sure to raise more questions about safety in the nuclear industry in Japan.
"Kepco has been planning to put plutonium into its reactors, even though that has met some opposition from local people," said Suzuki. "After this latest incident, that opposition is certain to be stronger. People just don't trust nuclear power."
Power of future?
Kepco has been a strong advocate of using a pluthermal programme using MOX - mixed plutonium uranium oxide - fuel as the power of the future, with Fukui Prefecture Governor Issei Nishikawa in March giving the company permission to restart a programme that has been dogged by problems.
On 12 August, that permission was rescinded after the governor said the accident "resulted from your company's failure to place emphasis on taking safety measures".
The broken pipe at Mihama killed
four workers and injured seven
Suspending the project, Governor Nishikawa said: "The safety of nuclear power plants is taken for granted, but it is no longer something that we can rely upon," adding that Kepco "should have known better".
There have been criticism that the MOX provided by British Nuclear Fuels does not meet technical specifications and it has also been pointed out that the programme will lead to the commercialisation of tonnes of weapons-grade nuclear fuel.
Third largest producer
Japan has 52 nuclear reactors that generate 45,740 megawatts of electricity, making Japan the third largest producer of nuclear power in the world, behind the United States and France.
Because it has very few natural resources of its own, the Asian country relies on atomic energy for more than one-third of its power needs.
"Japan has very few resources, with all the oil that we need, for example, imported from the Middle East or other sources," said Gotoh.
"Nuclear energy is a very good choice for us as it has many benefits and is important to our energy security."
But he accepts that the industry needs to clean up its act if it is to convince the public that the benefits of nuclear power outweigh the negatives.
The A-bomb dome in Hiroshima is
a reminder of 1945's atomic blast
"We do not yet know what caused the accident at Mihama, but it is clear that safety has to be the first priority," he said.
"Half a century ago, the cities of Hiroshima and Nagasaki were the targets of atomic bombs so we believe that safety at private companies, universities and research institutes involved in nuclear power have to be very careful because one little accident can be very powerful for those who oppose atomic energy."
Despite the official line that safety is of the utmost concern, anti-nuclear campaigners are not convinced that enough is being done to protect the public.
"It is too early to know for sure, but we suspect the official report into the Mihama incident will be a whitewash because they are just looking at old, inadequate data," said Aileen Mioko Smith, of the Kyoto-based Green Action environmental group.
"We say they should shut all their plants down to conduct checks now; if they're not willing to do that, then already it's a whitewash," she said.
"We hope that this accident will have a trickle-down effect on the Japanese public, when they see the relatives of the dead crying and hitting out at the head of Kepco"
Aileen Mioko Smith,
Representatives of Green Action held three hours of talks with senior officials of Kepco on 11 August, with Smith saying that while the company has been very quick to apologise to the families of the dead and injured, management has been equally speedy about trying to avoid legal responsibility.
Kepco's top echelons are "fleeing their responsibility" by saying that managers of individual plants should be held accountable for any mishaps, Smith said.
Minor impact feared
"We hope that this accident will have a trickle-down effect on the Japanese public, when they see the relatives of the dead crying and hitting out at the head of Kepco," she said.
"But we fear it will have a very minor impact on the government's nuclear power policy.
"There will be a big upheaval now, but then it will quickly go back to business as usual," she added.
"In Japan, there is a very big gap between what the public wants and feels comfortable with, and the need to have power and feed industry and the domestic market."