The BBC news reported today that “Japan PM says Fukushima nuclear site finally stabilised”:
The crippled nuclear reactors at Japan’s Fukushima power plant have finally been stabilised, Prime Minister Yoshihiko Noda has announced.
The article continues:
“The nuclear reactors have reached a state of cold shutdown and therefore we can now confirm that we have come to the end of the accident phase of the actual reactors,” Mr Noda told a news conference. […]
This [cold shutdown] is where water that cools nuclear fuel rods remains below boiling point, meaning that the fuel cannot reheat.
Tepco has also defined it as bringing the release of radioactive materials under control and reducing public radiation exposure to a level that does not exceed 1mSv/year at the site boundary.
The same BBC article cautions however:
But some nuclear experts have said that the repairs made to the plant after the accident are makeshift and could break down without warning.
On December 8th, Japanese Broadcasting Corporation NHK also reported on the efforts being made to establish ‘cold shutdown’ at Fukushima by the end of the year. Focusing on the situation with reactor number 1, where all of the nuclear fuel has melted, the report says that much of that fuel has since leaked into the containment vessel and eroded the concrete at the bottom.
According to Junichi Matsumoto, spokesman for the Tokyo Electric Power Company (Tepco), as the fuel in the reactors has cooled, this concrete container has finally stopped eroding. However, many experts remain concerned, pointing out that this latest release of information is probably unreliable. Noriyuki Mizuno, NHK Senior Science Commentator, gave his own assessment (through a translator):
The most important data is the temperature of melted fuel, but there’s no thermometer there, so there’s no way of knowing it first-hand. The government says that the two conditions that must be met to declare the state of ‘cold shutdown’: one is that the temperatures at the bottom of the reactor are kept under 100 degrees Celsius; and [two that] the release of radioactive material has been substantially reduced. But if much of the fuel has already melted through the reactor, the temperatures at the bottom of the reactor may not have much meaning.
Tepco says the air temperature in the containment vessel is 40 degrees Celsius, so the fuel must be cool enough. But I don’t know if such a statement can assure the people of Fukushima. After all, the ‘cold shutdown’ is the state of a healthy nuclear power plant being kept under 100 degrees Celsius, not the crippled plant like Fukushima Daiichi. Tepco and the government should explain the status inside the reactor more in detail by releasing such data as the gas concentration rates in the containment vessels.
Melted fuel is emitting very high levels of radiation, so it must be taken out from the reactor with remotely-controlled robot arms. But if fuel actually melts into the concrete shield of the containment vessel, Tepco would need to develop new technology to remove the concrete around the fuel. Experts estimate it [will] take 15 years to remove the fuel and another 15 years to decommission one unit of the reactor – total of 30 years. Or it may take longer.
Click here to watch the NHK report.
To follow the latest developments on the rescue operation at Fukushima, I also recommend fukushimaupdate.com