Zena Iovino, reporter
0150 GMT, 17 March 2011
In better times: This file photo from Aug. 21, 2010, shows plutonium-uranium mixed oxide fuel rods being placed in a storage pool at the No. 3 reactor of the Fukushima Daiichi nuclear power plant in northern Japan (Image: Kyodo/Reuters)
Japanese authorities are now focusing their attention on preventing the pools that store spent fuel rods at the numbers 3 and 4 reactors from overheating, Kyodo news agency reports.
Military helicopters were again used early Thursday to dump water on the damaged reactors. Clouds of steam had been seen issuing from the pool storing spent fuel rods in reactor 3, indicating they were boiling off water used to keep the rods cool. The helicopters have been fitted with lead plates to shield crew members from radiation during their mission, according to NHK.
Japanese officials have meanwhile raised hopes of preventing complete reactor meltdowns, saying that they were close to completing a new power line that could restore the reactors’ cooling systems, the AP reports. Officials did not say when the new power line might be be up and running.
On Wednesday morning (Japan time), officials in the city of Fukushima detected a small amount of radioactive substances - iodine and celsium - in the tap water. Later tests were free of the substances, according to Kyodo.
Michael Marshall, environment reporter
1820 GMT, 16 March 2011
The situation at Japan’s Fukushima Daiichi nuclear power plant has become extremely unnerving. The Tokyo Electric Power Company has now admitted that the spent fuel rods could go critical - that is, a nuclear chain reaction could restart.
We have known since yesterday that the reactors themselves were coming under control, and that the biggest threat came from the spent fuel ponds, where the water level has fallen and temperatures have risen. That could lead to the stored fuel rods breaking open, releasing their radioactive contents.
Kyodo News reports:
“Tokyo Electric Power Co. said Wednesday it is considering spraying boric acid by helicopter to prevent spent nuclear fuel rods from reaching criticality again, restarting a chain reaction, at the troubled No. 4 reactor of its quake-hit Fukushima No. 1 nuclear power plant.”The possibility of recriticality is not zero,“TEPCO said as it announced the envisaged step against a possible fall in water levels in a pool storing the rods that would leave them exposed.”
This is a real surprise. These ponds are a standard feature of nuclear reactors, and are typically designed to ensure that nuclear reactions cannot restart in the fuel rods. Among other things, the rods should be widely spaced in the pond.
The BBC explains that the company is now “caught between a rock and a hard place”:
“If the fuel rods are dry and hot, there could be damage to the cladding and the release of light radioactive nuclei. To prevent that, you would want to inject water. But water on its own is a neutron moderator and would enhance the chances, however small, of criticality... [water] reduces the speed of the neutrons, meaning they can be captured by uranium nuclei in the fuel rods, inducing them to split. Without water, the neutrons travel too fast, and are not captured.”
Hence the company’s proposal to add boric acid, which would mop up the neutrons and hopefully stave off the reactivation of a nuclear reaction. If this did happen, it does not mean there would be a nuclear explosion, but the rods would heat up, the zirconium cladding would probably split, and the likely release of radioactive material into the atmosphere would be significantly higher.
In the longer term, questions will be asked about how the ponds wound up in this condition, when it should have been completely avoidable.
Michael Marshall, environment reporter
1335 GMT, 16 March 2011
Radiation exposure is a major worry inside the 20-kilometre zone around the Fukushima Daiichi nuclear plant (Image: The Asahi Shimbun/Getty)
A helicopter that would have dropped water onto reactor 3 of the Fukushima Daiichi nuclear power plant in an attempt to prevent overheating has been forced to turn back by radiation. Nikkei reports:
“The Ground Self-Defense Force helicopters were on standby to drop water on the reactor as it is feared the reactor may have released radioactive steam due to damage to its containment vessel.”
The Japan Broadcasting Corporation (NHK) explains that the reactor’s storage pond, which houses 514 highly radioactive spent fuel rods, has been heating up:
“White plumes started rising from the reactor on Wednesday morning. Tokyo Electric Power Company says the vapor was steam caused by water evaporating from the reactor’s storage pool for spent fuel rods, which is heating up. In an effort to avert the fuel rods’ exposure, a Self Defense Force CH47 helicopter took off from the Sendai base hauling a large container of water on Wednesday afternoon. But the plan was aborted after radiation levels above the plant were found to have largely exceeded 50 millisieverts - the maximum permissible for SDF personnel on a mission.”
This is yet another problem for the beleaguered plant. Reuters notes that workers at the plant temporarily evacuated it yesterday because of the high radiation levels.
Yesterday the waste pond at reactor 4 was causing much of the concern. At one point it had heated up to 83 °C, and experienced two fires. The fear is that water levels in the pond will fall so far that the spent fuel rods will become exposed, releasing a significant amount of radioactive material.
It now seems that attention has shifted back to reactor 3, which yesterday seemed to be coming under control, despite a hydrogen explosion on Monday that damaged the outer building. The BBC now reports that smoke and steam were seen rising from the reactor yesterday, and that part of the containment system may have been damaged.
That would make it the second reactor to suffer containment damage, hot on the heels of reactor 2. The containment systems contain steam that has previously been passed through the reactor core. It is therefore radioactive, so its escape means further radioactive contamination.
It seems reactor 3 now has both a damaged containment system, and an overheating spent fuel pond.
With the helicopter plan scrapped, at least for now, Kyodo News reports that an alternative method for cooling the ponds is being proposed:
“The National Police Agency is considering using a special water cannon truck held by the Metropolitan Police Department to cool a pool storing spent fuel rods at the troubled No. 4 reactor at the Fukushima No. 1 nuclear power plant, police sources said Wednesday. The operation could start as early as Wednesday night, they said.”
Peter Aldhous, San Francisco bureau chief
0630 GMT, 16 March 2011
With smoke rising and workers being temporarily evacuated, the latest news from the stricken Fukushima Daiichi nuclear plant presents a confusing and alarming picture.
According to Kyodo News, Japan’s chief cabinet secretary, Yukio Edano, told reporters on Wednesday morning local time: “There is a possibility that the no. 3 reactor’s containment vessel is damaged.”
If so, that means that containment vessels for two reactors at the site may now be compromised. Concern also surrounds spent fuel rods at the site’s No. 4 reactor, which has experienced a second fire, Kyodo News reports.
Edano’s comments followed TV pictures showing white smoke rising above the No. 3 reactor, as radiation levels at the plant’s entrance briefly reached 10 millisieverts per hour at 10:40 am local time - considered hazardous to human health.
The remaining workers were temporarily evacuated, according to The Guardian, although readings later fell to around 6 millisieverts per hour.
Michael Marshall, environment reporter
1745 GMT, 15 March 2011
It looks like the damaged nuclear reactors at Japan’s Fukushima Daiichi plant are being brought under control. But another threat is coming to the fore: storage areas - cutely referred to as ponds - full of spent fuel rods.
Temperatures have risen in three of the ponds, with one of them coming close to boiling point. That means the radioactive material trapped in the ponds could escape into the atmosphere, contaminating the region far more than the relatively small amount of radioactive material that has escaped so far.
The pools are housed on the top floors of the reactor buildings. Spent fuel rods are transferred to them as soon as they come out of the reactor itself, and are kept under water to cool them down and trap the radioactive material within them. Once they have cooled down enough, the rods are then transferred to outdoor pools for long-term storage.
It is the pools inside the reactor buildings that are causing the problem. Two of the reactor buildings - 1 and 3 - have lost parts of their roofs, thanks to the hydrogen explosions that have taken place over the last few days. While these explosions apparently did not damage the reactors within, they have left the pools exposed to the outside air.
However it is the pool over reactor 4 that currently poses the biggest threat. Kyodo News reported earlier today that water levels in the pool storing the spent fuel rods at the No. 4 reactor may have dropped, exposing the rods:
[TEPCO] said it has not yet confirmed the current water levels or started operations to pour water into the facility. Unless the spent fuel rods are cooled down, they could be damaged and emit radioactive substances. The government’s Nuclear and Industrial Safety Agency urged TEPCO to inject water into the pool soon to prevent heating of the fuel rods... Water temperature in the pool stood at 84 °C as of 4 a.m. Monday, higher than the normal level of 40 to 50 degrees. Usually, the upper tip of the fuel rods is at a depth of 10 meters from the surface of the pool, it said.
The pond caught fire around 9:40 this morning (Japanese time), following another hydrogen explosion. It was extinguished, but is likely to have carried some radioactive material into the air.
The New York Times reports that the pools have not been cooled properly since Friday’s quake:
“The company, Tokyo Electric, has not been able to cool the spent fuel pools because power has been knocked out.”
And the BBC’s live coverage reported earlier that the company “may start pouring water from a helicopter over Fukushima Daiichi’s reactor four in the next few days, to cool the spent-fuel pool.”
The ponds above reactors 5 and 6, which were deactivated at the time of the quake and have so far caused no problems, have also warmed up though not as much as number 4.