Reactor 1 in worse shape than thought
Cracks suspected in containment after fuel rods found fully exposed
Tokyo Electric Power Co. said Thursday that the water level in the No. 1 reactor’s pressure vessel at the Fukushima No. 1 power plant is much lower than thought and that some of the fuel rods have melted and sunk to its bottom.
But Tepco also said the temperature in the pressure vessel has been kept at around 100 to 120 degrees, which is cool enough to keep the reactor safe, for now.
The low water level, however, indicates that the pressure vessel could have holes or cracks through which radioactive water is leaking.
Tepco’s latest discoveries were found after workers entered the main building, where they were finally able to check and fix water-level indicators damaged when the March 11 monster earthquake and tsunami crippled the nuclear power plant.
The workers also learned that the water in the containment vessel, which Tepco has kept spraying to cool down the pressure vessel from the outside, is also much lower than earlier believed. There might be some holes or cracks in the vessel or components connected to it, Tepco said.
Any holes or cracks in the pressure and containment vessels are sure to make repair work more difficult.
The utility plans to keep cooling the reactor’s core by attaching air-cooling machines that will recirculate the water inside the containment vessel.
Tepco has poured about 10,000 cu. meters of water into the containment vessel so far. But only about 7,500 cu. meters should be necessary to cool the fuel rods if a way can be found to reuse the water instead of letting it flow away.
“We’re not sure how much of the fuel rods fell down to the bottom and in what form, but the temperature shows that they are being cooled,” Tepco spokesman Junichi Matsumoto said at a news conference.
On March 12, reactor No. 1 suffered a hydrogen explosion after the fuel rods were fully exposed for hours and generated hydrogen, which reacted violently with oxygen.
But Matsumoto said Tepco believes such an explosion at this time is unlikely because nitrogen has already been poured inside the containment vessel to purge explosive gases.
Matsumoto also said that, considering the situation with the No. 1 reactor, the water level data from reactors 2 and 3 may not be credible.
Tadashi Narabayashi, a professor of reactor engineering at Hokkaido University, also said it is likely that the melted fuel rods are being cooled with coolant water at the bottom of the pressure vessel.
According to Narabayashi, the zirconium encasing the fuel rods becomes weak and brittle when it is fully exposed to oxygen and then covered with cold water again.
According to Tepco, the water-level indicators of the pressure vessel had indicated the water surface was about 1.65 meters below the top of the fuel rods. But as of Thursday morning the reading was more than 5 meters below the top. The fuel rods, if undamaged, are only 4.5 meters in height.
Although the water is leaking from somewhere in the pressure vessel, Tepco officials don’t believe the melted fuel has penetrated the bottom of the pressure vessel and dropped into the containment vessel below, given that the thermometer placed at the bottom of the pressure vessel is still working. If a big chunk was missing, it would malfunction, Tepco said.
There are actually no tools specially designed to check the water level in the containment vessel, but Tepco said it made estimates based on other factors, including the pressure in the containment vessel.
By KAZUAKI NAGATA, Staff writer, the Japan Times, May 13, 2011
[information from kyodo added]
* http://search.japantimes.co.jp/cgi-bin/nn20110513a1.html
Melting of reactor 1 fuel ’no surprise’
Experts were not surprised Thursday to find that most, if not all, of the fuel rods in reactor No. 1 at the Fukushima No. 1 power plant had been fully exposed, melted and fell to the bottom of the pressure vessel.
Tokyo Electric Power Co. announced the finding Thursday after workers entered the reactor building earlier this month and fixed equipment to monitor the water level in the pressure vessel.
“It’s neither a surprise nor bad news,” Kunihiko Takeda of Chubu University told The Japan Times. “This means Tepco has been pumping lots of water in the reactor without knowing what exactly is happening in it, which is the best thing Tepco could do.” He added that reactors No. 2 and 3 may also be in the same situation.
The new finding doesn’t increase the likelihood of a hydrogen explosion because the temperature in the pressure vessel is still low, experts said.
Hydrogen explosions can occur if zirconium, material used in fuel-rod casings, melts at around 1,200 degrees, said Ken Nakajima of the Kyoto University Research Reactor Institute.
The temperature in the pressure vessel is 100 to 120 degrees.
Tepco has injected nitrogen gas into the containment vessel to dilute the hydrogen density, further decreasing the chance of a hydrogen explosion, he said.
Takeda said the pressure vessels and containment vessels in reactors 1, 2 and 3 are likely to have cracks where hydrogen can escape, another factor reducing the likelihood of a hydrogen explosion. The fact remains, however, that the reactors at the Fukushima plant keep leaking radioactive substances into the air and are irradiating the water cooling them as well. Thus, the most important thing, as it has always been, is to build a system to recirculate the water being used to cool the reactors, Takeda said.
By MINORU MATSUTANI, Staff writer, the Japan Times, May 13, 2011
* http://search.japantimes.co.jp/cgi-bin/nn20110513a8.html
Plant springs new radioactive leak
Another leak at the Fukushima No. 1 power plant has been pouring radioactive water filled with high amounts of cesium into the Pacific Ocean for an unknown period of time, but Tokyo Electric Power Co. was able to plug the leak.
Radioactive materials are also apparently penetrating the silt fence installed in the sea near the crippled plant, raising concern that a wider area of the Pacific may be contaminated.
The leak was found Wednesday at a pit close to the seawater intake for the No. 3 reactor. Seawater sampled in the area found concentrations of cesium-134 at a 32,000 times the permissible level and concentrations of cesium-137 to be 22,000 times the limit, Tepco said Wednesday.
The water in the pit contained cesium-134 at 620,000 times the legal level and cesium-137 at 430,000 times the limit, it said.
The radiation on the water’s surface was giving off scorching readings of 1.5 millisieverts per hour.
The leak was stopped Wednesday night after filling the pit with concrete and other materials. A worker there heard the sound of water flowing nearby at around 10:30 a.m., but it is not known when the leak began. No data were provided on the leak’s size or rate.
“This is an extremely serious problem,” Goshi Hosono, special adviser to Prime Minister Naoto Kan, told a news conference Wednesday attended by Tepco and government officials to explain their ongoing efforts to contain the nuclear crisis.
In April, the utility found that highly contaminated water was leaking into the sea near a seawater intake for the No. 2 reactor at the plant, which was crippled by the quake and tsunami.
Kyodo, May 13, 2011
* http://search.japantimes.co.jp/cgi-bin/nn20110513a9.html