Troubles at Fukushima

TEPCO struggles to cool Fukushima plant's No. 2 reactor

Tokyo Electric Power Co. is taking steps to prevent a possible self-sustaining nuclear chain reaction at the crippled Fukushima No. 1 nuclear power plant.

Asahi Shimbun. 7 February, 2012

Readings from a thermometer at the bottom of the No. 2 reactor's pressure vessel rose from 50.8 degrees at 5 a.m. on Feb. 1  to  73.3 degrees at 7 a.m. on Feb. 6.

Melted fuel is believed to have accumulated at the bottom of the reactor, but high radiation levels have prevented workers from checking the exact situation within the reactor

After the flow of cooling water was increased to 10.6 tons per hour on Feb. 6, up from 8.6 tons two days earlier, the temperature fell to 69.2 degrees at 5 p.m. on Feb. 6. That night, TEPCO injected boric acid into the reactor to prevent criticality, the point at which a nuclear fission reaction becomes self-sustaining. Boric acid absorbs neutrons, which induce nuclear fission.

The Nuclear and Industrial Safety Agency (NISA) instructed the company to consider injecting boric acid earlier in the day.

TEPCO also plans to increase the amount of cooling water by 3 tons per hour.

At a news conference on Feb. 6, Haruki Madarame, chairman of the Nuclear Safety Commission of Japan, told TEPCO and NISA to keep the public informed.

“We expect them to address the public’s concerns by methodically explaining what could happen and how they plan to deal with it,” Madarame said.

A TEPCO official said there were no signs that the melted fuel had reached criticality. The official said the level of radioactive xenon, an element with a short half-life, remained below a measurable detection limit, and that monitoring devices around the nuclear power plant have not detected a rise in radiation levels.

TEPCO officials said the rise in temperatures was not steep enough to indicate that criticality had been reached.

However, a temperature of 80 degrees or more at the bottom of the pressure vessel would ring alarm bells. TEPCO has assumed a margin of error of up to 20 degrees for the thermometers in the reactor because it is not clear what damage was done to them by the Great East Japan Earthquake.

Therefore, a reading exceeding 80 degrees could mean an actual temperature of more than 100 degrees, compromising the reactor’s status as being in cold shutdown.

The rise in temperatures appears to coincide with changes in the flow of water through two separate systems for cooling the No. 2 reactor: the feed water system and the core spray system. 

TEPCO temporarily increased the amount of water being pumped through the feed water system and reduced the amount of water going through the core spray system as it strengthened outdoor piping in late January.

After that work was completed, it gradually decreased the amount of water flowing through the feed water system and increased the flow through the core spray system in an effort to restore flows to the setup before the strengthening.

The thermometer that has produced the high readings is located just under the feed water system. Its temperature readings rose when the water passing through the feed water system was reduced and water going through the core spray system was increased. Readings from two other thermometers at the same height in the reactor have been stable at 44-45 degrees.

“The temperature may have risen because water has not reached part of the fuel since the amount of water through the feed water system decreased and the flow of water changed,” said an official at TEPCO’s Nuclear Power and Plant Siting Division.

This article was written by Takashi Sugimoto and Jin Nishikawa.



This article may be more up-to-date

Rising temperatures at Fukushima raise questions over stability of nuclear plant

The amount of cooling water being injected into the No. 2 reactor was increased after its temperature soared to 73.3 C

the Guardian,

8 February 2012 04.52 GMT

Workers at the Fukushima Daiichi nuclear power plant say they are regaining control of a reactor after its temperature rose dramatically this week, casting doubt on government claims that the facility has been stabilised.

The plant's operator, Tokyo Electric Power [Tepco] was forced to increase the amount of cooling water being injected into the No. 2 reactor after its temperature soared to 73.3 C earlier this week.

By Tuesday night, the temperature had dropped to 68.5 C at the bottom of the reactor's containment vessel, where molten fuel is believed to have accumulated after three of Fukushima Daiichi's six reactors suffered meltdown following last year's tsunami disaster.

The temperature at the bottom of the No. 2 reactor vessel had risen by more than 20 C in the space of several days, although it remained below the 93 C limit the US nuclear regulatory commission sets for a safe state known as cold shutdown. Tepco said it had also injected water containing boric acid to prevent an accident nuclear chain reaction known as re-criticality.

The operator said the sudden rise in temperature did not call in to question the government's declaration in December that all three damaged reactors had achieved cold shutdown.

"The temperature of the reactor pressure vessel seems to be close to peaking out," Junichi Matsumoto, a Tepco spokesman, told reporters.

Late last year, however, the minister in charge of the response to the Fukushima disaster, Goshi Hosono, conceded that officials had no idea about the exact location of molten uranium fuel, but assumed that it had come to rest at the bottom of its containment vessels.

Hosono said the temperature rise may have been triggered by work to replace a cooling pipe, after freezing weather in north-east Japan caused a number of water leaks at the site.

The use of bigger volumes of water to cool the No. 2 reactor presents Tepco with the additional problem of a build-up of radioactive water. The utility said recently it had processed more than 220,000 cubic metres of contaminated water using treatment facilities, but added that as much as 95,000 cubic metres - enough to fill 38 Olympic-sized swimming pools - may have accumulated in the reactors' basements.

Tepco workers started injecting water into overheating reactors after the 11 March tsunami crippled the back-up power supply to cooling systems.

The Fukushima Daiichi accident has led to the closure of all but three of Japan's 54 reactors to undergo regular maintenance and new stress tests designed to gauge their ability to withstand powerful earthquakes and tsunami.

The Yomiuri Shimbun said this week that the government planned to restart two reactors in Ohi, western Japan, before the last reactor is due to go offline at the end of April. If the restarts do not take place by then, Japan will be without a single working nuclear reactor.

But the trade minister, Yukio Edano, said that no deadline had been set to restart any reactors, a move some experts say should wait until the official investigation into the Fukushima accident has been completed.

Edano acknowledged that an early restart would be difficult, given lingering public concern over the safety of nuclear power. "The only standard is whether we can gain a certain level of understanding from the local people and the public," he said.