Problems non stop

March 3, 2012

From broken temp sensors to leaky pipes, Fukushima nuke plant plagued with problems

http://mdn.mainichi.jp/mdnnews/news/20120303p2a00m0na008000c.html

Nearly a year has passed since the meltdowns at the Fukushima No. 1 nuclear plant, and while some progress has been made in decommissioning the power station, operations continue to be plagued with problems from broken temperature sensors to leaky piping.

Plant operator Tokyo Electric Power Co. (TEPCO) took a major step forward in the decommissioning and dismantling process -- expected to take at least 30 years -- in January when it inserted an endoscope into the plant's No. 2 reactor vessel, beginning the first direct internal observations since the crisis began in March 2011.

Cooling at the plant's No. 1 through 4 reactors and their spent fuel pools broke down when the March 11, 2011 tsunami knocked out all power to the coolant pumps. As a result, nuclear fuel was exposed, generating hydrogen gas that built up inside the reactor buildings, eventually causing explosions that destroyed the No. 1, 3 and 4 reactor buildings and releasing a massive amount of radioactive substances.

The government and TEPCO set up a temporary water recycling system to purify water contaminated with radioactive substances and re-inject it into reactors and fuel pools.

Nine months after the crisis broke out, the government declared that the plant had achieved "cold shutdown" after judging that regular cooling of the reactors and fuel pools had been guaranteed and that there were no longer radioactive substances being emitted.

However, the nuclear plant has since been plagued by various technical problems. In particular, a sensor suddenly began registering alarmingly high temperatures at the bottom of the No. 2 reactor's pressure vessel. The temperature sensor had been registering around 40 degrees Celsius until late-January, but after the water injection method was changed the reading began to rise, topping 70 degrees on Feb. 6.

Unsure of whether there was a problem with the sensor or if the reactor really was heating up again, TEPCO gradually increased the amount of water being pumped into it and also poured in boric acid to prevent the melted core from going critical again. Still, the reading continued to rise and surpassed 80 degrees on Feb. 12 -- the maximum "safe" temperature for maintaining a cold shutdown.

The sensor eventually registered temperatures surpassing 400 degrees, leading TEPCO to conclude it was faulty. However, the entire episode revealed how little the company actually understood of the conditions inside the plant's reactors and the fragility of the cold shutdown.

The loss of the sensor also meant that TEPCO's grasp of conditions had got that much worse, while the increase in the water injected into the reactor resulted in even more contaminated water.

After the government declared that the plant had been brought to cold shutdown, water leaked from 44 locations at the plant, including seven sections of the water injection equipment -- the core of the water recycling and re-injection system -- as well as from spent fuel pools at its No. 3 and 4 reactors. On Jan. 29, cooling at the No. 4 reactor stopped entirely for about two hours.

Moreover, a weed was found growing through a pressure resistant hose of the water recycling and re-injection system, causing a pinhole-sized opening. Also, water containing radioactive strontium leaked from a tank holding enriched saline -- generated after radioactive water is purified -- on two occasions. Up to 2 millisieverts of radiation was detected around the tank following the leaks.

TEPCO wrapped the piping in insulation in a desperate attempt to prevent it from freezing, which was the cause of the leak.

Pipes freezing in winter, however, was a perfectly foreseeable difficulty.

"We've taken countermeasures against freezing of important devices, but they were insufficient," said plant manager Takashi Takahashi, and workers there are expected to continue to face problems with contaminated water.