Tag Archives: Fukushima Daiichi

How seawater could corrode nuclear fuel

Japan used seawater to cool nuclear fuel at the stricken Fukushima-Daiichi nuclear plant after the tsunami in March 2011 — and that was probably the best action to take at the time, says Professor Alexandra Navrotsky of the University of California, Davis.

But Navrotsky and others have since discovered a new way in which seawater can corrode nuclear fuel, forming uranium compounds that could potentially travel long distances, either in solution or as very small particles. The research team published its work Jan. 23 in the journal Proceedings of the National Academy of Sciences.

“This is a phenomenon that has not been considered before,” said Alexandra Navrotsky, distinguished professor of ceramic, earth and environmental materials chemistry. “We don’t know how much this will increase the rate of corrosion, but it is something that will have to be considered in future.”

Japan used seawater to avoid a much more serious accident at the Fukushima-Daiichi plant, and Navrotsky said, to her knowledge, there is no evidence of long-distance uranium contamination from the plant.

Uranium in nuclear fuel rods is in a chemical form that is “pretty insoluble” in water, Navrotsky said, unless the uranium is oxidized to uranium-VI — a process that can be facilitated when radiation converts water into peroxide, a powerful oxidizing agent.

Peter Burns, professor of civil engineering and geological sciences at the University of Notre Dame and a co-author of the new paper, had previously made spherical uranium peroxide clusters, rather like carbon “buckyballs,” that can dissolve or exist as solids.

In the new paper, the researchers show that in the presence of alkali metal ions such as sodium — for example, in seawater — these clusters are stable enough to persist in solution or as small particles even when the oxidizing agent is removed.

In other words, these clusters could form on the surface of a fuel rod exposed to seawater and then be transported away, surviving in the environment for months or years before reverting to more common forms of uranium, without peroxide, and settling to the bottom of the ocean. There is no data yet on how fast these uranium peroxide clusters will break down in the environment, Navrotsky said.

SOURCE:  http://www.physorg.com/news/2012-01-seawater-corrode-nuclear-fuel.html

Radiation, rust seen in tsunami-hit Japan reactor

The first look inside one of Japan’s tsunami-hit nuclear reactors showed radiation, steam and rusty metal surfaces scarred by 10 months’ exposure to high temperatures and humidity.

The steam-blurred photos taken by remote control Thursday found none of the reactor’s melted fuel but confirmed stable reactor temperature and showed no major damage or ruptures caused by the earthquake last March, said Junichi Matsumoto, spokesman for the plant operator, Tokyo Electric Power Co.

TEPCO workers inserted the endoscope — an industrial version of the kind of endoscope doctors use —through a hole in the beaker-shaped containment vessel at the Fukushima Dai-ichi plant’s No. 2 reactor, hoping the first look inside since the crisis would help them better assess reactor conditions and make repairs.

Results of the 70-minute operation were mixed.

Some parts that were photographed were not identifable, and experts are still trying to identify what the photos show, Matsumoto said. Radiation was apparent as it interferred with the electronic device and was visible as static on the images.

The photos also showed inner wall of the container heavily deteriorated after 10 months of exposure to high temperature and humidity, he said.

‘Given the harsh environment that we had to operate, we did quite well. It’s a first step,’ Matsumoto said. ‘But we could not spot any signs of fuel, unfortunately.’

He said it would take more time and a better technology to get to the melted fuel, most of which has fallen straight down into the area that the endoscope could not reach. TEPCO hopes to use the endoscope to look inside the two other reactors that had meltdowns but that also would require customization of the equipment and further reduction of radiation levels.

Better assessment will help workers know how best to plug holes and cracks in the containment vessel — a protective chamber outside the core — to contain radiation leaks and gradually work toward dismantling the reactors.

Three of six reactors at the Fukushima plant melted down after the March 11 earthquake and tsunami knocked out the plant’s cooling systems and set off the world’s worst nuclear accident since Chernobyl.

TEPCO and nuclear officials have said that melted fuel probably fell to the bottom of the core in each unit, most likely breaching the bottom of the core and falling into the primary containment vessel, some dropping to its concrete floor.

Experts have said those are simulation results and that exact location and condition of the fuel could not be known until they have a first-hand observation inside.

The probe Thursday successfully recorded the temperature inside the containment vessel at 44.7 Celsius (112 F), confirming it stayed below the boiling point and qualifying a ‘cold shutdown state,’ the stable condition that the government had declared in December despite skepticism from experts.

The probe failed to find the water surface, which indicate the water sits at lower-than-expected levels inside the primary containment vessel and questions the accuracy of the current water monitors, Matsumoto said.

The government has said that it would take 40 years until the Fukushima plant is fully decommissioned.

SOURCE: http://www.khaleejtimes.com/displayarticle.asp?xfile=data/international/2012/January/international_January715.xml&section=international&col=