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Posts Tagged ‘oxygen’

Friday, October 11, 2013 @ 03:10 PM gHale

A device that uses only sunlight and wastewater to produce hydrogen gas could end up being a sustainable energy source while improving the efficiency of wastewater treatment.

The solar-microbial device is now in development and it combines a microbial fuel cell (MFC) and a type of solar cell called a photoelectrochemical cell (PEC), according to a research team led by Yat Li, associate professor of chemistry at the University of California, Santa Cruz.

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In the MFC component, bacteria degrade organic matter in the wastewater, generating electricity in the process. The biologically generated electricity delivers to the PEC component to assist the solar-powered splitting of water (electrolysis) that generates hydrogen and oxygen.

The way it stands now, either a PEC or MFC device can work alone to produce hydrogen gas. Both, however, require a small additional voltage (an “external bias”) to overcome the thermodynamic energy barrier for proton reduction into hydrogen gas. The need to incorporate an additional electric power element adds significantly to the cost and complication of these types of energy conversion devices, especially at large scales.

In comparison, Li’s hybrid solar-microbial device is self-driven and self-sustained, because the combined energy from the organic matter (harvested by the MFC) and sunlight (captured by the PEC) is sufficient to drive electrolysis of water.

In effect, the MFC component is a self-sustained “bio-battery” that provides extra voltage and energy to the PEC for hydrogen gas generation.

“The only energy sources are wastewater and sunlight,” Li said. “The successful demonstration of such a self-biased, sustainable microbial device for hydrogen generation could provide a new solution that can simultaneously address the need for wastewater treatment and the increasing demand for clean energy.”

Microbial fuel cells rely on unusual bacteria, known as electrogenic bacteria, that are able to generate electricity by transferring metabolically-generated electrons across their cell membranes to an external electrode.

Li’s group collaborated with researchers at Lawrence Livermore National Laboratory (LLNL) who have been studying electrogenic bacteria and working to enhance MFC performance. Initial “proof-of-concept” tests of the solar-microbial (PEC-MFC) device used a well-studied strain of electrogenic bacteria grown in the lab on artificial growth medium. Subsequent tests used untreated municipal wastewater from the Livermore Water Reclamation Plant. The wastewater contained both rich organic nutrients and a diverse mix of microbes that feed on those nutrients, including naturally occurring strains of electrogenic bacteria.

When fed with wastewater and illuminated in a solar simulator, the PEC-MFC device showed continuous production of hydrogen gas at an average rate of 0.05 m3/day, according to LLNL researcher and research team member Fang Qian. At the same time, the turbid black wastewater became clearer. The soluble chemical oxygen demand — a measure of the amount of organic compounds in water, widely used as a water quality test — declined by 67 percent over 48 hours.

The researchers also noted hydrogen generation declined over time as the bacteria used up the organic matter in the wastewater. Replenishment of the wastewater in each feeding cycle led to complete restoration of electric current generation and hydrogen gas production.

Qian said researchers are optimistic about the commercial potential for their invention. Currently they are planning to scale up the small laboratory device to make a larger 40-liter prototype continuously fed with municipal wastewater. If results from the 40-liter prototype are promising, they will test the device on site at the wastewater treatment plant.

“The MFC will be integrated with the existing pipelines of the plant for continuous wastewater feeding, and the PEC will be set up outdoors to receive natural solar illumination,” Qian said.

Wednesday, September 4, 2013 @ 11:09 AM gHale

A new oxygen “sponge” can easily absorb or shed oxygen atoms at low temperatures, which would be useful in devices such as rechargeable batteries, sensors, gas converters and fuel cells.

Materials containing atoms that can switch back and forth between multiple oxidation states are technologically important but very rare in nature, said Oak Ridge National Laboratory’s (ORNL) Ho Nyung Lee, who led the international research team which also wrote a paper on the subject.

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“Typically, most elements have a stable oxidation state, and they want to stay there,” Lee said. “So far there aren’t many known materials in which atoms are easily convertible between different valence states. We’ve found a chemical substance that can reversibly change between phases at rather low temperatures without deteriorating, which is a very intriguing phenomenon.”

Many energy storage and sensor devices rely on this valence-switching trick, known as a reduction-oxidation or “redox” reaction. For instance, catalytic gas converters use platinum-based metals to transform harmful emissions such as carbon monoxide into nontoxic gases by adding oxygen. Less expensive oxide-based alternatives to platinum usually require very high temperatures — at least 600 to 700 degrees Celsius — to trigger the redox reactions, making such materials impractical in conventional applications.

“We show that our multivalent oxygen sponges can undergo such a redox process at as low as 200 degrees Celsius, which is comparable to the working temperature of noble metal catalysts,” Lee said. “Granted, our material is not coming to your car tomorrow, but this discovery shows that multivalent oxides can play a pivotal role in future energy technologies.”

The team’s material consists of strontium cobaltite, known to occur in a preferred crystalline form called brownmillerite. Through an epitaxial stabilization process, the ORNL-led team discovered a new recipe to synthesize the material in a more desirable phase known as perovskite. The researchers have filed an invention disclosure on their findings.

“These two phases have very distinct physical properties,” Lee said. “One is a metal, the other is an insulator. One responds to magnetic fields, the other does not — and we can make it switch back and forth within a second at significantly reduced temperatures.”

The international team’s design and testing of this advanced material from scratch required multidisciplinary expertise and sophisticated tools from places such as Argonne National Laboratory and ORNL, including Argonne’s Advanced Photon Source and ORNL’s Center for Nanophase Materials Science, Lee said.

“As we showed in this study, only through the study of a well-defined system can we build a framework for the design of next generation energy materials,” said coauthor John Freeland of Argonne. “This insight was made possible by merging the capabilities at Oak Ridge and Argonne national labs for advanced synthesis and characterization of novel materials.”

Tuesday, March 5, 2013 @ 03:03 PM gHale

An “artificial leaf” system under development would be able to use sunlight to produce a storable fuel, such as hydrogen, instead of electricity for immediate use.

The hydrogen could then work on demand to generate electricity through a fuel cell or other device. This process would liberate solar energy for use when the sun isn’t shining, and open up a host of potential new applications, according to a team of researchers at MIT.

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Researchers discuss the new work is in a paper by associate professor of mechanical engineering Tonio Buonassisi, former MIT professor Daniel Nocera (now at Harvard University), MIT postdoc Mark Winkler (now at IBM) and former MIT graduate student Casandra Cox (now at Harvard). It follows up on 2011 research that produced a “proof of concept” of an artificial leaf — a small device that, when placed in a container of water and exposed to sunlight, would produce bubbles of hydrogen and oxygen.

The device combines two technologies: A standard silicon solar cell, which converts sunlight into electricity, and chemical catalysts applied to each side of the cell. Together, these would create an electrochemical device that uses an electric current to split atoms of hydrogen and oxygen from the water molecules surrounding them.

The goal is to produce an inexpensive, self-contained system they could build from abundant materials. Nocera has long advocated such devices as a means of bringing electricity to billions of people, mostly in the developing world, who now have little or no access to it.

“What’s significant is that this paper really describes all this technology that is known, and what to expect if we put it all together,” Cox said. “It points out all the challenges, and then you can experimentally address each challenge separately.”

This is a “pretty robust analysis that looked at what’s the best you could do with market-ready technology,” Winkler said.

The original demonstration leaf, in 2011, had low efficiencies, converting less than 4.7 percent of sunlight into fuel, Buonassisi said. But the team’s new analysis shows efficiencies of 16 percent or more should now be possible using single-bandgap semiconductors, such as crystalline silicon.

“We were surprised, actually,” Winkler said. Conventional wisdom held the characteristics of silicon solar cells would severely limit their effectiveness in splitting water, but that turned out to not be the case. “You’ve just got to question the conventional wisdom sometimes,” he said.

The key to obtaining high solar-to-fuel efficiencies is to combine the right solar cells and catalyst — a matchmaking activity best guided by a roadmap. The approach presented by the team allows for each component of the artificial leaf to undergo testing individually, then combined.

The voltage produced by a standard silicon solar cell, about 0.7 volts, is insufficient to power the water-splitting reaction, which needs more than 1.2 volts. One solution is to pair multiple solar cells in series. While this leads to some losses at the interface between the cells, it is a promising direction for the research, Buonassisi said.

An additional source of inefficiency is the water itself — the pathway the electrons must traverse to complete the electrical circuit — which has resistance to the electrons, Buonassisi said. So another way to improve efficiency would be to lower that resistance, perhaps by reducing the distance that ions must travel through the liquid.

“The solution resistance is challenging,” Cox said. But, she added, there are “some tricks” that might help to reduce that resistance, such as reducing the distance between the two sides of the reaction by using interleaved plates.

“In our simulations, we have a framework to determine the limits of efficiency” that are possible with such a system, Buonassisi said. For a system based on conventional silicon solar cells, he said, that limit is about 16 percent; for gallium arsenide cells, a widely touted alternative, the limit rises to 18 percent.

Models to determine the theoretical limits of a given system often lead researchers to pursue the development of new systems that approach those limits, Buonassisi said. “It’s usually from these kinds of models that someone gets the courage to go ahead and make the improvements,” he said.

“Some of the most impactful papers are ones that identify a performance limit,” Buonassisi said. But, he said, there’s a “dose of humility” in looking back at some earlier projections for the limits of solar-cell efficiency: Some of those predicted “limits” have already exceeded expectations, he said.

“We don’t always get it right,” Buonassisi said, but such an analysis “lays a roadmap for development and identifies a few ‘levers’ that can be worked on.”

Friday, December 21, 2012 @ 01:12 PM gHale

It wasn’t that long ago where the United States imported much of its natural gas, but the tables have turned where the country is producing more than it can handle and is now exporting.

With this abundance of natural gas, there is now a need to develop ways to convert that form of energy into other practical uses.

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“With petroleum reserves in decline, natural gas production is destined to increase to help meet worldwide energy demands,” said Matthew Neurock, a chemical engineering professor in the University of Virginia’s School of Engineering and Applied Science. “But petroleum – in addition to being used to make fuels – is also used to make ethylene, propylene and other building blocks used in the production of a wide range of other chemicals. We need to develop innovative processes that can readily make these chemical intermediates from natural gas.”

That is where the problem comes in. There are currently no cost-effective ways to do this. Methane, the principal component of natural gas, is fairly inert and requires high temperatures to activate its strong chemical bonds; so a practical conversion of methane to useful chemical intermediates has eluded chemists and engineers so far.

For Neurock, that is the beginning of a challenge as he is now working with colleagues at Northwestern University to invent novel ways and catalytic materials to activate methane to produce ethylene.

Along those lines, they just published a paper in the online journal Nature Chemistry detailing the use of sulfur as a possible “soft” oxidant for catalytically converting methane into ethylene, a key “intermediate” for making chemicals, polymers, fuels and, ultimately, products such as films, surfactants, detergents, antifreeze, textiles and others.

“We show, through both theory – using quantum mechanical calculations – and laboratory experiments, that sulfur can be used together with novel sulfide catalysts to convert methane to ethylene, an important intermediate in the production of a wide range of materials,” Neurock said.

Chemists and engineers have attempted to develop catalysts and catalytic processes that use oxygen to make ethylene, methanol and other intermediates, but have had little success as oxygen is too reactive and tends to over-oxidize methane to common carbon dioxide.

Neurock said sulfur or other “softer” oxidants that have weaker affinities for hydrogen may be the answer, in that they can help to limit the over-reaction of methane to carbon disulfide. In the team’s process, methane reacts with sulfur over sulfide catalysts used in petroleum processes. Sulfur removes hydrogen from the methane to form hydrocarbon fragments, which subsequently react together on the catalyst to form ethylene.

Theoretical and experimental results indicate the conversion of methane and the selectivity to produce ethylene end up controlled by how strong the sulfur bonds to the catalyst. Using these concepts, the team explored different metal sulfide catalysts to ultimately tune the metal-sulfur bond strength in order to control the conversion of methane to ethylene.

Chemical companies consider methane a particularly attractive raw material because of the large reserves of natural gas in the U.S. and other parts of the world.

“The abundance of natural gas, along with the development of new methods to extract it from hidden reserves, offers unique opportunities for the development of catalytic processes that can convert methane to chemicals,” Neurock said. “Our finding – of using sulfur to catalyze the conversion of methane to ethylene – shows initial promise for the development of new catalytic processes that can potentially take full advantage of these reserves. The research, however, is really just in its infancy”

Tuesday, November 13, 2012 @ 07:11 PM gHale

Using the sun and ultrathin films of iron oxide, better known as rust, there is now a new way to split water molecules into hydrogen and oxygen.

This could lead to less expensive, more efficient ways to store solar energy in the form of hydrogen-based fuels and could be a major step forward in the development of viable replacements for fossil fuels.

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“Our approach is the first of its kind,” said lead researcher Associate Prof. Avner Rothschild, of the Technion-Israel Institute of Technology Department of Materials Science and Engineering. “We have found a way to trap light in ultrathin films of iron oxide that are 5,000 times thinner than typical office paper. This is the enabling key to achieving high efficiency and low cost. ”

Iron oxide is a common semiconductor material, inexpensive to produce, stable in water, and – unlike other semiconductors such as silicon – can oxidize water without itself becoming oxidated, corroded, or decomposed. But it also presents challenges, the greatest of which was finding a way to overcome its poor electrical transport properties. Researchers have struggled for years with the tradeoff between light absorption and the separation and collection of photogenerated charge carriers before they die out by recombination.

“Our light-trapping scheme overcomes this tradeoff, enabling efficient absorption in ultrathin films wherein the photogenerated charge carriers are collected efficiently,” Rothschild said. “The light is trapped in quarter-wave or even deeper sub-wavelength films on mirror-like back reflector substrates. Interference between forward- and backward-propagating waves enhances the light absorption close to the surface, and the photogenerated charge carriers are collected before they die off.”

The breakthrough could make possible the design of inexpensive solar cells that combine ultrathin iron oxide photoelectrodes with conventional photovoltaic cells based on silicon or other materials to produce electricity and hydrogen. Rothschild said these cells could store solar energy for on demand use, 24 hours per day. This is in strong contrast to conventional photovoltaic cells, which provide power only when the sun is shining (and not at night or when it is cloudy).

The findings could also reduce the amount of extremely rare elements the solar panel industry uses to create the semiconductor material in their second-generation photovoltaic cells. The Technion team’s light trapping method could save 90% or more of rare elements like Tellurium and Indium, with no compromise in performance.

Wednesday, October 24, 2012 @ 07:10 PM gHale

Nicholas Creager just pointed to one of Iowa State University’s latest biofuel machines.

The 6-inch diameter, stainless steel pipe is the pressure vessel, which is essential for the system’s operation, said Creager, a doctoral student in mechanical engineering and biorenewable resources and technology. It’s a little over three feet long and about a foot across. It can contain pressures up to 700 pounds per square inch.

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Then Creager picked up a dark gray pipe that’s a few inches across, wrapped in insulation and fits inside the pressure vessel. It’s the system’s reactor. It’s made of silicon carbide and can operate at temperatures exceeding 1,800 degrees Fahrenheit.

Next was a finger-sized nozzle that mixes bio-oil with oxygen and sprays it into the top of the reactor.

Add a bunch of toggle switches, electronics, pipes, a sturdy frame and some very thick bolts and you have a bio-oil gasifier. It will allow Iowa State researchers to combine two thermochemical technologies to produce the next generation of fuels from renewable resources such as corn stalks and wood chips.

First, biomass feeds into a fast pyrolysis machine where it quickly heats without oxygen. The end product is a thick, brown oil that can divide and further process into fuels. Researchers sometimes describe bio-oil as densified biomass that’s much easier to handle and transport than raw biomass.

Second, the bio-oil sprays into the top of the gasifier where heat and pressure vaporize it to produce a combination of (mostly) hydrogen and carbon monoxide called synthesis gas.

That gas can process into transportation fuels. It can also see use as boiler fuel to create the steam that turns turbines to produce electricity.

“We hope to be able to use cellulosic biomass as opposed to using corn grain for the production of fuels,” said Robert C. Brown, the director of Iowa State’s Bioeconomy Institute, an Anson Marston Distinguished Professor in Engineering and the Gary and Donna Hoover Chair in Mechanical Engineering. “This helps us move toward cellulosic biofuels.”

Brown said researchers have yet to perfect ways to biologically break down plant cellulose to get at the sugars that are converted to fuels. And so the Iowa State researchers are turning to nature’s solution.

“Nature uses high temperatures to quickly decompose biomass,” Brown said.

The bio-oil gasifier has been fully operational since June and has been converting bio-oil made from pine wood into synthesis gas. As the project moves beyond its startup phase, researchers will use bio-oil produced by Iowa State researchers and fast pyrolysis equipment.

The gasifier is part of a two-year, nearly $1 million grant from the U.S. Department of Energy. Another three-year, $450,000 grant from the Iowa Energy Center will allow researchers to study and refine bio-oil gasification.

Song-Charng Kong, an associate professor of mechanical engineering who’s leading the latter project, will build a computer simulation model of bio-oil gasification. The model will take into account changes in temperature, pressure and biomass. It will allow researchers to understand, predict and ultimately improve the gasification process.

The project will also develop a systems simulation tool that allows researchers to examine the technical, economic and big picture implications of bio-oil gasification. And finally, the project will develop a virtual reality model of a full-size plant that will allow researchers to see, study and improve a plant before construction crews are ever hired.

“The physics and chemistry will be behind all these models and images,” Kong said. “This is a very new area to study. We can use these models as a tool to understand what will happen as this technology is scaled up.”

Monday, March 26, 2012 @ 03:03 PM gHale

An explosion involving a chemical reactor at a semiconductor factory left two people injured in Northwest Portland, OR.

Two men doing maintenance Thursday night near the chemical reactor at Wacker Siltronics ended up injured, fire department spokesman Paul Corah said. The reactor combines chemicals for manufacturing.

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“The 9-1-1 call came in as a Hazmat explosion at the facility — an explosion of a chemical reactor which resulted in a fire and the release of chemical gases (reportedly phosgene gas and trichloroethylene),” American Medical Response reports. “With the assistance of Portland Fire & Rescue, two patients were identified and, after assessment at the scene, AMR paramedics transported both patients to Legacy Emanuel Medical Center for further treatment.”

Phosgene gas is a major industrial chemical used to make plastics and pesticides. It is poisonous at temperatures about 70 degrees Fahrenheit, according to the U.S. Centers for Disease Control.

Trichloroethylene is a colorless liquid used as a solvent for cleaning metal parts. Drinking or breathing high levels of trichloroethylene may cause nervous system effects, liver and lung damage, abnormal heartbeat, coma, and possibly death, the CDC reports.

A 56-year-old man and a 58-year-old man reported respiratory problems after the explosion and personnel took them to the hospital.

Two plant technicians and two Portland Fire hazmat technicians assessed the chemical reactor’s condition. The explosion drew more than 40 Portland Fire personnel.

Corah said the explosion remained confined to one part of the plant, which makes semiconductor wafers.

Wednesday, March 21, 2012 @ 12:03 PM gHale

A robotic jellyfish, oddly named Robojelly, not only works for underwater search and rescue operations, but could also never run out of energy because it uses hydrogen fuel.

Constructed from a set of smart materials, which have the ability to change shape or size as a result of a stimulus, and carbon nanotubes, Robojelly mimics the natural movements of a jellyfish when placed in a water tank and gets power by chemical reactions taking place on its surface.

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“To our knowledge, this is the first successful powering of an underwater robot using external hydrogen as a fuel source,” said Virginia Tech’s Yonas Tadesse, lead author of a study on the subject.

The jellyfish is an ideal invertebrate to base the vehicle on because of its simple swimming action: It has two prominent mechanisms known as “rowing” and “jetting”.

A jellyfish’s movement is down to circular muscles located on the inside of the bell – the main part of the body shaped like the top of an umbrella. As the muscles contract, the bell closes in on itself and ejects water to propel the jellyfish forward. After contracting, the bell relaxes and regains its original shape.

They were able to replicate the movement in the vehicle using commercially-available shape memory alloys (SMA) – smart materials that “remember” their original shape – wrapped in carbon nanotubes and coated with a platinum black powder.

The robot gets its power by heat-producing chemical reactions between the oxygen and hydrogen in water and the platinum on its surface. The heat given off by these reactions transfers to the artificial muscles of the robot, causing them to transform into different shapes.

This green, renewable element means Robojelly can regenerate fuel from its natural surroundings and therefore doesn’t require an external power source or the constant replacement of batteries.

At the moment, the hydrogen-powered Robojelly has been functioning while clamped down in a water tank. Researchers said the robot still needs development to achieve full functionality and efficiency.

“The current design allows the jellyfish to flex its eight bell segments, each operated by a fuel-powered SMA module. This should be sufficient for the jellyfish to lift itself up if all the bell segments are actuated.

“We are now researching new ways to deliver the fuel into each segment so that each one can be controlled individually. This should allow the robot to be controlled and moved in different directions,” Tadesse said.

Monday, January 16, 2012 @ 02:01 PM gHale

For the first time, a superconducting current limiter is now working at a power plant that could help enhance intrinsic safety of the grid.

At the Boxberg power plant of Vattenfall, Germany, the current limiter, based on YBCO strip conductors, protects the grid against damage due to short circuits and voltage peaks. The new technology, co-developed by Karlsruhe Institute of Technology and made by Nexans SuperConductors, enhances the intrinsic safety of the grid and may help reduce the investment costs of plants.

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“For a long time, high-temperature superconductors were considered to be difficult to handle, too brittle, and too expensive for general industrial applications,” said project manager Wilfried Goldacker from Karlsruhe Institute of Technology. “The second generation of high-temperature superconductor wires based on YBCO ceramics is much more robust. Properties have been improved.”

Superconducting current limiters work reversibly. In case of current peaks after short circuits in the grid, no components end up destroyed. The limiter automatically returns to the normal state of operation after a few seconds. Consequently, the power failure is much shorter than in case of conventional current limiters, such as household fuses, whose components usually end up ruined and you have to replace them.

“Superconducting current limiters have a number of advantages for the stability of medium- and high-voltage grids,” said Mathias Noe, head of the Institute of Technical Physics of Karlsruhe Institute of Technology.

Reliable, compact current limiters enhance the operation stability of power grids and allow for a simplification of the grid structure. They end up protected against current peaks. In addition, decentralized energy generators, such as wind and solar systems, can integrate quite a bit easier into rids. Expensive components in the existing grid enjoy greater protection. In the future, components can undergo a design for smaller peak currents, and transformers will no longer be necessary. Investment costs of power plants and grids will be lower. Superconducting current limiters on the basis of YBCO can also apply to high-voltage grids of more than 100 kilovolts for better protection against power failures in the future.

YBCO stands for the constituents of the superconductor: Yttrium, barium, copper, and oxygen. An YBCO crystal layer of about 1 micrometer in thickness grows directly on a stainless steel strip of a few millimeters in width that gives the ceramics the necessary stability.

Below a temperature of 90° Kelvin or minus 183° Celsius, the material becomes superconductive. However, superconductivity collapses abruptly when the current in the conductor exceeds the design limits. This effect sees use by the current limiter. In case of current peaks in the grid, the superconductor loses its conductivity within fractions of a second and the current will flow through the stainless steel strip only, which has a much higher resistance and, thus, limits the current. The heat ends up removed by the cooling system of the superconductor. A few seconds after the short circuit, it returns to normal operation in the superconducting state. YBCO superconducting layers on stainless steel strips are more stable and operation-friendly than first-generation superconductors based on BSCCO ceramics. Moreover, their production does not require any noble metals, such as silver, and cost much less.

A field test is underway at the Vattenfall utility company.

Tuesday, September 27, 2011 @ 04:09 PM gHale

Hydrogen has accumulated to a level higher than previously thought in pipes connected to the No. 1 reactor containment vessel at the Fukushima No. 1 nuclear power plant, Tokyo Electric Power Co. (TEPCO) officials said.

But TEPCO officials said an explosion was unlikely because they were pumping nonflammable nitrogen in to prevent oxygen from entering the containment vessel and triggering a blast. TEPCO also said the discovery of the hydrogen buildup will not affect the road map toward achieving a cold shutdown and other schedules.

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TEPCO is looking at the possibility hydrogen has also accumulated in a similar manner at the plant’s No. 2 and No. 3 reactors.

However, the company acknowledged there was no way to tell whether the hydrogen in the pipes came right after the onset of the crisis on March 11 or in later stages. Nor could TEPCO measure how much hydrogen generated in the containment vessel.

Nitrogen injections have lowered the hydrogen concentration considerably, but some hydrogen, being lighter than nitrogen, may be accumulating near the top of the containment vessel.

The discovery of the hydrogen accumulation came when TEPCO was analyzing gas in the pipes connected to the interior of the No. 1 reactor containment vessel while installing a device to reduce the amount of radioactive substances leaking from the vessel.

The hydrogen concentration exceeded one percent, the threshold of the measurement device. TEPCO said it was conducting a more detailed analysis on the concentration level.

TEPCO said most of the accumulated hydrogen was the result of a reaction under high temperatures between water vapor and the surface of nuclear fuel rods exposed after water was lost following the March 11 earthquake and tsunami.

Even now, the damaged reactors may be generating small amounts of hydrogen as water decomposes through irradiation from the melted fuel rods.

An explosion can occur in a gas containing more than 4 percent hydrogen and more than 5 percent oxygen.

TEPCO has been pumping nitrogen into the No. 1, No. 2 and No. 3 reactor containment vessels to drive out hydrogen from their interiors. The injection of nitrogen also should create higher pressure levels than those outside to prevent oxygen in the air from entering the containment vessels.

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