Nuclear

Tungsten Shielding Helps at Fukushima Daiichi

Issue 3 and Volume 4.

A Tokyo Electric power worker in Unit 1 at the Fukushima Daiichi Nuclear Power Station in may 2011. Photo courtesy of TEPCO.

By Brian Wheeler, Editor

Technology implemented at over 40 percent of the operating nuclear power plants in the United States is now being used to help in the efforts to contain the stricken reactors at the Fukushima Daiichi nuclear power plant in Japan.

Tungsten shielding, first developed in 2006 with a partnership between Entergy Nuclear and American Ceramics Technology, has been used by Entergy employees since 2008 for protection from radiation while performing maintenance during refueling outages. The shielding is a flexible heat-resistant material made of tungsten and iron metal powder immersed in a silicone polymer.

Traditionally, lead has been the product of choice for radiation shielding. Tungsten shielding has the ability to field-fit, providing for attenuation of radiation totaling from 5 to 10 person-Rem/year than provided by the equivalent weight of traditional lead blanket. And since tungsten is thinner and weighs as much as 50 percent less than lead, it may be more forgiving when workers are performing the physical activities required in a nuclear power plant.

“It gives them better protection at a lighter weight and better mobility,” said Dick Culbertson, CEO of American Ceramics Technology.

Vests made of tungsten worn by employees working in areas where there is exposure to radiation were successful when first used, but did have limitations. The vests had the ability to protect only the front of the body, so the source of radiation had to be directly in front of the worker. Then, American Ceramics Technology received a request from the 810 MW Cooper Nuclear station, which Entergy provides support services to, for a vest that covered the entire body, front and back. Workers at Cooper were entering areas of the plant that had a high dose field coming from all directions.

Workers at Entergy’s Arkansas Nuclear One install New Generation Sheilding on the Control Element Assembly Motor Tubes during a Reactor Vessel Closure Head Lift Rig Project in 2008.

Entergy and American Ceramics Technology developed the vest that covers both the front and back of the body as well as 88 percent of the blood-forming organs about a month before the Japanese earthquake and tsunami. The new model has given workers at the Fukushima Daiichi plant the ability to work in areas that before may have been impenetrable.

“When the workers went in initially to try to get power and protective systems going, and to find out exactly what was going on, TEPCO had to turn their workers around because the dose rates were so high,” said Jim Bacquet, radiation protection supervisor, ALARA, at Entergy’s Arkansas Nuclear One plant.

Bacquet said if the workers had been wearing the tungsten vests, they could have doubled the amount of time they had to make their initial assessments of the plant’s condition and next steps to start recovering the facility.

Workers at Fukushima Daiichi had been using the single-sided tungsten vests. With the new, two-sided vests a dozen workers entered the reactor building of Unit 1 in May for the first time since the March 11 disaster to install a ventilator to help lower radiation levels inside the reactor building.

“As it worked out, we had just developed a new vest that was exactly what the Japanese needed because they were dealing with a source coming from all directions; ground and all sides,” said Bacquet.

There are now 180 tungsten vests being used in Japan. American Ceramics Technology has orders for 30 additional vests and quoted a price for another 200 to be used by a second utility.

“The initial use is proving effective in Japan and people are coming back for a second wave of ordering,” said Culbertson.

Additional Uses

To date, American Ceramics Technology has sold over 35,000 pounds of tungsten shielding products, not including the vests, for use as pipe wraps and blankets for shielding large areas. The largest blankets shipped to Fukushima Daiichi are 12-inch by 18-foot-long rolls that are half an inch thick. With the ability to “field-fit” the product, the tungsten can be cut with scissors or knives to create custom sizes in hot spots.

Tungsten shielding is a flexible radiation shield that can be cut in the field to fit various needs. Photo courtesy of Entergy and American Ceramics Technology.

Along with the large, and heavy, 18-foot-long rolls, hundreds of 1-foot by 3-foot blankets have been shipped to Japan. Working with Entergy, American Ceramics Technology also developed a magnetic product that can be applied. Three-hundred tungsten pieces with embedded magnets are now in use. The magnetic pieces are being used at temporary facilities around the plant that are constructed with steel walls. Culbertson said workers on-site can apply the magnets on the walls for added protection. Shielding the temporary facilities is the first step toward building custom “safe haven” facilities at Fukushima Daiichi with enhanced shielding and air filtration systems.

Proven Technology

The tungsten vests have been used at Entergy plants for protection during maintenance. During a 2008 Arkansas Nuclear One refueling outage, for example, eight shield sites were installed using about 5,800 pounds of the shielding. Of that, 168 pieces were installed in less than an hour on the control element assembly motor tubes during a reactor vessel closure head lift rig project. The dose rate for the lift rig project with tungsten in use fell from an estimated 21 Rem to an actual dose of 8.1 Rem. During the 2008 outage, radiation protection also increased.

Former Process: Dose estimate = 1550 person-mRem
With tungsten vest: Dose estimate = 908 person-mRem
Savings: 642 mRem
Annual Savings: 642 mRem
Savings life of plant (2034): 15408 mRem

At Entergy, all emergency planning drills are set up to have plant workers make entry into a high dose area to recover some piece of equipment to stop the release of radiation in order to protect the public. Now, Entergy is implementing the use of the vests for all emergency planning drills.

“This is an ideal piece of equipment to protect the worker that now is not under normal 10CFR20 limits for exposure,” said Bacquet. “When going back in to protect the public the limits are raised dramatically.”

Bacquet said Entergy sees the shielding as a strength in its emergency plan. It will now be automatic that every team member has the vest on when they make entry into the plant, as is being done at the Fukushima Daiichi nuclear power plant.


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