Plant personnel at Arkansas Nuclear One drove development of a method to attach a flexible shield to various sizes of pipes and surfaces.
By David Wagman, Chief Editor
Arkansas Nuclear One is identifying opportunities to create and deploy technologies that achieve breakthrough for the plant and ultimately, the entire Entergy fleet. An innovative shielding design using tungsten allows the plant to achieve higher levels of safety, worker efficiency and other advantages.
In looking beyond conventional materials such as lead, steel and water for more effective materials to use in shielding applications, tungsten has been identified as a viable radiation shielding material. Tungsten is used effectively and economically in innovative piping and surface applications.
Jim Bacquet, radiation protection supervisor, ALARA, at Arkansas Nuclear One (ANO), said lead can be a “big concern” because of its toxicity. “When it becomes contaminated with radioactive contaminants, lead becomes a mixed hazardous waste, which is extremely difficult to get rid of,” he said. “Contaminated lead is a very bad liability for a company to have much of it.” Tungsten, by contrast, is benign and faces no handling or storage restrictions. What’s more, tungsten’s price has fallen over the past 10 years, largely erasing what had been a factor favoring lead.
ANO drove development of a method to attach a flexible shield to various sizes of pipes and surfaces. Working with American Ceramics Technology (formerly Absorbtek), plant personnel successfully implemented the shielding at Arkansas Nuclear One during a 2008 refueling outage.
A major contributor of the outage exposure success was development and implementation of an effective shielding plan. ANO took a step forward in shielding technology using new generation shielding (NGS) in several first-time applications. Eight shield sites were installed using some 5,800 pounds of shielding material. The company said this was a big part of the reason why the radiation exposure for the outage came in 12 Rem below the outage exposure stretch goal.
Defining the Shield
The Entergy tungsten radiation shielding is a flexible heat-resistant shielding made of tungsten and iron metal powder immersed in a silicone polymer. Lead blankets have been the backbone of shielding applications for years but, in comparison, is a less effective and efficient shielding choice. Due to the ability to place the maximum amount of weight between the source and the worker, the new generation tungsten shielding has proven to be up to twice as effective as lead in lowering exposure rates.
Designed in custom shapes, this 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. Additionally, the tungsten shield weighs 25 to 50 percent less than lead while removing the accompanying toxicity hazard and mixed waste processing costs. As a primary driver of this shielding approach, safety improvements included:
- During the U2 RVCH shroud modification, the new generation shielding avoided 10 person-Rem
- The weight reduction results in improved industrial safety conditions and, specifically, reduced worker fatigue
- Transport to the worksite proved safer and easier with tungsten shielding
- Floor surface shielding uses (such as on the refueling bridge or on work platforms) are safer due to the uniform thickness across the surface. This eliminated tripping hazards and provided a cushion to reduce the stress of standing or kneeling for long periods on steel surfaces.
- The tungsten shielding does not generate a hazardous waste material.
Over 4,000 pounds of the 7/8-inch-thick shielding was installed for this project. Dose rates on the CEA Motor Tubes were reduced from 800 mRem/hr contact and 250 mRem/hr at 30 cm to 250 mRem contact and 120 mRem/hr at 30 cm.
The dose performance for the RVCH Lift Rig Project was 8.1 Rem compared to an estimated 21 Rem. Since the 6-inch pipe wrap shielding was purchased for the RVCH Lift Rig Project, the same shielding has maintained its effectiveness and has been used for multiple applications at both ANO-Unit 1 and ANO- Unit 2, including decay heat system piping, core flood system piping, RCP bleed-off piping, gas collection header piping and pressurizer spray piping.
The shielding remains in good condition and is ready for the next application, demonstrating what the company said is the versatility and durability of the NGS material. The material is also able to be readily decontaminated.
Flexible in the Field
The system allows for shielding difficult-to-shield projects that include pipe diameters from ¾-inch and larger. The shielding can be ordered in custom sizes and thicknesses, such as sheets or a roll of duct tape. The ability to cut shielding with scissors or knives allows for a “field fit” that works well around obstructions.
The time savings in the field because of this adaptability converts to dose savings and ALARA is realized at Entergy’s ANO facility. Specific examples include:
- Shielding is low in profile and therefore can be used in areas where there are clearance issues, such as with automated welders
- Pipe shielding can be formed to position entire engineered weight between the radius of the pipe and the occupied work area
- Overall flexibility to field-needs means that installation of shielding is completed in a shorter period of time
- Use of this shielding on floor surfaces provides additional safety measures eliminating uneven surfaces and lessens the physical stress of standing for long periods on steel surfaces
- There is less dose to workers hauling lead blankets during the outage or in normal operations.
The Entergy tungsten radiation shielding uses a new method of attaching shielding using imbedded magnets. As a polymer material, this shielding becomes form fitting.
The ability to secure shielding in this attachment avoids the need for ty-wraps or other means of attachment, therefore foreign material exclusion is enhanced. This shielding technology also lessens exposure by reducing the time required to install.
Saving Time and Dollars
The savings realized as a result of using the new generation tungsten shielding are as follows:
- $300,000/outage equating to 24 hours critical path time
- $15,000/outage in outage labor hours saved to install shielding
- $4,000/year in non-outage labor hours saved for installing shielding
- $319,000 TOTAL COST SAVINGS
The new generation shielding that Entergy has used at its ANO facility ranges in weight from 0.5 pounds to 60 pounds per piece. One advantage of the NGS is that it can be fabricated to weigh whatever is allowable by the associated engineering evaluation. This helps take advantage of the full allowable weight and improves the efficiency of a given approved weight. Conventional lead blankets are typically available in a single weight rate of 10 pounds per linear foot. This limitation doesn’t take full advantage of an engineering analysis that would allow (for example) 18 pounds per linear foot.
Additionally, with custom fit application, the new generation shielding allows the worker to put the weight of the shielding where it’s needed the most.
“Because we can form the tungsten, we can concentrate the weight on a specific location,” said Dan Stoltz, RP ALARA coordinator. For example, a hot spot may be only as big as one square inch. Using a lead blanket as a shielding tool consumed much of the weight that engineering allowed. “The way the tungsten is formed, sometimes like a roll of tape, we can concentrate the weight where the hot spot is,” he said.
The tungsten shielding is lighter and easier to handle than lead blankets required for similar exposure reduction. Moving and installing the shielding, therefore, proved to be quicker and required less manpower. The Entergy tungsten shielding is acceptable up to 300 C, which allows applications close to weld sites.
New applications have already been implemented for the Auxiliary Building at ANO and include upper and lower north piping rooms in Unit One; and upper and lower south piping rooms in Unit Two. The Entergy shielding has been shipped to and applied to piping at Pilgrim Nuclear Power Station and River Bend Nuclear Station.
Custom-ordered new generation tungsten shielding is applied to negate gamma- streaming from the reactor vessel reducing the radiation from approximately 120 mRem to 40 mRem. Demonstrating versatility, the tungsten shielding is fitted around the pipe to form an outer square. The additional tungsten shielding is then fitted to the square.
For the Entergy fleet, this method in its many variations will be used across all facilities. The new generation shielding leads to lower collective radiation exposure for the Arkansas Nuclear One work force.
This project was a team effort to enhance productivity and efficiency. It involved Phillip Killane and Sam Bradshaw from Radiation Protection. The Refuel Team, headed by John Meyers, has been involved in managing the benefit from the shielding during the outage period. Engineering has been involved including work by Cowper Chadbourn, David McKenny and Maqbool Bhatti. Mechanics were involved with application of the NGS on the Reactor Coolant Pump Seal replacements for “B” and “C” RCPs, including supervisor Jeff Erwin, Lynn Muncy as superintendent and Randy Butts.