operations & maintenance
A vested interest
2 July 2010A team at Entergy’s Arkansas Nuclear One have developed a personal shielding tungsten vest. When trialled during outages at the plant it decreased radiation exposure by up to 40%, leading to significant cost savings.
During an outage at Arkansas Nuclear One unit 2 in 2009, Entergy employees wore six radiation-shielding tungsten vests for the first time in the industry’s history. Following the success of the project, which saw a decrease in the number of staff required, as well as a reduction in dose and cost savings, Entergy decided to purchase additional vests (six for use at a Waterford outage and another ten for ANO). It has continued to utilize the vests in other projects, for example as shielding in the pump cavities during the replacement of the seal cartridges. Other applications for the vest are in projects such as work under the reactor head, in inspections of Alloy 600 and steam generators.
“Working with the possibilities of tungsten brought us to this solution for shielding that is really revolutionary,” says Dan Stoltz, RP/ALARA Coordinator. The tungsten vests were developed as part of a project at ANO to come up with new generation shielding (NGS) technology. Initially ANO staff developed tungsten shielding to replace conventional methods. Unlike lead blankets, whose shape and size are limited by manufacturing constraints, the tungsten material can be custom-ordered to the desired weight and dimensions allowed by engineering calculations. The material was first used during an ANO outage in 2008.
The vest works on the principle that shielding the abdomen area will significantly reduce overall dose. An effective dose equivalent (EDE) approach recognises that a radiation dose to one part of the body does not have the same potential health effect as a dose to another part. For the thorax and abdomen the percentage damage is 88%, compared with 10% for the head and 2% for other areas. In contrast, current US estimating methods base the entire received dose on what the head receives.
Entergy was granted approval by the NRC to use an Electric Power Research Institute (EPRI) two-badge method for estimating EDE at its sites. This approval is based upon experience and expertise extending from calculations used in the medical field.
Looking at the numbers, if the exposure was 100mRem, with the tungsten vest covering the thorax and abdomen, the dose received would be 88% or 88mRem. Tungsten is twice as effective as lead at shielding because it is almost twice as dense, so the exposure would be 44%. Therefore, it a 30-40% reduction in dose is experienced.
Heat exchanger project
The first use of the tungsten vests, during a project to install shielding in ANO unit 2 indicated an actual reduction of dose of close to 40%.
The project entailed building and installing a permanent shield comprised of a steel plate, one inch thick, around a regenerative heat exchanger in the chemical volume and control system. The project was scheduled for 10 days, 24 hours per day, with seven to ten employees assigned to the project during the 2009 outage, not on the critical path timeline. The heat exchanger is the highest dose-contributor for the ANO unit 2 containment building.
Welders and fitters on scaffolds, up on rigging or some within inches of the exchanger were exposed to approximately 200-600mRem/hour. ANO put in additional monitoring devices to capture dose readings on this project.
Overall, the exposure to the welders was reduced by 39% and the total exposure avoided was 642 person-mRem. This calculation was based on an initial dose estimate of 1550 person-mRem, reduced to 908 person-mRem by the tungsten vests.
The vest
The Entergy tungsten vest is made of a flexible heat-resistant shielding made of tungsten and iron metal powder immersed in a silicone polymer. It was developed together with American Ceramic Technology.
As well as lowering exposure rates to the workers, wearing tungsten vests eliminates the need to build temporary shielding racks and it avoids exposure for short duration projects where shielding the source is not possible. Since all the material used in the tungsten-shielding vest is non-toxic, it may be disposed of in a different radioactive waste stream than the toxic lead material, whose waste is classed as an expensive mixed hazardous waste.
The 6.8kg/15 lb vest sells for approximately $3700. At first glance, tungsten seems more expensive than its alternative, iron; finished tungsten shielding costing $67.50 compared with $32.50 for the iron equivalent. But the new generation shielding more than pays off the price premium through dose savings and labour savings during project work.
